Cable and Wire Harness Assembly Handbook Ground Rules:
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
14)
Why do we do what we do
Technical History
Some How, but only for that which SHOULD not be in the standard
Further explain the processes already in the standard
Lessons Learned to be as part of each section it applies to.
Not be a J/STD-001 or A-610 book, more like the A&J Handbook or Conformal Coating but referencing back to the standard to
see the end results.
Structure to follow TECHNOLOGY
Include ancillary technology.
Audience – WIRE AND CABLE MANUFACTURES and others who are craving information, Designer, ME, QA, Assemblers.
This is stand alone with the A620.
If the Information for a section resides in another document it’s OK to reprint into this document. Must add statement that if
they want to learn more than what is applicable to this document you see the base reference.
Always think about new technology insertion
Pb-Free issues.
Include basic material types and use selection information in each section.
3 TYPES OF HARNESS (INFO FOR INTRO SECTION)
Type 1 - Unprotected Harness
No overbraiding, jacketing etc, held together by lacing/ties
Type 2 - Protected Harness
Has Jacket/Cloth overbraiding, no overall shielding
Type 3 - Protected/Shielded Harness
Jacketed/Cloth overbraid and overall shielded
IPC Midwest
September 2008
TABLE OF CONTENT for IPC-HDBK-620
1
2
3
4
5
6
7
8
9
10
Cable and Wire Harness Assembly Handbook
$Scope
$Purpose
$Approach To This Document
$Uncommon or Specialized Designs
$Terms And Definitions
#Shall or Should
#Classes of Product
#Document Hierarchy
#Tool and Equipment Control
#Observable Criteria
#Defects and Process Indicators
#Inspection Conditions
#Measurement Units and Applications
#Verification of Dimensions
#Visual Inspection
#Contamination
#Materials and Processes
APPLICABLE DOCUMENTS
Cable and Wires Layout and Length measurement
 wire measurement
 termination type measurement points and
process strip allowances.
 wire types and selection of (important) <Apr
08>
Wire Preparation
 Stripping
 Tinning
Wire Termination Methods
a) Mechanical
 Crimp Terminations (Contacts and Lugs)
 Insulation Displacement Connection (IDC)
b) Thermal
 Splices (including hot air melt)
 Soldered Terminations (solder cups, turret,
pierced, J-Hook)
c) Other Technology
 Ultrasonic Welding
 Wire Wrap
d) Coaxial and Twin Axial Cable Assemblies
Connectorization
Molding/Potting
Marking/Labeling
Securing
 Lacing tape vs plastic types
 knots, anti-knot loosening
 location of ties, use of lacing
 tape as securing of tapes and sleeving
Harness/Cable EMI/RFI Shielding
 EMI/RFI shielding theory
 methods of shielding
 shield jumpers [with how to of using shrink
sleeves in wire term methods/splices)
Subjects marked with ‘$’ are to be
looked at and where necessary expand
or explain the category.
Covers Section 1
Those subjects marked with ‘#’ will be
rewritten to agree with actual
handbook use and ground rules.
Teresa Rowe?
IPC STAFF
Brett Miller – USA Harness<Sept 08>
John Laser – L3 <Sept 08>
Covers Section 2
Covers Section 11
Richard Rumas – Honeywell <Apr 08>
Covers Sections 3, 4,
13.1
Subsection & Author
a)Richard Rumas – Honeywell <Apr 08>
d))John Laser – L3 <Apr 08>
c)Brett Miller – USA Harness <Sept 08>
b)Teresa Rowe ????
Covers Sections
A) Section 5, 6, 8
B) Section 4, 8
C) Section 7, 18
D) Section 13
Richard Rumas – Honeywell <Apr 08>
Brett Miller – USA Harness <Sept 08>
Gordon Sullivan <Sept 08>
Les Bogart – Bechtel <Apr 08>
Randy McNutt - Northrop
Covers Sections 9, 13
Covers Section 10
John Laser – L3 <Sept 07>
Covers Section 12
Covers Section 14
Covers Section 13,
15
IPC Midwest
September 2008
11
Harness Jacketing Methods
 Mechanical Braiding (discussions of machines
and braiding materials).
 Open loose Sleeving
 Closed Sleeving
 Tapes
12
Finished Assembly Installation
 wire routing rules
 box installations rules
 hardware termination
 Terminal Blocks
 clamping
 ESD caps
13
Measurement/Testing
14
METHODS OF STD REPAIR & MODIFICATION
(proposed)
APP A
A-620A to this HDBK pointer Chart
Randy McNutt - Northrop
Covers Sections 15,
16
Les Bogert <Sept 07>
Covers Sections 14,
17
Les Bogert<Sept 07>
On Hold
Section 19
NEW
Leadership prior to publishing
IPC Midwest
September 2008
This handbook is a companion reference to and was prepared using IPC/WHMA-A-620A.
Format of this Handbook
The section and paragraph numbers in this handbook refer and correspond to the section and paragraph numbers in Revision A of
IPC/WHMA-A-620. Where used verbatim, text of IPC/WHMA-A-620 is identified by being boxed.
Foreword
1.1
Scope The scope of the IPC/WHMA-A-620A provides visual, electrical and mechanical acceptability requirements. This
document can be used by manufacturers or as a stand-alone for purchasing products. Activities such as in-process and end product
inspection are not defined in the document.
1.2
Purpose This document does not address assembly methods.
1.3
Approach to This Document The document is organized such that the title of each section includes the criteria for that
topic. In some cases, the same or similar figures are shown throughout the document, and the user is advised to select the correct
section when reading the document.
When product is compliant to Class 3, the manufacturers are required to use a documented process control system. Documentation
may occur in any format compliant with a user’s internal requirements.
For all Class 3 product and where a document process control system is used for Class 2, process control and corrective action limits
are required. There is no requirement for Class 2 to have a documented process control system, however when one exists, these
additional requirements apply.
The focus in this section is on the process control. As stated in the document, there is no requirement for a statistical process control
system. The concern is about managing the processes to produce hardware that meets the requirements. The user may decide that
statistical process control is necessary for a particular situation, and in these situations, they may select this as the type of process
control system to use.
Class 2 and Class 3 manufacturers are required to use process control methodologies in the planning, implementation and evaluation
of the processes. Unlike the earlier requirement in this section that may result in documentation depending on the product class, the
approach to using process control methodologies is more of a technique used to achieve an end result.
1.4
Shall or Should The word “shall” is used throughout the document for mandatory requirements. In some cases, the
requirement is applicable to all classes as a process-related requirement, but in some cases, the requirement is not applicable to all
Classes.
In each case, a text box with the associated requirement is listed near the paragraph in which the word appears. Each Class
requirement is stated in the text box, and the user will select the appropriate class to determine whether a hardware defect exists for the
relevant product.
Conditions such as “Defect,” “Process Indicator,” Acceptable” and “Not Established” could be stated in the text boxes. Where the
condition is “Acceptable,” no further action is required by the user when the condition exists. Where the condition is “Not
Established,” the document does not provide any criteria. If a condition exists where the criteria is “Not Established,” the user is
encouraged to determine if additional action is necessary for the particular product.
Where it is used, the word “should” is providing guidance to the user. Even though no requirement exists, the document developers
provide this as useful information to the users.
1.5
Uncommon or Specialized Designs The document developers recognize that industry consensus documents typically
address common technologies. There may, however, be times when the user needs to have additional requirements definition for
particular applications. Users are encouraged to develop these additional criteria for their application and to include the definition for
acceptance of each characteristic. Users are also encouraged to provide this information, where feasible, to the IPC Technical
Committee for consideration in future revisions of the document.
1-1
1.6
Terms and Definitions Definitions for some of the terms used in the document can be found in the Terms and Definitions
section of the standard or in Appendix A of the standard.
1.7
Classes of Product The product classes are provided in this section of the standard. Definition of the product class is
necessary in order to determine which requirements are applicable. The standard provides guidance to the manufacturer where the
manufacturer and user have not established the product class requirements. In these situations, the manufacturer is permitted to select
the product class.
1.8
Document Hierarchy There are many documents that may be invoked when manufacturing this product. The document
hierarchy or order of precedence is established in this section of the standard.
There are various standards available to the industry that include topics also discussed in the IPC/WHMA-A-620A, including J-STD001, “Requirements for Soldered Electrical and Electronic Assemblies” and IPC-A-610, “Acceptability of Electronic Assemblies.”
IPC/WHMA-A-620A users are not required to use these documents unless contractually required. Although information provided may
appear to be similar, the documents have different scope statements and conflicts with the requirements of IPC/WHMA-A-620A may
be introduced it the documents are used incorrectly.
The user does have the option to select alternate acceptance criteria. Where such criteria are specified, however, procurement
documentation must also include the order in which the documents are used. This provides a standard hierarchy that, where a conflict
exists, eliminates confusion on which takes precedence.
1.9
Tool and Equipment Control Manufacturers are required by the standard to have tool and equipment control processes in
place. This is to ensure that tools are in good working condition and are used as intended. These processes are in addition to
calibration requirements that are also defined in this section.
Manufacturers are required to have a documented calibration system as stated in the standard. Where a National or International
standard other than ANSI/NCSL Z540-1 is used, the standard selected for the calibration system is required to meet minimum criteria
as established in IPC/WHMA-A-620A.
1.10
Observable Criteria This document establishes acceptance criteria for the subject matter. Measurements are not typically
required, however, they may be made to supplement an inspection. There is no requirement for this.
Not every condition stated in the standard can be shown in the figures provided. Many times, the conditions shown are worse-case
conditions in order to over-emphasize the condition. This is an aid to the user of the standard in understanding the requirement as
stated. Hence, the written requirements always take precedence over the figures in the standard.
1.11
Defects and Process Indicators Defects are defined in the standard as conditions that fail to meet the acceptance criteria of
the document and affect form, fit or function of the assembly in its end use environment. Process indicators also fail to meet the
acceptance criteria, but they do not affect the form, fit or function of an assembly. Since process indicators do not affect the form, fit
or function of an assembly, disposition is not required. The recommendation, however, is that process indicators be monitored.
Since a process may be unique to a manufacturer or type of product, there are situations where defects or process indicators may exist
that are not listed in the standard. The manufacturer is responsible for identifying those situations.
In many cases, the user is more knowledgeable of the product and its end use. For this reason, the user is tasked with the responsibility
for identifying any defects that are unique to the product.
1.12
Inspection Conditions When inspecting a product, the inspector will need to know the product class of the product under
inspection in order to appropriately evaluate the product. The standard requires documentation be provided to the inspector which
identifies the product class and states the inspector can not select it.
1-2
1.12.1
Target Definition
1.12.2
Acceptable Definition
1.12.3 Process Indicator IPC/WHMA-A-620A requires processes for Class 3 products where the number of process indicators
indicates an abnormal variation in the process, an undesirable trend or conditions that indicate the process is nearing or is out of
control to be analyzed. This implies that the number of process indicators be counted as part of the process control system even though
no requirement for this exists. The manner in which these situations are identified is the responsibility of the manufacturer.
1.12.4 Defect Manufacturers are required to document and disposition each defect for all three product classes. There is no
requirement for when this documentation is to be prepared, and the manufacturer is responsible for determining this as part of process
definition.
1.12.5 Disposition The most common dispositions, i.e., the way the product with defects is handled, are rework, repair, scrap and
use-as-is. Where the disposition is “repair,” Class 3 manufacturers are required to conduct the repairs in accordance with documented
procedures.
1.12.6 Product Classification Implied Relationship Some criteria given in the document are for either Class 2 or Class 3. By
implied relationship, these criteria are also applicable to any lower classes.
1.12.7 Conditions Not Specified Not all conditions can be included in the standard. For this reason, where conditions are not
specified as either a defect or process indicator, the condition is considered acceptable. Where this decision leads to a condition that
affects the user-defined form, fit, function or reliability, the manufacturer is responsible for identifying those conditions.
1.13
Electrical Clearance Electrical clearance measurement is defined by the design activity and may be on the design
documentation. Although the design will consider the minimum electrical clearance measurement and provide clearances to include
this amount, there may be instances where an assembly process causes a violation of this clearance. Violation of electrical clearance is
a defect condition in all cases.
Where no electrical clearance measurement is defined, the value can be calculated using Table 1-1. This number takes into account the
operating volt-ampere dating and the voltage.
1.14
Measurement Units and Applications The dimensions in the document are given in SI (System International) units. These
units are commonly referred to as “metric.” The IPC policy is to provide the Imperial English units to three decimal places, and these
are presented in the document in brackets following the dimensions in metric units. Because the conversion is not one-to-one, there are
instances where a conflict exists between the two numbers. In these situations, the procurement documentation defines the order of
precedence between the two documents.
1.15
Verification of Dimensions Dimensions provided in this standard are need definition of absolute limits per ASTM E29.
1-3
1.16
Visual Inspection
1.16.1 Lighting The lighting recommendation for 1000 lm/m2 at the surface of workstations is a practice used throughout the
industry. This is not a requirement, and lighting may be altered depending upon the working conditions and the product. This does not
address lighting in an area other than at the workstation surface.
1.16.2 Magnification Aids and Lighting Magnification aids may be required for assembly inspection. Table 1-2 provides the
magnification power requirements, and the manufacturer selects the magnification power based on the item to be inspected.
Two magnification power ranges are provided for each wire size. The first is the “inspection range,” and it is used when the product is
being inspected. If there is a question about whether or not a defect condition exists that cannot be determined at this magnification,
the referee magnification power may be used. If, after inspection at the referee magnification power, no defect is identified, the
product is considered acceptable.
Where mixed wire sizes are found in an assembly that would require two different magnification powers, the greater magnification is
allowed, but not required, for the entire product. This is an exception to the requirements for magnification power when only one
magnification power is required.
1.17
Electrostatic Discharge (ESD) Protection The introduction of an electrostatic event to an unprotected component can lead
to failures. These failures may lead to component degradation (latent failures) or immediate failure. ESD protection, as defined in the
required ANSI/ESD-S-20.20-1999 or an equivalent document, will provide protection for the components when implemented.
ESD protection is required for all product classes when the assemblies contain ESD sensitive components.
1.18
Contamination When this standard is used, a defect condition exists when assemblies with materials that are not a part of the
assembly are present. This may require process development to provide this level of control, but such processes are not a requirement.
Where soldered assemblies are present, a separate requirement for cleanliness is found in section 4.2.
1.19
Materials and Processes When using this document for assembly or manufacturing purposes, selection of the materials and
processes that are used is important. The desired end result is an acceptable assembly, and the items selected that comprise this
activity need to work together to produce that assembly.
From time to time, major changes may be required for an assembly process. These changes may be a result of many things, including
technology changes or process improvement efforts. For Class 3 products, where major changes are made to proven processes, it is
important to determine if the changes will affect the end product. IPC/WHMA-A-620A requires validation of the changes for this
product. The manufacturer may select the method of validation.
1-4
8
Marking and Labeling of Cable and Wire Harness Assemblies
IPC/WHMA-A-620 provides accept/reject criteria and process information for the manufacture of various
types of cable and wire harness assemblies. Section 12 of the document entitled, Marking and Labeling,
provides the accept/reject criteria for the various types of marking and/or labeling that one may encounter.
For simplicity, the terminology “marking and/or labeling” is referred to herein as “marking”.
This Handbook section provides tutorial information on the most commonly used methods for marking of
cable and wire harness assemblies and discusses the importance of implementing this methodology in a
manner that provides marking that is legible and permanent, and that the marking methods do not damage
or otherwise degrade the performance of the product the markings are applied to.
This Handbook section does not mandate any marking of cable and wire harnesses. When marking is
required it is normally specified by the customer and identified on the applicable documentation (e.g.,
assembly drawing, electrical schematic, wiring diagram, wire list, etc.)
The following topics are addressed in this section:
8.1
8.1.1
8.1.2
8.1.3
8.1.4
8.1.5
8.1.5.1
8.1.5.2
8.1.6
8.1.7
8.1.7.1
8.1.7.2
8.1.7.3
8.1.8
8.1.8.1
8.1.8.2
8.1.9
8.1.10
8.1.11
Scope
Reference Designation Marking
Part Number Marking
Drawing Revision Level Marking
Manufacturer Identification Marking
MIL-STD-130 Marking
Unique Identification (UID) Marking
Machine Readable Information (MRI) Marking
Serial Number and Other Traceability Marking
National or International Regulations Marking
CE Marking
Underwriters Laboratories (UL) Marking
Canadian Standards Association (CSA) Marking
Environmental Markings
WEEE Marking
RoHS Marking
Temporary Marking
IPC/JEDEC J-STD-609 Marking
National Stock Number (NSN) Marking
8.2
8.2.1
8.2.2
8.2.3
8.2.4
8.2.5
8.2.6
8.2.7
8.2.8
Wire Marking Methods
Hot Stamp Marking
Inkjet Marking
Dot Matrix Marking
Laser Marking
Hand Ink Pen Marking
Label Marking
Heat Shrink Marking
Thermal Marking
8.3
8.3.1
8.3.2
8.3.3
Fundamental Marking Principals
Correctness of required Marking
Marking Process (s)
Marking Robustness
8-1
8.1
Scope Markings, when required, are normally provided for the applications noted below; however,
reference to the type of marking herein does not mandate that the marking be provided unless otherwise
required by the applicable contract/documentation.
8.1.1
Reference Designation Marking This marking provides an easy method of identifying where the
completed wire and/or cable harness terminates to when installed in the next higher assembly, or otherwise
within itself. For example, J1 designates a connector receptacle, P1 designates a plug that mates with a
connector, F1 designates a fuse, XF1 designates a fuse-holder for the F1 fuse, R designates a resistor
termination point, K designates a relay contact termination, TB1-1 designates the wire is connected to
terminal 1 of terminal board TB1, etc.
8.1.2
Part Number Marking This marking identifies the part number (if assigned) for the completed
cable and/or wire harness assembly. For example, part number (P/N) 123456 (G1, GR1, GP1 or dash 1)
designates the complete assembly; 123456G2(G2, GR2, GP2 or dash 2) designates one branch of the
completed assembly.
Some assemblies may not have a unique P/N assigned since they may be manufactured on a wire harness
board or otherwise manufactured at the next higher assembly (e.g., within a chassis, enclosure or cabinet),
by using the parts/materials specified on the bill-of-material (BOM) or parts list of the assembly drawing.
8.1.3
Drawing Revision Level Marking The revision level of the assembly drawing may be marked
adjacent to the assigned P/N for configuration control. For example, the assembly is marked with Revision
A to designate that it was manufactured to drawing Revision A. If a subsequent change was implemented
via drawing Revision B, future manufactured assemblies would be marked Revision B to maintain
configuration control. However, when the change impacts form, fit or function such that the Revision A
assembly is no longer suitable for use, most users require that a new P/N be assigned to the assembly.
Other methods of providing configuration control marking are sometimes used. For example, the assembly
may be marked with P/N 123456G1, Revision A, EO1. The EO1 marking designates that the Revision A
assembly was modified by Engineering Order # 1 (EO 1).
8.1.4
Manufacturer Identification Marking The manufacturer may mark the manufacturer name,
LOGO or Commercial and Government Entity (CAGE) code on the assembly.
The CAGE code is a five-position alphanumeric code with a numeric in the first and last positions (e.g.,
27340, 2A345, 2AA45 OR 2AAA5, etc.) used extensively within the federal government. The CAGE
Code is used to support a variety of mechanized systems throughout the government and provides for a
standardized method of identifying a given facility at a specific location. A listing of CAGE Codes may be
found at http://www.dlis.dla.mil/cage_welcome.asp.
Non U. S. companies can obtain a North Atlantic Treaty Organization (NATO) CAGE (NCAGE) Code
form the appropriate source. The NCAGE Code can be obtained directly from the Codification Bureau in
your country. For most countries contact the following site for the NCAGE Code information;
http://www.dlis.dla.mil/Forms/Form_AC135.asp. For a list of addresses go to:
http://www.dlis.dla.mil/nato_poc.asp.
The CAGE CODE and/or the NCAGE Code are required for the Central Contractor Registration (CCR)
discussed below.
Manufacturers interested in doing business with the U. S. Federal Government must be registered in the
Central Contractor Registration (CCR). Information on the CCR may be obtained at: http://www.ccr.gov.
In some instances, a Data Universal Numbering System (DUNS) number may be assigned. This is a ninedigit number assigned by Dun & Bradstreet to each business in their global database.
8-2
8.1.5
MIL-STD-130 Marking If the cable and/or wire harness assemblies are destined for ultimate
shipment to a military customer, the marking requirements of MIL-STD-130 may have been mandated on
the contract/documentation.
MIL-STD-130 may be obtained from: http://assist.daps.dla.mil.
8.1.5.1 Unique Identification (UID) Marking The UID number included in MIL-STD-130 is a system
of establishing globally unique and unambiguous identifiers within the Department of Defense, which
serves to distinguish a discrete entity or relationship from other like and unlike entities or relationships. A
UID may apply to a cable or wire harness assembly.
Marking of a UID symbol normally takes up little space. A 50-character concatenated UID symbol only
requires a square space of from 0.25 inch square to a 0.50 inch square depending of the size of the Data
Matrix “cell”. Most concatenated UID’s contain less than 35 characters.
8.1.5.2 Machine-Readable Information (MRI) Marking The MRI referenced in MIL-STD-130 is a
pattern of bars, squares, dots, or other specific shapes containing information interpretable through the use
of equipment specifically designed for that purpose. The patterns may be applied for interpretation by
digital imaging, infrared, ultra-violet, or other interpretable reading capabilities.
MIL-STD-130 requires that items be marked with a machine readable 2D Data Matrix bar code.
8.1.6
Serial Number and Other Traceability Marking A serial number, lot number, batch number,
manufacturing number, or other form of traceability marking may be required by contract/documentation,
or otherwise may be used by the manufacturer. This type of marking provides for traceability to
manufacturing, inspection and test records for the item. The marking can also assist in root cause
determination and corrective action for any defects found on shipped product. In some cases, a unique
serial number may have been assigned by the customer. Date code marking and bar code marking are also
sometimes used as traceability marking.
The manufacturer should employ a positive system for precluding duplication of serial number markings.
It is recommended that serial numbers be assigned from a computer generated listing/database that creates a
new unique serial number each time the program is accessed.
8.1.7
National or International Regulations Marking Cable and/or wire harness assemblies may
require certification markings of one or more of the types discussed below. Other types of these markings
also exist but are not discussed herein.
8.1.7.1 CE Marking CE marking is a European marking of conformity that indicates that a product
complies with the essential requirements of the applicable European laws or Directives with respect to
safety, health, environment and customer protection. Generally, this conformity to the applicable directives
is done through self-declaration.
The CE Marking is required on products in the countries of the European Economic Area (EEA) to
facilitate trade between the member countries.
Unlike the UL mark, the CE Marking:
a)
Is not a safety certification mark;
b)
Is generally based on self-declaration rather than third party certification, and
c)
Does not demonstrate compliance to North American safety standards or installation Codes.
8.1.7.2 Underwriters Laboratories (UL) Marking The UL Mark on a product means that the UL has
tested and evaluated representative samples of that product and determined that they meet UL’s
requirements. In addition, products are checked by UL at the manufacturing facility to make sure they
continue to meet UL requirements.
8-3
8.1.7.3 Canadian Standards Associating (CSA) Marking A CSA marking, like the UL marking, deals
with the issue of safety and safe use of products with a focus on the Canadian market. CSA Marks appear
on over one billion products worldwide. Each mark tells you that an authorized testing laboratory has
evaluated a sample of the product to determine that it meets applicable national standards.
8.1.8
Environmental Marking Because of environmental concerns, various environmental regulations
have been issued. These include the EU RoHS (Restriction on the use of Certain Hazardous Substances) in
electrical and electronic equipment, and the WEEE (Waste Electrical and Electronic Equipment). The
main environmental concern that applies to cable and/or wire harness assemblies pertains to the elimination
of lead (Pb). Manufacturers who design their products to be Pb-free may choose to provide Pb-free
markings, or otherwise the contract may mandate such markings. The following discussion reviews some,
but not all, of the various environmental markings that exist:
8.1.8.1 WEEE Marking Electrical and electronic equipment (EEE) plays an ever-increasing role in our
daily lives. Our kitchen appliances, mobile phones and computers offer us many benefits during their
working lives but when this equipment is thrown away it affects the environment. Waste electrical and
electronic equipment (WEEE) is one of the fastest growing waste streams in multiple countries. Some
WEEE contains hazardous substances and parts such as mercury in some switches, lead in solder, and
cadmium in batteries. Recycling rates for most types of WEEE (other than large ‘white goods’ such as
fridges and washing machines) are very low.
The WEEE Directive covers a wide range of electrical and electronic products, although some are exempt
from certain requirements. The types of products covered are:
a) large and small household appliances;
b) IT and telecommunication equipment;
c) consumer equipment such as TVs, videos, hi-fi;
d) lighting, electrical and electronic tools (except large stationary industrial tools);
e) toys, leisure and sports equipment;
f) automatic dispensers;
g) medical devices (these are exempt from the WEEE recycling and recovery targets);
h) monitoring and control instruments.
Many of these products may have cable and/or wire harness assemblies installed and as such, they may
be subject to the WEEE requirements.
If the cable and/or wire harnesses fall within the scope of the WEEE Directive then products must be
marked with the “crossed out wheelie bin” symbol designed to specifications set forth in EN
50419:2005.
8.1.8.2 RoHS Marking The restriction of the use of Certain Hazardous Substances in Electrical and
Electronic Equipment Directive (RoHS), which went into effect on July 1 2006, places restrictions on the
use of mercury, lead, and other materials. The most significant impact for cable and/or wire harness
assemblies involves the prohibition on the use of lead (Pb). Many cable and/or wire harnesses contain
lead-bearing solder which must be phased-out if a manufacturer intends to sell product to the EU market, or
otherwise where a customer mandates Pb-free. Other countries are adopting their own version of the RoHS
requirements.
Manufacturers may be required by contract/documentation, or otherwise may elect to mark their cable
and/or wire harness assemblies to designate RoHS compliance.
8.1.9
Temporary Marking Manufacturers may elect to use temporary markings for ease of
manufacturing and/or for traceability during manufacturing, inspection and testing. For example, a wire
number may be marked on a temporary label affixed to individual wires on a harness board.
8-4
Whenever possible, it is recommended that the wire markings/labels installed on the wire harness board are
the same as required by the assembly documentation, in lieu of using a temporary marker, to help reduce
the potential for wiring errors during installation of the harness into its next higher assembly.
A label or indication of inspection/test status stamp may be applied to designate completion of required
inspections and/or tests. Such markings are normally removed prior to shipment. However, unless
otherwise prohibited by the customer (Class 3 product), some temporary markings, such as stamp marking
indicating acceptance by Quality Assurance and/or Test, may remain on the product. The manufacture
should select a temporary marking method that will not result in damage to the item or otherwise result in
functional problems.
8.1.10 IPC/JEDEC J-STD-609 Marking Normally, cable and/or wire harness assemblies will be
shipped as stand alone assemblies for ultimate installation into a next higher assembly. These next higher
assemblies may be marked in accordance with IPC/JEDEC J-STD-609, Marking of Labeling of
Components, PCBs and PCBAs to Identify Lead (Pb), Pb-Free and other Attributes.
It is recommended that manufacturers who include soldered terminations on their cable and/or wire harness
assemblies include the following “e” markings, as applicable, from IPC/JEDEC J-STD-609:
e0 – Designates intentionally added lead (Pb) (≥ 3 percent by weight)
e1 – Tin-silver-copper (SnAgCu)
e2 – Tin (Sn) alloys with no bismuth (Bi) nor zinc (Zn), excluding tin-silver-copper (SnAgCu)
e3 – Tin (Sn)
e4 – Precious metal [e.g., silver (Ag), gold (Au), nickel-palladium (NiPd), nickel-palladium-gold (NiPdAu)
(no tin (Sn)]
e5 – Tin zinc (SnZn), tin-zinc-other (SnZnX) [all other alloys containing tin (Sn) and zinc (Zn) and not
containing bismuth (Bi)]
e6 – Contains bismuth (Bi)
e7 – Low temperature solder (≤150ºC) containing indium (In) [no bismuth (Bi)]
e8 and e9 symbols - unassigned
8.1.11 National Stock Number (NSN) Marking If the cable and/or wire harness assembly is designated
for shipment for military use by the USA and/or NATO countries; it may have been assigned a National
Stock Number (NSN). The contract/documentation may require marking of the NSN on the cable and/or
wire harness assembly, or otherwise as part of the marking provided on the shipping containers.
The NNSN is a 13-digit number that is assigned by the Defense Logistics Information Service (DLIS) in
Battle Creek Federal Center in Michigan.
The 13-digit number is composed as follows:
6150-00-014-3579
The first four digits (6150) represent the Federal Supply Class (FSC).
 The first two digits (61) are assigned as the Federal Supply Group (FSG). The
“61” identifies
that the item is listed as; “Electric Wire, Power and Distribution Equipment.”
 The next two digits (50) identify the specific type of item classification within the “61” FSG. The
“50” designation represents “Miscellaneous Electric Power and Distribution Equipment.”
Digits 5 and 6 (00) represent the Country of origin, as noted below.
 USA
00 and 01
 Germany
12
 France
14
 Canada
20 and 21
 Slovenia
40
 Australia
66
8-5
The remaining digits seven through 13 (014-3579) are a unique number that is assigned to the item. In this
example, the number shown is a bogus number. These seven digits are also referred to as the NIIN
(National Item Identification Number).
8.2
Wire Marking Methods The following paragraphs identify the most commonly used methods for
marking of cable and/or wire harness assemblies.
8.2.1
Hot Stamp Marking Hot stamp marking is still the most inexpensive method for wire or cable
identification and it can be used to mark over Teflon insulation.
In order to ensure a quality marking you should have the correct air pressure, dwell time, wheel
temperature and foil. The air pressure is the pressure in which the wheels make contact with the wire or
cable. Control of this process is important to preclude affecting the integrity of the wire or cable insulation.
The dwell time is the length of time in which it takes to complete the whole stamping cycle. The wheel
temperature and foil types are chosen together. The foil consists of a backing and a pigment color. The
pigment is transferred to the wire or cable insulation via the heat from the character wheels. The backing
of the foil should be able to withstand the required temperature range and can be made of different
materials such as Mylar or Nylon. Certain pigments will stick to certain substrates and will require
different temperatures to transfer them. For example, one cannot use a foil that will mark on PVC at 275
degrees for Teflon that may require temperatures of 350-400 degrees.
Hot stamp marking is specifically prohibited for some wire types; specifically, wire conforming to AS
81044 and AS50881. This wire is used for aerospace application and the concern is that this marking
method could sufficiently penetrate the wire insulation and result in a dielectric breakdown of the
insulation.
8.2.2
Inkjet Marking Inkjet technology has improved greatly over the last few years. With less
maintenance and quicker start-ups, inkjet marking systems have grown much more reliable and user
friendly.
For the wire and cable industry, it is usually a dye or pigmented ink, with an MEK (Methyl Ethyl Ketone)
base. Although rare, alcohol based inks can be used; however, drying time is increased.
Depending on the interfacing wire processing equipment and software, one can mark on the fly and vary
text strings throughout the length of the wire or cable. You can also vary font sizes and bold font, tower
print, and invert the text. Inkjets are dot matrix printers, and the ink is directed onto the wire or cable via
deflector plates once it is electrically charged.
Inkjet marking, however, does have its limitations. Certain substrates (insulations) require certain ink
types, and marking Teflon is not an option. In addition, if you have an automated printer with black ink,
you normally cannot clearly mark on black wire. Since changing ink is normally not a possibility, one
would need to purchase an additional printer to mark using a different color ink.
8.2.3
Dot Matrix Marking A dot matrix printer is used to apply marking information to a label. The
Dot Matrix terminology refers to the method the printer uses to create the marking images. This is
accomplished by several small pins, aligned in a column. These pins strike an ink ribbon positioned
between the pins and the label, creating dots on the label. Characters are formed by patterns of these dots
by moving the print-head laterally across the page in tiny increments.
The pins are contained in the print-head and are driven by solenoid actuated small hammers which force
each pin to contact the ink ribbon and label.
A Dot Matrix printer has an advantage in being able to print letters in italics or bold by changing the way
dots are arranged on the label. These printers are more economical than laser printers.
8-6
8.2.4
Laser Marking Laser wire marking provides a permanent, non-contact, permanent mark for wire
and multi-core cable identification. It is the preferred, and most often specified, method of marking wire
and cable today for aerospace and military applications. Equipment is available that is capable of marking
wire in accordance with the AS 50881 International Standard.
Although laser marking is the commonly preferred marking method, there are certain mil-spec wire and
cable that cannot be laser marked. This includes Kapton insulated wire and cable. Ink Jet marking may be
used for marking this type of wire and cable.
Laser marking is not a viable option for some manufacturers because of the high cost ($125-$400K), and/or
the time it takes to apply the marking.
The following different laser types are available:
a) Vector Based Laser – The wire needs to stop and the laser uses x-y coordinates.
b) Mask Type Laser – The mask acts as a type of stencil.
c) Carbon Dioxide Laser – Destructive and not typically used for wire and cable.
d) UV Laser – Ideal for marking Teflon. However, the Teflon jacket must contain Titanium Dioxide in
order for a color change to occur.
8.2.5
Hand Ink Pen Marking Hand marking of information using a commercially available ink marker
pen is sometimes used to modify markings in fielded product. However, this marking method is not
recommended for initial marking because of legibility and longevity concerns.
8.2.6
Label Marking Applying marking using a label is especially useful when hot stamp or inkjet
cannot provide satisfactory results. Labels are printed and then applied automatically, or manually. Some
labels can also withstand harsh environments such as gasoline and oil. Some labeling systems allow you to
program the text to be printed via a PC and use a master machine to send a print signal for the label
location.
Wrap-around markers, including a self-laminating marker, are an easy method for marking that does not
require the termination to be removed in event a wire marker replacement is needed. These markers have a
clear portion that will wrap around and laminate the marking legend. This protects the marking from
damage.
The gauge (size) of the wire or cable determines the length of the self-laminating/wrap-around marker or
the diameter of the sleeve to be used. Normally the length of the label should be five times the outer
diameter of the wire or cable to be marked.
8.2.7
Heat Shrink As with labeling, heat shrink can also be an effective method when hot stamp or
inkjet marking is not an option. Heat shrink can be marked prior to it being applied to the wire or cable and
then heated to shrink. However, once the wire is terminated, heat shrink cannot normally be used in event
a marking change is needed unless the termination is removed and replaced with new heat shrink.
8.2.8
Thermal Marking Thermal marking requires a foil and heat, but unlike hot stamp marking, it
does not require “impacting the insulation”. This method uses heat to transfer the pigment from the foil to
the wire while it is rolled on. It can mark on both flat and round cables with either black or white
markings. Color foil changeover is quick and it can mark logos and other bitmaps.
8.3
Fundamental Marking Principals – The process(s) involved in the marking of cable and/or wire
harness assemblies should be robust and under process control to the extent necessary to assure that the
following marking principals are met to achieve compliance with the accept/reject criteria of Section 12 of
IPC/WHMA-A-620.
8-7
8.3.1
Correctness of Required Marking – The manufacturer should ensure that all applied marking
contains the correct information (text, numbers, color, font size, etc.) specified in the
contract/documentation, and that the marking was provided in the correct location, using the specified
materials (e.g., shrink sleeve, labels, ink, etc.).
8.3.2
Marking Process (s) – The manufacturer should ensure that the contractually specified marking
process (s), and/or applicable process (s) specified by the manufacturer were used to apply the marking and
are under acceptable process control.
8.3.3
Marking Robustness – Completed marking should be permanent and free from damage,
including any evidence of damage that may have occurred from use of the incorrect marking process (s).
8-8
10 Harness/Cable EMI/RFI Shielding.
10.1
EMI/RFI Shielding Theory
Electro-magnetic interference can be composed of both magnetic fields and
electric fields that cause a disruption in the desired
signals to and from an
electronic device. Radio Frequency Interference is by virtue of its higher
frequency is primarily an
electric field causing the disruption of the desired signals. These fields can come from the signals with in the cable or harness and
from outside source.
10.1.1
Magnetic Field shielding
Magnetic fields are not easy to shield, magnetic fields must be contained by continuous metal enclosures of magnetically permeable
materials, and may include highly permeable rare earth metals and alloys. The effects of a magnetic field on a cable or harness can be
reduced by increasing the distance between the field generator and the cable and harness or by twisting the conductors. When the
cable is generating the field as in a high current power cable, the magnetic field is mitigated by running the source and return together
in a twisted wire, the sum of the opposite magnetic fields is zero where the currents are equal.
10.1.2
Electric Field Shielding
Electric fields are a much more common problem and can be shielded by placing a terminated conductive shield around the cable or
harness. The electric field induces currents in the shield and those currents must be dissipated into ground so effective grounding of
the shield is also required. The higher frequencies of RFI may be reflected off an un-terminated shield but non grounded shields only
partially reduce the effects of the electric fields. The shielding may be terminated on one or both ends and may be terminated to the
ground at intermediate points as determined by engineering. A round wire terminating the shield is acceptable only at low frequencies
because the impedance of the round wire goes up with frequency do to the self inductance of the round wire, the low resistance path
for DC current is now a high impedance path for the high frequency currents making a ineffective ground path.
10.1.3
Shield Braid Condition
Braided wire should be uniform over the wire bundle with at least 80% braid coverage.
Photo 46 (less than 80% coverage)
Unacceptable
Photo 47 (minimum of 80% coverage)
Acceptable
Photo 48 (90% + coverage)
Preferred
10.2
Shield Termination
10.2.1 Terminating Over Shield, Adapter
The shield must be terminated to the Chassis ground by a very low impedance path to be the most effective. There are cable to
connector adapters that are designed to terminate the shield around the cable circumference (360 degrees). When using an adapter the
shield terminating surfaces must be cleaned and free of oils or other contaminants. The shield material must also be cleaned and
adapter assembled per manufacture’s instructions. The DC resistance between the connector body through the adapter to the shield
should be less than 2.5 milliohms.
Clean
(Compression Fitting)
Clean
(Bandit ®)
10.2.2
Clean
(Crimp Ring)
Terminating Over Shield, Molded
The molded part should be wrapped in a copper foil or other shielding material. The foil must be connected to
the connector body and the cable shield and seams in the foil soldered or over lapped at least 50%. The DC
resistance between the connector body through the foil mold to the shield should be less than 2.5 milliohms.
Solder foil around
connector body
Patch
Solder foil to connector body completely around and patch any openings in the foil with a small piece of foil and solder.
10.2.3 Terminating Over Shield, Other Methods
Connectors with strain reliefs or other rear appliances may terminate the shield by the methods specified on the engineering drawing.
Power cable shields may be terminated to the chassis ground and drain wires may be connected to the shield and terminated on a
ground lug or clamp screw, but shield drain wires terminating the shield are not effective at higher frequencies and may not have the
desired results.
10.3
Sub-Cable Shield Termination
Components of a cable may be also be shielded. When shielded cable components like twisted shielded paired wires are used the
shields must also be terminated for best results. These shields may be terminated to chassis ground at the cable connector adapter, or
they may terminate in a pin or pins of the connector to be carried continuously through the cable.
10.3.1 NEED TITLE OR NOT A SUBCLAUSE
To terminate the component shields to the connector adapter the inner conductors may be pulled through the braid and the braid
terminated in the adapter. There are specialized adapter designs for terminating component shield to the adapter and installation shall
follow the manufactures instructions. The component shields may be soldered to the over braid to terminate to the adapter, caution
must be used to prevent wire damage from excessive heat.
10.3.2 NEED TITLE OR NOT A SUBCLAUSE
When terminating the component shield to pins, a wire must be soldered to the shield and terminated with a pin either directly or by
using a leaded solder ferrule to terminate the shield. The lead must be kept short, preferably the same length as the signal leads and
going in the same direction.
Not Preferred
Pick off wire is too long
Preferred
Pick off wire is oriented to be as short as possible
10.3.3 NEED TITLE OR NOT A SUBCLAUSE
When multiple shields are terminated together in a pin the leads may come out the rear side of the ferrule except the lead with the pin.
Wiring should be done to keep the lead length short and minimize the additive wire length.
Figure 4 Excess Length
Excessive length in daisy chain, wire twisting not maintained
Not preferred
Pick off wire exits from the end of the daisy chain
Figure 5 Preferred
Short daisy chain wires, wire twisting maintained
Preferred
Pick off wire exits from the center one of the daisy
chain.. .
10.3.4 NEED TITLE OR NOT A SUBCLAUSE
Component cables may use a foil shield with low coverage braid or drain wire. The braid is terminated as any braid in the cable. The
drain wire is terminated the same way the pick off wires are terminated, sleeving may be required by the drawing to cover the drain
wire.
Section 11 – Harness/Jacket Method
Covers Sections 15 (machine braiding method only), all of Section 16
General
Jacketing and overall harness shielding materials are applied either by machine, molding/extrusion, or by hand.
Environment, harness materials, type of connectors and cost drive the method and type of construction that should
be specified by the Designer.
When electrical shielding is applied the requirements for its application is more controlled. Here shielding
effectiveness is the objective. It is normally specified in percentage (%) coverage since its effectiveness is based on
the amount and size of gaps of the applied material. Formulas based on wire gauge, number of yarn ends and the
number of carries verses the print shield percentage coverage requirement will drive the setup. Sometimes a total
resistance value may be specified, this is for …………. See xxxx for converting % coverage to machine setups and
from the setup to calculate D.C. resistance of the braid per unit length.
Separation tape is required over the wire between machine braided protective or shield coverings. The spiral of the
tape wrapping is always in the opposite direction from the wire twist to maintain bundle configuration and twist.
For braiding non-metallic protective materials (protective coverings) overlapping of the separation tape is not
required. Its function is to hold the bundle shape during the braiding operation. For metal shield braid the
separation tape also provides additional abrasion resistance. Here tape application require a minimum of a 25%
overlap of the separation tape, however, overlapping of the tape which exceeds 50% will lead to decrease flexibility
and should be avoided. Where the wire bundle reaches a braid stop, the spiral wrap shall be stopped and terminated
with 1-1/2 to 2 straight wraps of tape. Separation tapes should be a thin smooth surface tape, i.e. Teflon, Mylar,
Impregnated Fiberglass, etcetera, with or without a weak adhesive backing. Strong adhesive backing will decrease
flexibility by preventing the tape from sliding over itself during flexure. Adhesive bond failure after assembly is not
an issue since its function during assembly has been fulfilled.
For cut ends of braid materials, any material, apply a layer of a thin smooth surface adhesive backed tape, i.e.,
Teflon, Mylar, etcetera, to stop fraying of the end.
Definitions
Pick. A Pick is the point in a braid at which one carrier goes over another carrier along the long axis of the cable.
Picks per Inch. Picks per inch is the number of picks in a distance of 1 inch.
Cordage. Cordage is the product formed by twisting together two or more ply yarn.
Yarn. Yarn is the product formed by two or more single continuous filament threads when twisted together.
Monofilament. Monofilament is a single continuous filament.
Ply yarn. Ply yarn is the product formed by twisting together two or more yarns.
Denier. Denier is the unit weight of yarn or cordage based upon a skein 450 meter. long, weighting 0.05 gram. It is
numerically equal to the number of grams per 9,000 meters.

Mechanical Braided Materials
o Machines
 Wardwell, Steeger, etc(Insert pics of different machine and how each works in general)
 Number of carries for most machines are 8, 16, 24, 32, 36, 46, 54, 64 and xxx. The diameter
of the finished cable and the number of ends of the yarn being used normally determine what
size of machine to use. Here are some general guidelines: (see my specs on typical limits).
 The braider drives on what type of bobbins are used, and how much yarn can be put on the
bobbin. Bobbins with multiple yarn ends need be laid flat and parallel to each other with
equivalent tension to create a smooth flat surface. The layers also must be evenly applied and
free of loose turns or turns that could come loose during braiding operations. The starting and
ending thread must not be tied together.
11-1
Section 11 – Harness/Jacket Method
o
o
o
Materials
 Polyester
 Aramid
 Glass/Ceramic fibers
 Plated High Strength Synthetic Fibers
 Metal/Plated Fiber Blend a mixture of plated yarn and metal yarn to achieve a complete
protection to the entire EMI spectrum with a reduced weight over all metal braid.
 Metal
Shield Braiding
Braided Shielding verses Carriers Formulas
The shield braid shall be applied in such a manner as to provide the percentage coverage as shown on the
engineering drawing/model. In lieu of testing, the value may be determined by calculation from the listed formulas
below. The intended result is to determine a minimum number of picks per inch as a quick measure for production
to achieve the minimum required shielding.
Tan  = [2(D+2d)P]/C
Where:
When:
 = braid angle
C = number of carriers in braiding machine
d = gauge diameter of shield wire
D = Diameter of unbraided harness
F = shield over
K = percent coverage
N = number of wire ends per carrier
P = Picks per inch
K = (2F-F2)100
and
F = (NPd)/Sin 
(Values for P shall always be chosen so that F is always
less than or equal to 1.0.)
D.C. Shield Resistance Calculations
R = dR/[Cos(NC)]
Where:
 = braid angle
C = number of carriers in braiding machine
dR = D.C. resistance of 1 strand end, ohms/unit length
N = number of wire ends per carrier
R = D.C. resistance in ohms/unit length

Insert picture of what this means, see Alpha wire
technical data sht on Shielding.
Hand Applied Coverings
o Open/loose Sleeving
 Expandable sleeve
 Metallic braid sleeve
 Metal Mesh Tape
o Closed Coverings,.
 Shrink Sleeving
 Most Common Materials, other materials can be used depending on the capability to
be manufactured and meet end item requirements. Shrink material is normally an
extruded tube of material that when heated will return to its original diameter. This
is a beneficial material property that will allow materials to be hand applied over the
wire and cable easily to create a protective jacket, and then shrunk down to create a
slimmer and neat package, at lower cost than either over-molding or braiding.
Materials commonly used are:
o Polyolefin
11-2
Section 11 – Harness/Jacket Method
o
o
o
o
o
o
o
o
Polyethylene Terephthalate
Polyvinylidene Fluoride (Kynar)
Fluorosilicone Rubber
Silicone Rubber
Fluorinated Ethylene Propylene (FEP)
Polytetrafluoroethylene (PFE)
Ethylene-Tetrafluoroethylene Fluoropolymer (ETFE)
Tetrafluoroethylene Fluoropolymer (TFE)





Many of the materials listed above can have an adhesive coating to the interior of the
expanded tube normally called a melt liner.
 Conductive lined
Shrink Boots – w & w/o Sealant
Tapes that are used for protective coverings of harness assembles are normally made from
Silicone, though other materials such as Neoprene and fiberglass have been used, but the most
common type is self bonding silicone.
What is meant by self bonding silicone? It is where the tape is supplied with one side rough
and one side ultra smooth and clean. The tape roll comes with a release paper liner to kept the
clean side uncontaminated and away from the rough exposed side. When the two sides are
put in contact under a small compressive force, that created when the tape is wrapped over
itself tightly, the two surfaces will bond to each other in about 24 hours.
 Silicone Tapes, i.e. – Self-Sealing, Guideline, square cut, taper cut
 Electrician’s Tape which many people think can be used on any commercially
producted harness is made from PVC with an adhesive backing and is normally
black, but can be any color. Due to most commercial and military regulations is not
allowed to be used because of hazardous outgassing and burn products. All existing
military and industry specifications have been canceled to preclude its use.
11-3
12
Securing and Finished Assembly Installation
12.1 Scope IPC/WHMA-A-620 provides accept/reject criteria and process information
for the manufacture of various types of cable and wire harness assemblies. Section 14 of
the document entitled, Securing, provides criteria applicable to cable and wire harness
manufacture rather than criteria applicable to installation of the completed cable or wire
harness assembly into the next-higher assembly (e.g., chassis, drawer, panel, cabinet,
etc.).
Section 17 of IPC/WHMA-A-620 entitled, Finished Assembly Installation, provides
criteria that apply to the installation of the completed cable and wire harness assemblies
into the next-higher assembly (e.g., chassis, drawer, panel, cabinet, etc.).
This handbook Section 12 provides tutorial information on the methodology most
commonly used for securing (e.g., application of tie-wrap/lacing, breakouts and routing)
cable and wire harness assemblies during their manufacture. It also provides tutorial
information on the methodology most commonly used for installing the completed cable
and wire harness assembly into a next higher assembly.
This handbook section was developed as a companion document to IPC/WHMA-A-620
Sections 14 and 17, to assist the manufacturer in manufacturing and/or subsequently
installing completed cable and wire harness assemblies into a next higher assembly in a
manner that assures wiring system safety, performance, reliability, maintainability,
service life and minimizes life cycle costs.
The information provided herein can be used for all Product Classes of equipment.
This handbook section, by design, does not discuss criteria for manufacture of the next
higher assembly the completed cable and/or wire harness will be installed in, except in
general terms.
IPC/WHMA-A-620 is primarily structured to provide acceptance and/or rejection criteria
for cable and wire harness assemblies similar in structure to IPC-A-610, Acceptability of
Electronic Assemblies. However, unlike IPC-A-610, IPC/WHMA-A-620 also includes
process information. The majority of cable and wire harness assemblies manufactured to
IPC/WHMA-A-620 requirements are sold as individual completed assemblies. However,
in many instances, the manufacturer of the cable and/or wire harness assemblies is the
same manufacturer for the next-higher assemblies (e.g., chassis, drawer, panel, cabinet,
etc.) the cable and/or wire harness assemblies will be installed in. Therefore, this
handbook section provides tutorial information pertaining to the next-higher assemblies
as related to the installation of cable and/or wire harnesses, and discusses related topics
(e.g., personnel safety, electrical bonding, etc.) that are not directly addressed in
IPC/WHMA-A-620.
Since this document is a handbook, as such, non-mandatory terminology “should” is used
throughout this section of the handbook rather than mandatory terminology “shall”.
1
However, the user of this handbook is cautioned that much of the information contained
in this section of the handbook may be invoked as a mandatory requirement by other
contractual documents such as the applicable design specifications, other IPC documents,
or via the applicable contract documentation such as drawings and parts lists.
Much of the information herein is based on existing recognized military and aerospace
documents that are also acceptable for use for non-military or non-aerospace applications
as they contain information of a generic, primarily tutorial, nature. Reference to these
documents herein should not be construed as a mandate for their use, or otherwise as a
mandate to use military and/or aerospace mandated parts. Parts and materials required
for manufacture of wire and cable harnesses, and for installation of these harnesses into
the next higher assembly are controlled by the applicable contractual design requirements
and/or as mandated by the user. The following documents, in addition to IPC/WHMAA-620, were used in creating this handbook section and may be consulted for additional
information:
(a)
(b)
(c)
MIL-E-917, Electric Power Equipment Basic Requirements
MIL-W-5088 (Superseded by AS50881), Wiring Aerospace Vehicle
MIL-HDBK-419, Grounding, Bonding, and Shielding for Electronic Equipments
and Facilities
2
The following topics are addressed in this section:
12.
12.1
Securing and Finished Assembly Installation
Scope
12.2 Selection of Parts, Materials and Tools
12.2.1 Standard Parts and Materials
12.2.2 Non-Standard Parts and Materials
12.2.3 Sealing Materials
12.2.4 Fastening Devices
12.2.4.1 Nuts, Bolts, and Screws
12.2.4.2 Self-Locking Threaded Fastener
12.2.4.3 Flat-Head Screws
12.2.4.4 Blind Fasteners
12.2.5 Thread-Cutting Screws (Self-Tapping Screws)
12.2.6 Washers
12.2.6.1 Lock Washers
11.2.6.2 Flat Washers
11.2.7 Utilization of Standard Tools
12.2.8 Materials
12.2.8.1 Non-Recommended Materials (May be Specifically Prohibited by the User)
12.2.8.1.1 Toxic Pyrolytic Materials
12.2.8.1.2 Flammable Materials
12.2.8.1.3 Fragile or Brittle Materials
12.2.8.1.4 Mercury
12.2.8.1.5 Asbestos
12.2.8.1.6 Silicone
12.2.8.1.7 Polychlorinated Biphenyls (PCB)
12.2.8.1.8 Polyvinyl Chloride
12.2.8.1.9 Cadmium and Cadmium Plating
12.2.8.2 Other Non-Recommended Materials (May be Specifically Prohibited by the
User)
12.2.9 Metals
12.2.9.1 Selection of Metals in Direct Contact
12.2.9.2 Corrosion-Resisting Metals
12.2.10 Plastics
12.2.11 Insulation Materials
12.2.11.1 Arc and Tracking Resistance
12.2.11.2 Laminated Plastics
12.2.11.3 Molded Thermosetting Plastics
12.2.11.4 Thermoplastics
12.2.12 Classes and Definitions of Insulating Materials
12.2.12.1 Class 90
12.2.12.2 Class 105
12.2.12.3 Class 130
12.2.12.4 Class 155
3
12.2.12.5 Class 180
12.2.12.6 Class 200
12.2.12.7 Class 220
12.2.12.8 Class 240
12.2.12.9 Class Over 240
12.2.12.10 Electrical Tape
12.2.12.11 Sleeving
12.2.12.12 Straps and Clamps
12.2.12.13 Lacing Cord
12.2.13 Terminal Lugs
12.3 Wiring Selection
12.3.1 Conductor Degradation
12.3.1.1 Tin-plated Conductors
12.3.1.2 Silver-plated Conductors
12.3.1.3 Conductor Solderability
12.3.2 Aluminum Wire
12.3.3 Insulation Compatibility with Sealing and Servicing
12.3.3.1 Wire Diameter
12.3.3.2 Potting Seal on Wire or Cable
12.3.3.3 Insulation Degradation
12.3.4 Wire Size and De-rating
12.3.5 Wire and Cable Identification
12.3.5.1 Wire Size Color Code System
12.3.6 Wire for Electromagnetic Interference (EMI)
12.4
Service Life
12.5 Safety and Personnel Protection
12.5.1 Electric Shock
12.5.1.1 Levels of Electric Shock
12.5.1.2 Exposed Metal or Other Conductive Parts
12.6 Electrical Creepage and Clearance Distances
12.6.1 Distance from Enclosure
12.7
Accessibility
12.8
Maintenance and Repair
12.9
Smoke and Fire Hazards
12.10 Cable and Wire Harness Installation
12.10.1 Arrangement and Harnessing
12.10.2 Bundle and Group Size
12.10.3 Dead Ending
12.10.4 Splicing
12.10.5 Routing
4
12.10.6 Stress Relief and Mechanical Support
12.10.7 Slack in Cable and Wiring
12.10.7.1 Connector Termination
12.10.7.2 Lug Termination
12.10.7.3 Strain Prevention
12.10.7.4 Free Movement
12.10.7.5 Cable and Wire Shifting
12.10.8 Inspection and Maintenance
12.10.9 Protection and Support
12.10.10 Bend Radius
12.10.11 Drip Loop
12.10.12 Routing Near Moving Parts or Controls
12.10.13 Routing Near Fluid Lines
12.10.14 Ground Return
12.10.15 Shielded Wire Grounding
12.10.16 Multiple Grounds
12.10.17 Connectors
12.10.17.1 Environment Resisting Connectors
12.10.17.2 Contacts
12.10.17.2.1 Spare Contacts
12.10.17.3 Connector Installation
12.10.17.3.1 Circular Connector Installation
12.10.17.3.2 Rectangular Connector Installation
12.10.17.4 Potting
12.10.17.5 Safety Wiring
12.10.17.6 Dust Protection
12.10.17.7 Connector Accessories
12.10.18 Splices
12.10.19 Terminal Lugs
12.10.20 Terminal Boards and Terminal Junction Modules
12.10.21 Wiring Mockup
12.10.22 Screw Thread Standards for Fastening Devices
12.10.22.1 Fastening of Harnesses and Associated Parts
12.10.22.2 Threads in Aluminum
12.10.22.3 Threads in Plastic
12.10.22.4 Inserts
12.10.22.5 Thread Projection
12.10.22.6 Bolt and Screw Thread Engagement
12.10.22.7 Thread Locking of Mechanical Assemblies
12.10.22.8 Flexible Wiring
12.10.22.9 Wire Connections and Terminals
12.10.22.10 Spare Terminals
12.10.22.11 Manufacturing Processes
12.11 Bonding
12.11.1 Purposes of Bonding
5
12.11.2 Resistance Criteria
12.11.3 Direct Bonds
12.11.3.1 Contact Resistance
12.11.3.1.1 Surface Contaminants
12.11.3.1.2 Surface Hardness
12.11.3.1.3 Contact Pressure
12.11.3.1.4 Bond Area
12.11.4 Direct Bonding Techniques
12.11.4.1 Welding
12.11.4.2 Brazing
12.11.4.3 Soft Solder
12.11.4.4 Bolts
12.11.4.5 Rivets
12.11.4.6 Conductive Adhesives
12.11.5 Indirect Bonds
12.11.5.1 Resistance
12.11.5.2 Frequency Effects
12.11.5.2.1 Skin Effect
12.11.5.2.2 Bond Reactance
12.11.5.2.3 Stray Capacitance
12.11.6 Surface Preparation
12.11.6.1 Solid Materials
12.11.6.2 Organic Compounds
12.11.6.3 Plating and Inorganic Finish
12.11.6.4 Corrosion By-Products
12.11.7 Completion of the Bond
12.11.8 Bond Corrosion
12.11.8.1 Chemical Basis of Corrosion
12.11.8.1.1 Electrochemical Series
12.11.8.1.2 Galvanic Series
12.11.8.1.3 Relative Area of Anodic Member
12.11.8.1.4 Protective Coatings
12.11.9 Workmanship
12.12 Electrostatic Discharge (ESD) Control Program
6
12.2 Selection of Parts, Materials and Tools Normally, the parts and materials
needed to manufacture cable and wire harness assemblies, or otherwise needed to install
the assemblies into their ultimate next-higher level assembly, will have been specified by
the user activity, or otherwise, will have been defined on the applicable documentation
(assembly drawings, wire lists, parts lists, etc.), which may or may not require user
approval. Consult Section 7 of this handbook for additional information pertaining to
materials.
However, this handbook section will provide some general guidance that may be useful
when parts and materials have otherwise not been previously selected or otherwise
mandated.
Tools needed are normally selected by the manufacturer; however, some tooling may
have been selected or otherwise mandated by the user.
12.2.1 Standard Parts and Materials Whenever possible, standard parts and materials
(items purchased to recognized commercial, industrial or military/NASA specifications)
should be used. Such items should be suitable for their intended purpose.
12.2.2 Non-Standard Parts and Materials Any parts and materials not selected in
accordance with paragraph 12.2.1 are considered non-standard and as such user approval
may be required for their use.
12.2.3 Sealing Materials Some cable and wire harness assemblies may need to function
in a humid environment or otherwise in applications where water or other liquid can
exist. In such cases, it may be necessary to provide a means of sealing of components
such as connectors, etc.
Materials used for sealing should be elastomeric and reversion resistant. Materials should
also be fully cured at the time of delivery of the assembly. If not fully cured, the material
may out-gas unacceptable chemicals, or otherwise may not have developed the necessary
mechanical and sealing properties needed for the application.
Various sealing materials are available. Examples include cured synthetic rubber
(operating range -60ºF to + 200ºF)(MIL-PRF-8516) which acts as a deterrent to fatigue,
corrosion, and contamination, as well as an aid in reducing arc-over between electrical
connector pins.
Another example is a silicone rubber sealing compound with accelerator (operating range
-80ºF to +400ºF) (MIL-PRF-23586), for use in applications where tear resistance is not
critical. Proper curing is essential and in some cases, vacuum de-airing may be needed to
eliminate air bubbles. Volatile by-products formed during cure should be removed.
Factors affecting the rate and degree of cure are numerous, such as; catalyst
concentration, humidity, diluents (if used to lower the viscosity), thickness of section area
exposed for the release of volatiles and post-cure employed. Because of the high volume
expansion of RTV silicones they are not recommended for use for the total filling of
7
confined items. Pressures developed during heating are of a high order. Literature
pertaining to the manufacturer’s product should be thoroughly understood prior to
consideration of RTV silicones, and all applications for use of this material should be
thoroughly evaluated prior to use.
12.2.4 Fastening Devices Fastening devices (e.g., nuts, bolts, screws, lock washers, flat
washers, clips, pins, lock wire, etc.) should be made of corrosion-resisting material, or
otherwise treated to resist corrosion, without using paint. Spring type locking devices,
such as lock washers and retaining rings, that are made of precipitation hardened semiaustenitic corrosion-resisting steel, do not require additional protection against corrosion.
Aluminum alloy fasteners are not normally considered to be corrosion resistant.
Fastener galling is an important consideration as certain fastener combinations such as
using a stainless steel fastener in a tapped hole in stainless steel material can create
galling of the fastener. In many cases, this galling causes the fastener to bind up in the
machined hole and it eventually may break when trying to remove the fastener.
Acceptable methods exist to prevent galling. These include, but are not limited to,
plating the fastener (e.g., chrome plating, etc.), or using an anti-galling compound. If
anti-galling compound is used, it is important that the compound selected be compatible
with the fastener material. Additionally, the recommended fastener torque value (if
specified) may require adjustment to the “lubricated”, rather than “dry” thread value if
anti-galling compounds are used.
12.2.4.1 Nuts, Bolts and Screws Nuts, bolts and screws should be selected from
recognized federal, military or industry standards; examples of such standards include,
but are not limited to; FF-S-85, FF-S-86, FF-S-92, FF-S-200, MIL-DTL-1222,
NASM17828, NASM17829, NASM17830, NASM21250, etc.
When lock washers are required, they are normally supplied as stand alone items, or
otherwise as part of an assembled fastener.
12.2.4.2 Self-Locking Threaded Fastener Self-locking bolts and screws are sometimes
used in a design. These are standard UNC, UNJC, UNF, UNJF, UNRC or UNRF threads
that have a special self-locking feature provided in the fastener. A recommended
specification is MIL-DTL-18240. This provides for three types of self-locking features;
Type N (Plug/Pellet), Type L (Strip) and Type P (Patch).
The self-locking elements are incorporated in external screw used in applications where
maximum temperature does not exceed 250°F.
Type N element, plug/pellet configuration is installed via a hole drilled into the fastener.
Type L element, strip configuration is installed via a strip cut through the threads parallel
to the length of the fastener.
8
Type P element, patch configuration is installed without removal of any material of the
fastener.
Normally, a specific type should only be specified when required by design or application
requirements.
The plug/pellet is not recommended for sizes below 0.190.
Self-Locking fasteners are not recommended for the following applications:
1. Temperatures above 250ºF.
2. Applications where the item being fastened must be removed and/or reinstalled
and/or opened and closed multiple times (e.g., panels, doors, drawers, plug-in
modules, etc.). Normally self-locking fasteners are only qualified to withstand
five removal and/or reinstallations.
3. Safety related applications where the failure of the fastener can cause injury or
death to personnel or product damage.
4. Applications where the self-locking mechanism will encounter keyways, slots,
cross-holes or thread interruptions.
5. Fasteners that have had the self-locking mechanism reworked or reprocessed
6. Electrical connections.
7. Other considerations as indicated in the applicable self-locking fastener
specification.
12.2.4.3 Flat-Head Screws Flat head screws should not be used in material of a
thickness less than one and one-half times the height of the screw head. Flat-head screws
should be properly and completely seated in the material.
12.2.4.4 Blind Fasteners A blind fastener is the type of fastener used when only one side
of an assembly is accessible for installation of the fastener.
Blind fasteners are generally provided in two different types; Type I - pull type-positive
mechanically locked, and Type II - threaded-self-locking type. See NASM8975 for one
type of blind fastener that may be used.
The Type I fastener is installed by the spindle being pulled into the sleeve, forming a
blind head on the back side of the assembly, and by subsequently removing the pulling
portion of the spindle.
The Type II fastener is a multiple piece construction furnished as an integral assembly
which consists of a nut body, a core-bolt, and a sleeve.
Special commercially available tooling is used to install these fasteners.
9
12.2.5 Thread-Cutting Screws (Self-Tapping Screws) These types of fasteners should
only be used for non-critical and non-structural applications such as for mounting
information and/or identification plates.
12.2.6 Washers Two types of washers are used; lock washers and flat washers. Washers
are used to distribute the load over a larger area, and to provide a hardened bearing
surface. Additionally, lock washers compensate for developed looseness between
component parts of an assembly.
12.2.6.1 Lock Washers Lock washers are normally available in two different types,
helical spring-lock washers, and tooth-type. Helical spring-lock washers include: regular,
heavy, extra duty, and high-collar types. Tooth-lock washers include internal tooth,
external tooth, countersunk external tooth, internal/external tooth, and others. ASME
B18.21.1 covers one type of acceptable lock washer; Lock Washers (Inch Series), and
NASM35338, is an example of a Regular Medium Series Helical-Spring Lock Washer.
Split ring (helical spring) are the preferred type.
External tooth lock washers (tin-brass, copper alloy 425) are preferred for electrical
connections (see MS35335 for one example that conforms to ASSME B18.21.1).
External-tooth type lock washers are used to bite through protective coatings of
aluminum parts if they are grounded or electrically bonded through the fastening device.
Internal-tooth lock washers are used instead of external-tooth lock washers only where
necessitated by space limitations, appearance, or other special conditions.
If internal-tooth lock washers are used, it is important that the size of the washer and
diameter of the fastener are chosen so that the serrations make satisfactory contact.
12.2.6.2 Flat Washers One recommended specification for procurement of flat washers
is NAS1149 (supersedes MS16208, FF-S-92, and AN960). Flat washers are
recommended for use for the following applications:
a)
b)
c)
d)
Between screw heads and soft materials, unless a washer head screw or
similar type is used to provide a bearing surface equivalent to the bearing
surface of the appropriate flat washer.
Between a nut or lock washer and a soft material.
Where lock washers are used for securing a soft material, flat washers are
used to prevent marring or chipping of the material and the applied protective
coating, except in areas where an electrical ground is required.
Between an organically finished material and lock washers, bolt and screw
heads, or nuts, except where their use conflicts with electromagnetic
interference considerations.
10
12.2.7 Utilization of Standard Tools Whenever possible, only standard tools should be
needed to install and/or to maintain cable and wire harness assemblies. A standard tool is
defined as wrenches, crimp tools, soldering equipment, and other types of tooling that is
normally available as listed in the Federal Supply Catalog.
All other tooling is normally considered special tooling, and as such, the user may require
the manufacturer to provide this tooling, including a place to store the tooling in the
delivered equipment, when appropriate.
12.2.8 Materials Various types of materials are used during the manufacture of cable and
wire harness assemblies, or otherwise, when installing these assemblies in their nexthigher assembly. Normally, the materials will have been mandated for use via the
applicable documentation (e.g., drawings, parts lists, etc.), or otherwise as specified by
the user. The following guidance is provided to aid the manufacturer in event that
materials have not yet been selected, or otherwise mandated for use.
12.2.8.1 Non-Recommended Materials (may be specifically Prohibited by the User)
Various materials exist that are not recommended for use because of potential personnel
hazards, restrictions by Government regulations, or for other reasons. Some of these
materials may have been specifically prohibited from use by the user. The following are
examples of non-recommended materials. This list may not be all inclusive.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
. (l)
Toxic pyrolytic materials. (see 12.2.8.1.1)
Flammable materials (see 12.2.8.1.2).
Fragile or brittle materials (see 12.2.8.1.3).
Mercury (see 12.2.8.1.4).
Asbestos (see 12.2.2.8.1.5).
Silicone (see 12.2.8.1.6).
Polychlorinated biphenyls (PCB) (see 12.2.8.1.7).
Polyvinyl chloride (PVC) (see 12.2.8.1.8).
Cadmium and cadmium plating (see 12.2.8.1.9).
Freon solvents.
Radioactive materials
Magnesium or magnesium base alloys
12.2.8.1.1 Toxic Pyrolytic Materials Toxic pyrolytic materials include those
materials which emit toxic gases or other harmful products when exposed to high
temperatures, including fire.
Toxicity of materials can be determined by performing a pyrolysis test. One such test
method is DTIC AD 297457, Procedure for Determining Toxicity of Synthetic
Compounds.
12.2.8.1.2 Flammable Materials Flammable materials include any material in a form
which will ignite or explode from an electric spark, flame, or from heating, and which, if
so ignited, will independently support combustion in the presence of air. Material
11
flammability can be determined by various test methods. Some example test methods
include; DTIC AD 297457, ASTM D5948, Standard Specification for Molding
Compounds – Thermosetting, and ASTM D 635, Standard Test Method for Rate of
Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position.
12.2.8.1.3 Fragile or Brittle Materials Fragile materials include any materials which are
fragile in the form, size, and manner in which they would be used.
Brittle materials, in general, fall within this category from the point of use as structural
members. However, certain brittle materials may be used in small quantities, within a
part, when the materials is so mounted, constrained or otherwise disposed within the part
that it will not be strained under any processing, environmental, and handling conditions
to which the part reasonably may be subjected. (For example, glass and ceramic terminals
seals and bushings have been employed successfully in packaging certain semiconductor
devices).
Any material in a frail form which is not positively protected against mechanical damage
as used in a part or subassembly is considered fragile.
Cast iron, semi-steel, porcelain, and similar brittle materials are not recommended for use
as frames, brackets, mounting panels, spacers, or enclosures.
12.2.8.1.4 Mercury Mercury is highly toxic to humans and is corrosive to metals.
Therefore, equipment, components and cable and wire harness assemblies should be free
of mercury. Mercury should not be used in the manufacture or testing of cable and wire
harness assemblies.
Manufacturers should evaluate materials to confirm that they do not contain intentionally
added mercury. For example, some potting compounds may use a catalyst that contains
mercury as a method of minimizing the presence of air bubbles during the potting
process.
12.2.8.1.5 Asbestos Materials containing asbestos should not be used since asbestos is a
known human health hazard.
12.2.8.1.6 Silicone This material is acceptable when used in conjunction with the
manufacture and/or installation of cable and wire harness assemblies. Silicone can also
contaminate a solder joint in event it is inadvertently allowed to come in contact with
surfaces designated for soldering.
12.2.8.1.7 Polychlorinated Biphenyls (PCB) PCBs are a type of organic compound that
should not be used because they are an environmental pollutant. Historically, their main
use was within components such as certain types of capacitors, and transformers.
12.2.8.1.8 Polyvinyl Chloride (PVC) PVC, commonly referred to as vinyl, is hazardous
to human health and the environment throughout its entire life cycle. PVC is useless
12
without the addition of a plethora of toxic additives, which can make the PVC product
itself harmful to consumers. These chemicals can leach out of PVC, posing risks to
children and consumers. New car smell? New shower curtain smell? That’s the smell of
poisonous chemicals out-gassing from the PVC. One of the most toxic additives is
DEHP, a phthalate that is a suspected carcinogen and reproductive toxicant readily found
in numerous PVC products.
When heated in a fire, PVC releases toxic hydrogen chloride gas, forming deadly
hydrochloric acid when inhaled.
PVC cannot be effectively recycled due to the many different toxic additives used to
soften or stabilize PVC, which can contaminate the recycling batch.
Based on the above, PVC, in particular, PVC insulated wire is not recommended for use.
12.2.8.1.9 Cadmium and Cadmium Plating Cadmium and its compounds are extremely
toxic even in low concentrations, and will bio-accumulate in organisms and ecosystems.
When working with cadmium, it is important to do so under a fume hood to protect
against dangerous fumes. For example, silver solder, which contains cadmium, should be
handled with care. Serious toxicity problems have resulted from long-term exposure to
cadmium plating baths.
Based on the above, manufacturers should evaluate the components, fasteners, and other
items they intend on using to verify that they do not contain cadmium. One common
source of cadmium is fasteners that have been cadmium plated.
12.2.8.2 Other Non-recommended Materials The following other types of materials
should not be used:
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
(l)
Linen
Cellulose acetate
Cellulose nitrate
Regenerate cellulose
Wood
Jute
Leather
Cork
Paper and cardboard
Organic fiberboard
Hair or wool felts
Plastic materials using cotton, linen or wood flour as a filler
12.2.9 Metals Metals should be selected or processed and applied in a manner
that provides corrosion-resistance. Metals that are not inherently corrosion resistant
should be processed (treated, plated, or painted) to provide corrosion-resistance.
13
12.2.9.1 Selection of Metals in Direct Contact Equipment should meet
guidelines for minimizing attack due to electrolytic action between dissimilar
metals in contact with each other. MIL-STD-889 can be used for guidance. See
paragraphs 12.11.8.1 and subparagraphs thereto for additional information.
Metal-to-metal contact is not normally considered to exist if one of the contact surfaces is
hard-coat sulfuric acid anodized aluminum that has not been previously exposed to a
corrosive environment.
If a metal is coated or plated, the coating or plating metal rather than the base
metal should be considered.
12.2.9.2 Corrosion-Resisting Metals The following commonly used metals,
when properly applied, are considered to be inherently corrosion-resistant without
further processing when the service environment precludes immersion, condensation,
or periodic wetting of the surface. These metals are suitable except where otherwise
specified for severe environmental conditions.
(a) Brass – Brasses containing 20 to 40 percent zinc are highly susceptible to stress
corrosion cracking in marine environments when highly stressed
(b) Bronze
(c) Copper
(d) Copper-nickel alloy
(e) Copper-beryllium alloy
(f) Copper-nickel-zinc-alloy
(g) Nickel-copper-alloy
(h) Nickel-copper-silicon alloy
(i) Nickel-copper-aluminum alloy
(j) Aluminum alloys, types 3003, 3004, 5052, 5056, 5083, 5085, 5086, 5154, 5456, 6061
(k) Titanium
(1) Austenitic steels, corrosion-resisting types 202, 301, 302, 303, 304, 304L, 309, 310,
316, 316L, 321, 324A, 347 – Austenitic stainless steels are susceptible to stress corrosion
cracking in marine environments when service temperatures exceed 150ºF.
12.2.10 Plastics It is recommended that plastics which serve as electrical insulation be
selected in accordance with 12.2.11.
It is recommended that plastics which do not serve as electrical insulation (structural
parts, and so forth) meet all physical and mechanical properties required for plastic
insulating materials, including non-flammability and non-toxicity; however, these plastics
may not need to meet the arcing and tracking resistance requirements.
12.2.11 Insulation Materials Various types of materials are suitable for use in the
manufacture of cable and wire harness assemblies, and when installing these assemblies
into their next-higher assembly. The following guidance is recommended regarding
insulation materials:
14
12.2.11.1 Arc and Tracking Resistance Structural insulators, such as laminates,
molding compounds, encapsulating materials, bus bar coverings, and similar materials
subject to arcing conditions should have an arc resistance of not less than 130 seconds
and a track resistance of not less than 70 minutes for low voltage (< 2000 V) equipment.
For equipment rated at 2000 V and higher a minimum track resistance of 300 minutes is
recommended.
Arc resistance may be determined via ASTM D495, and tracking resistance may be
determined by ASTM D3638.
12.2.11.2 Laminated Plastics. Laminated plastics in the form of sheets, rods,
or tubes are recommended for use where rigid materials with dielectric properties are
needed. Such laminates should meet the temperature, mechanical and electrical
requirements of each application.
Other forms of plastics should be used only when suitable for the particular application.
It is recommended that laminates be chosen to meet the minimum
requirements for toxicity, flame resistance, and arc and tracking resistance.
Machined edges on glass based laminates may require sealing to prevent moisture
infusion.
12.2.11.3 Molded Thermosetting Plastics Molded thermosetting plastics are generally
used in electrical equipment where a rigid dielectric is needed and where the form or
shape is such that fabrication of the part out of sheet stock is too costly, or the part too
complex in design. It is recommended that the molding compound meet minimum
requirements for toxicity, flame resistance and arc resistance and the mechanical and
electrical requirements for each application.
See ASTM D5948 for some recommended molding compounds.
Consideration should be given to specifying the color of thermosetting plastics depending
on the voltage rating. For example, red color is normally used for voltages rated at 2000
V or higher and gray color is used for lower voltages.
12.2.11.4 Thermoplastics In general, thermoplastics are not recommended for
any molded part because of temperature rating considerations.
However, if the application is such that only a thermoplastic material can be
used, then polyamide (nylon) (ASTM D4066), or polycarbonate (ASTM D3935),
molding compounds are recommended as long as they are suitable for the application.
12.2.12 Classes and Definitions of Insulating Materials Temperature classes of
insulating materials have traditionally been established by definition based on a chemical
composition of the materials. Methods of temperature classification based on the results
15
of thermal evaluation tests are coming into use. Since the temperature classification of a
material that has been accepted for a long time will have been established by field
experience, its life temperature characteristics determined by test provides a basis for
comparison with the thermal life of a new material.
The purpose of assigning each material to a definite temperature class, therefore, is to
facilitate comparisons between materials and to provide a single number to designate
each class for purposes of standardization. The life expectancy under the test conditions
may be shorter than, and has no direct relation to, the life expectancy of the material in
actual service. The classes and definitions of insulating materials are grouped according
to the classifications noted below. New classifications may be created, and new materials
may be developed for existing classifications. Therefore, the listing in the subsequent
paragraphs may not be all inclusive.
12.2.12.1 Class 90 Materials or combinations of materials such as cotton,
silk, and paper without impregnation. Other materials or combinations of
materials may be included in this class if by experience or accepted tests they
can be shown to have comparable thermal life at 90ºC.
12.2.12.2 Class 105 Materials or combinations of materials such as cotton,
silk, and paper when suitably impregnated or coated or when immersed in a
dielectric liquid such as oil. Other materials or combinations of materials may
be included in this class if by experience or accepted tests they can be shown to
have comparable thermal life at 105ºC.
12.2.12.3 Class 130 Materials or combinations of materials such as mica,
glass fiber, and so forth, with suitable bonding substances. Other materials or
combinations of materials may be included in this class if by experience or.
accepted tests they can be shown to have comparable thermal life at 130ºC.
12.2.12.4 Class 155 Materials or combinations of materials such as mica,
glass fiber, and so forth, with suitable bonding substances. Other materials or
combinations of materials may be included in this class if by experience or
accepted tests they can be shown to have comparable thermal life at 155ºC.
12.2.12.5 Class 180 Materials or combinations of materials such as
silicone elastomer, mica, glass fiber, and so forth, with suitable bonding
substances such as appropriate silicone resins. Other materials or combinations
of materials may be included in this class if by experience or accepted tests they
can be shown to have comparable thermal life at 180ºC.
12.2.12.6 Class 200 Materials or combinations of materials such as mica,
glass fiber, asbestos, and so forth, with suitable bonding substances. Other
materials or combinations of materials may be included in this class if by
experience or accepted tests they can be shown to have comparable thermal life at
200ºC.
16
12.2.12.7 Class 220 Materials or combinations of materials which by
experience or accepted tests can be shown to have the required thermal life at
220ºC.
12.2.12.8 Class 240 Materials or combinations of materials which by
experience or accepted tests can be shown to have the required thermal life at
240ºC.
12.2.12.9 Class over 240 Materials consisting entirely of mica, porcelain,
glass, quartz, and similar inorganic materials. Other materials or combinations,
of materials may be included in this class if by experience, or accepted tests they
can be shown to have the required thermal life at temperatures over 240°C.
12.2.12.10 Electrical Tape Electrical tape is sometimes used in lieu of sleeving for
protection from chaffing and/or for maintaining required electrical clearances between
non-common conductors. Any tape that provides the necessary electrical properties may
be used. However, the tape used should be suitable for the maximum operating
environment, and the adhesive should have a shelf life and robustness to remain attached
without unraveling or otherwise coming loose over the expected life time of the
equipment in the worst case operating environment. Some available tapes include MIL-I631, MIL-I-24391, MIL-I-19166 and A-A-59770.
Tapes selected should be non-flammable. Tape specifications should be carefully
evaluated for flammability ratings since a specification may cover both flammable and
non-flammable materials.
12.2.12.11 Sleeving Sleeving is commonly used to protect against chaffing and for
maintaining required electrical clearances between non-common conductors. Any
sleeving that provides the necessary electrical properties may be used. However, the
sleeving used should be suitable for the maximum operating environment. Some
available sleeving includes MIL-I-3190, MIL-I-631, MIL-I-22129 and AMS-DTL-23053.
Sleeving selected should be non-flammable. Sleeving specifications should be carefully
evaluated for flammability ratings since a specification may cover both flammable and
non-flammable materials.
One disadvantage of sleeving is that it normally cannot be easily replaced after the
conductors are terminated with components such as terminal lugs or connectors unless
the sleeving is cut and subsequently tied back together after installation; or unless an
alternate type of sleeving such as “zipper” or “spiral” sleeving is used.
Heat-shrink sleeving can also be an issue in event it is damaged via a nick or tear since in
time, the damage can eventually cause the sleeving to completely tear and separate from
the conductors due to the internal stresses from the heat shrink process.
17
12.2.12.12 Straps/Clamps Various types of straps and/or clamps are normally used to
restrain and/or route cable and wire harness assemblies. Their primary function is to
preclude damage during a shock and vibration environment and to facilitate routing in an
organized fashion. They also preclude interference with moving components such as
cooling fan blades, etc.
Straps and clamps that include a non-metal insert should be non-flammable. The
applicable specifications should be carefully evaluated for flammability ratings since a
specification may cover both flammable and non-flammable materials. The materials of
the inserts should be verified for chemical compatibility with the conductor insulation
since some materials may interact chemically and leave a residue on the conductor
insulation or sleeving.
It is also recommended that insert materials have an unlimited shelf-life (i.e., 20 years)
whenever possible and appropriate for the end-item application.
See SAE-AS23190 and SAE-AS21919 as examples.
12.2.12.13 Lacing Cord Lacing cord and/or lacing tape should be selected based on the
maximum operating/environmental temperature range since some materials are restricted
to 105ºC whereas other materials are available for 200ºC or greater temperature rating.
Materials selected should not have a wax coating.
Typical materials include; MIL-T-713, MIL-I-3158, MIL-Y-1140 and A-A-52080
through A-A-52084.
12.2.13 Terminal Lugs Whenever possible, it is recommended that terminal lugs be of
the insulation restricting ring-tongue type such that the lug is designed to accommodate
only a single wire size (the inside diameter of the terminal lug barrel in which the bare
wire is to be inserted is smaller than the outside diameter of the insulation of the wire).
This precludes crimping down over the wire insulation and also helps ensure the correct
wire size has been used with the correct size lug. Normally these type lugs have the wire
size embossed in the lug barrel and the insulation portion of the lug is color coded with a
bi-color stripe for identification purposes. See SAE AS7928/1 or SAE AS7928/2 for this
type of lug. SAE AS7928 ring type lugs are recommended for larger wire sizes not
included in the other two specifications.
Class 1 ring type terminal lugs per the cited specifications are preferred for reference on
drawings; however, Class 2 lugs may be used by the manufacturer. Other types of lugs
may be used when specified or approved.
When required, terminal lugs should be of the insulated type as long as the temperature
rating of the terminal lug is capable of withstanding the worst case equipment operating
temperature in the worst case environment. Another alternative is to select an uninsulated lug and add properly temperature rated insulation over the lug.
18
Terminal lugs should be plated with either tin or silver; however, tin whisker concerns
should be considered if a pure tin plated lug is selected. Only tin plated lugs are
recommended for connections to aluminum material. Un-plated copper or copper alloy
lugs are not recommended and may require specific user approval.
12.3 Wiring Selection Wiring should be of a type suitable for the application. Wire
should be selected so that the rated maximum conductor temperature Is not exceeded for
any combination of electrical loading, ambient temperature, and heating effects of
bundles, conduit and other enclosures.
Typical factors to be considered in the selection are voltage, current, ambient
temperature, mechanical strength, abrasion, flexure and pressure altitude requirements,
and extreme environments such as Severe Wind and Moisture Problem (SWAMP) areas
or locations susceptible to significant fluid concentrations.
The wire selection should take into account all requirements of the application and the
following design considerations:
12.3.1 Conductor Degradation Degradation of tin and silver plated copper conductors
wI1l occur If they are exposed to continuous operation at elevated temperatures. These
effects should be taken into account in the selection and application of wiring.
12.3.1.1 Tin Plated Conductors Tin-copper intermetallics will form resulting in an
increase in conductor resistance. The increase is inverse to size, being up to 4 percent for
the smallest gage.
12.3.1.2 Silver Plated Conductors Degradation in the form of inter-strand bonding,
silver migration, and oxidation of the copper strands will occur with continuous operation
near rated temperature, resulting in 1oss of flexibility.
Due to potential fire hazard, silver plated conductors should not be used in areas where
they are subject to contamination by ethylene glycol solutions. These potential problems
should be considered in the application of silver plated copper wire.
12.3.1.3 Solderability Both tin plated and silver plated copper conductors will exhibit
poor solderability after exposure to continuous elevated temperature. In event of a
maintenance action, it may be necessary to re-tin the conductors to restore their
solderability.
12.3.2 Aluminum Wire Aluminum wire should be avoided and may require user
approval for use. If aluminum wire has been authorized for use, it should be terminated
only by terminations specifically compatible with the application.
19
12.3.3 Insulation Compatibility with Sealing and Servicing Wiring terminations in
devices where the wiring must be sealed to provide an environment-resistant joint should
have insulation compatible with the sealing feature of the device.
After installation, the integrity of the sealing features of all such devices should be intact,
and able to perform their function. A device is considered as sealed if the outermost
sealing feature (web) is in full contact with the device when visually inspected.
The wiring should be installed so that transverse loads will not destroy the integrity of the
sealing feature of the wire.
12.3.3.1 Wire Diameter The finished wire outside diameter should be within the limits
specified for the grommet specified in the appropriate component specification and
should not exceed the capability of contact servicing tools to insert and release contacts.
12.3.3.2 Potting Seal on Wire or Cable The potting should be bonded to the outermost
surface of the wire or cable in such a way to ensure an environmental seal.
12.3.3.3 Insulation Degradation Wiring should be handled, stripped and
installed so as not to distort, roughen or damage the insulation on which
sealing is to be effected. Methods-of marking and identification should be applied so as
not to provide a track for moisture entry. The impression left on the insulation of shielded
and twisted wires can also cause unacceptable degradation of the insulation in relation to
the elastomer seal. Caution should be used to avoid this condition.
12.3.4 Wire Size and De-rating The required minimum wire size (AWG or conductor
diameter) depends on the maximum operating and/or transient current it will need to
carry in the application. It also depends on the worst case operating environment and
whether the wiring is in an open or closed enclosure, with or without added air flow or
cooling, and whether or not the wires are bundled together, how many wires are in a
bundle, and whether the bundle is covered with a protective material. The impact of
altitude should also be considered when determining the minimum wire size
Including specific minimum wire current rating information herein for all available types
of wire is beyond the scope of this handbook. However, generally wire ratings for
aerospace application are more conservative that for other applications and therefore, the
following wire de-rating values from NASA publication EEE-INST-002 are reproduced
in Table 12-1 below. The reader should consult applicable design standards for other
wire current ratings.
20
Table 12-1 Wire and Cable De-rating Requirements (Notes 1 and 2)
Wire Size (AWG)
De-rated Current (Amperes)
Single Wire
Bundled Wire or Multiconductor Cable
30
1.3
0.7
28
1.8
1.0
26
2.5
1.4
24
3.3
2.0
22
4.5
2.5
20
6.5
3.7
18
9.2
5.0
16
13.0
6.5
14
19.0
8.5
12
25.0
11.5
10
33.0
16.5
8
44.0
23.0
6
60.0
30.0
4
81.0
40.0
2
108.0
50
0
147.0
75.0
00
169.0
87.5
Note 1: De-rated current ratings are based on an ambient temperature of 70ºC or less in a
hard vacuum 10-6 torr. For de-rating above 70ºC ambient, consult the NASA project
parts engineer.
Note 2: The de-rated current ratings are for 200ºC rated wire, such as TeflonTM insulated
(PTFE) wire, in a hard vacuum of 1 X 10-6 torr.
a. For 150ºC wire, use 80% of the values shown in Table 12-1.
b. For 135ºC wire, use 70% of the values shown in Table 12-1.
c. For 260ºC wire, use 115% of the values shown in Table 12-1.
Small wire sizes can save weight but require special handling in order to prevent
breakage. Conductors smaller than 26 AWG are discouraged for use and may require
specific user approval for use. When conductors smaller than 24 AWG must be used, it
is recommended that the conductor material be high strength copper alloy.
The number of different wire sizes should be minimized. For example, it is not desirable
to mix 22 AWG and 20 AWG wires unless some means such as color coding is used to
differentiate between the wires. It is relatively easy to visually mistake 22 AWG wire for
20 AWG wire. Standardizing on just a few different wire sizes minimizes the amount of
different tooling (e.g., crimp tools, contact insertion tools, etc.) needed thus saving costs.
12.3.5 Wire and Cable Identification In some applications, it may be necessary to
mark each wire and cable with an identification code on the jacket or sleeving of the wire
21
and cable via a method that will not damage the conductor or insulation. See Section 8 of
this handbook for additional guidance on marking and labeling.
Identification may include component reference designations (e.g., J for connector
receptacles, P for connector plugs, TB1-1for terminal 1 of terminal board # 1, etc.),
harness part numbers, drawing revision level, RoHS markings, and other types of
markings. In some applications it may be necessary to identify the criticality of the
circuit the wire or cable is a part of to ensure proper harness separation and protection
from damage, fire, etc.
It is recommended that identification marking be readable horizontally from left to right
and vertically from top to bottom.
Marking characters should be legible and permanent and the method of applying the
marking should not impair the characteristic of the wiring. Hot stamp marking directly
on wire or cable insulation is not recommended.
Identification should be located close to the termination end such that it is readily visible
on installation. Long wire or cable runs may require identification at periodic intervals
(e.g., every 12 inches or shorter).
12.3.5.1 Wire Size Color Code System Use of color coded wire provides an easy means
for differentiating between different wire sizes, as well as for facilitating separation of
wiring for critical versus non-critical circuits. However, color coding may require user
approval.
When wire color coding has been authorized, the color code scheme shown in Table 12-2
is recommended, unless otherwise specified.
Size
26
24
22
20
18
16
14
12
Table 12-2 Wire Size Color Code
Color
Size
Black (Not a recommended wire size) 10
Blue (Not a recommended wire size)
8
Green
6
Red
4
White
2
Blue
1
Green
0
Yellow
Color
Brown
Red
Blue
Yellow
Red
White
Blue
The color code can be assigned by using solid colored wire, wire with a distinctively
color band or distinctively stripped; however, the methods should not be mixed in the
harness or assembly for consistency and understanding. The use of black and green color
coding per table 12-2 also needs to consider whether this same color scheme is being
applied to ground wires and/or common return wires to avoid any confusion.
22
12.3.6 Wire for Electromagnetic Interference (EMI) Some wires and cables may
connect to circuits that are vulnerable to noise pickup or incorrect equipment operation
when subjected to external (e.g., lightning, operation of welding equipment, lighting
fixtures, magnetic components, etc.) or internal (e.g., within the chassis) electromagnetic
interference. In military applications, such as for Naval ship-based aircraft, wiring must
be such as to facilitate proper operation in EMI fields caused by the ship’s radar,
communication, and tracking equipment.
Vulnerability to EMI can be mitigated by using wires with shields, or by adding shielding
over the wire or cable harness. Twisting the conductors can also help alleviate EMI
concerns, and routing wires away from EMI sources (e.g., magnetic components,
switching power supplies, etc.) can also help mitigate EMI concerns.
In addition to providing shielded wires, or adding shielding over completed wire and
cable harnesses, special electrical cable that acts as distributed low-pass filters is
sometimes used in critical applications where EMI interference cannot be tolerated. One
such filter cable is MIL-C-85485. Filter line wires must have a metallic shield
surrounding them in order for the expected filtering action to occur. The method of
installing this type of cable is critical to performance of the product for its intended use.
Consult SAE AIR4465 for details on how to install this type cable.
12.4 Service Life Wire and cables and associated components used for the wire and cable
installation should be selected and installed to provide ease of maintenance and high
reliability over the expected service life of the equipment. The user normally defines the
required service life.
12.5 Safety and Personnel Protection Wire and cable harness installation can have a
significant adverse impact on personnel safety unless steps are taken to address personnel
safety considerations. When the harnesses are properly installed and the enclosure is
grounded, there should be no accessible way for operating personnel to receive an electric
shock even though an internal fault that may exist between any two circuits, between any
circuit and a structural member, or between any circuit or ground.
Installation should be such as to minimize the possibility of maintenance personnel being
exposed to electrical shock while servicing, adjusting, or checking out the equipment.
For access to such circuits, further positive action should be required to remove a cover
or open a portion of the guard means. A warning plate is recommended for prominent
display to remind personnel of appropriate precautions to ensure the circuit has been deenergized.
With regard to personnel protection from mechanical safety hazards, external moving
parts should be avoided. If their use is unavoidable, positive protection in the form of a
guard should be provided. Sharp corners and projections which may cause injury or
catch on clothing should be avoided. Excessive fastener length for fasteners used to
secure clamps, etc. can act as sharp surfaces and as such may cause personnel injury.
23
12.5.1 Electric Shock Electric shock occurs when the human body becomes a part of an
electric circuit. It most commonly occurs when personnel come in contact with energized
devices or circuits while touching a grounded object or while standing on a damp floor.
The major hazard of electric shock is death.
The effects of an electric current on the body are principally determined by the magnitude
of the current and the duration of the shock. The current is given by Ohm’s Law, which,
stated mathematically, is I= V/R where V is the open circuit voltage of the source and R
is the resistance of the total path including the internal source resistance, and not just the
body alone. In power circuits, the internal source resistance is usually negligible in
comparison with that of the body. In such cases, the voltage level, V, is the important
factor in determining if a shock hazard exists.
At the commercial frequencies of 50-60 Hz and at voltages of 120-240 volts, the contact
resistance of the body primarily determines the current through the body. This resistance
may decrease by as much as a factor of 100 between a completely dry condition and a
wet condition. Thus, perspiration on the skin has a great effect on its contact resistance
(For calculation purposes, the resistance of the skin is usually taken to be somewhere
between 500 and 1500 ohms).
At voltages higher then 240 volts, the contact resistance of the skin becomes less
important. At the higher voltages, the skin is frequently punctured, often leaving a deep
localized burn. In this case, the internal resistance of the body primarily determines the
current flow.
12.5.1.1 Levels of Electric Shock The perception current is that current which can just
be detected by an individual. At power frequencies, the perception current usually lies
between 0 and 1 milliamps for men and women, the exact value depending on the
individual. Above 300 Hz, the perception current increases, reaching approximately 100
milliamps at 70 kHz. Above 100-200 kHz, the sensation of shock changes from tingling
to heat. It is believed that heat or burns are the only effects of shock above these
frequencies.
The reaction current is the smallest current that might cause an unexpected involuntary
reaction and produce an accident as a secondary effect. The reaction current is 1-4
milliamps. The American National Standards Institute limits the maximum allowable
leakage current to 0.2 milliamps for portable two-wire devices and 0.75 milliamps for
heavy movable cord-connected equipment in order to prevent involuntary shock
reactions.
Shock currents greater than the reaction current produce an increasingly severe muscular
reaction. Above a certain level, the shock victim becomes unable to release the
conductor. The maximum current at which a person can still release a conductor by using
the muscles directly stimulated by that current is called the “let-go” current. The “let-go”
current varies between 4-21 milliamps, depending on the individual. A normal
24
person can withstand repeated exposure to his “let-go” current with no serious after
effects when total duration of each shock lasts only for the time required for him to
release the conductor.
Shock currents above about 18 milliamps can cause the muscles of the chest to contract
and breathing to stop. If the current is interrupted quickly enough, breathing will resume.
However, if the current persists, the victim will loose consciousness and death may
follow. Artificial respiration is frequently successful in reviving electric shock victims.
Above a certain level, electric shock currents can cause an effect on the heart called
ventricular fibrillation. For all practical purposes, this condition means a stoppage of the
heart action and blood circulation. Experiments on animals have shown that the
fibrillating current is approximately proportional to the average body weight and that it
increases with frequency.
In Table 12-3, the various hazardous current levels for ac and dc are summarized along
with some of the physical effects of each.
Table 12-3 Summary of the Effects of Electric Shock
Alternating Current
(60 Hz)(ma)
0-1
1-4
4-21
21-40
40-100
Over 100
Direct Current (ma)
0-4
4-15
15-80
80-160
160-300
Over 300
Effects
Perception
Surprise (Reaction Current)
Reflex Action (Let-Go Current)
Muscular Inhibition
Respiratory Block
Usually Fatal
12.5.1.2 Shock Prevention Most shock hazards can be divided into two categories:
unsafe equipment and unsafe acts. The most common hazards in each category can be
controlled as follows:
a.
b.
c.
d.
e.
f.
g.
h.
i.
Power cords and drop cords with worn and/or broken insulation should be
routinely replaced.
All spliced cords should be removed from service.
Exposed conductors and terminal strips at the rear of switchboards and
equipment racks should be enclosed and warning labels installed.
Rubber mats should be installed on the floor of all enclosures containing
exposed conductors and on the floor in front of high voltage switches.
High voltage switches should be of the enclosed safety type.
All wiring should comply with recognized electrical codes and it should be
large enough for the current being carried.
Temporary wiring should be removed as soon as it has served its purpose.
The noncurrent-carrying metal parts of equipment and power tools should be
grounded.
The main power switch to all circuits being worked on should be locked open
and tagged.
25
j.
k.
l.
Power switches should be opened before replacing fuses and fuse pullers
should be used.
Fuse boxes should be locked to prevent bridging or replacing with a heavier
fuse.
Care should be taken to prevent overloading of circuits.
12.5.1.2 Exposed Metal or Other Conductive Parts Design and construction of
the equipment should be such that all exposed parts or panels of metal or other
electrically conductive material are at ground potential at all times. Exposed metal
portions of electrical parts (switches, and so forth) or other parts located near electrical
circuits (including parts inside enclosures where access is required for operation
or adjustment) should be in intimate physical contact with the frame of the equipment or
electrically connected to the frame if these parts could touch the electrical circuits as a
result of deformation, wear, insulation failure, and so forth.
12.6 Electrical Creepage and Clearance Distances Wire and cable harness assemblies
should be designed and/or installed in their next higher assemblies in a manner that
precludes direct electrical short-circuits, or otherwise an electrical breakdown due to
insufficient spacing between non-common conductors, or via insufficient spacing along
insulating materials. This is assured by specifying a minimum electrical creepage and
clearance distance.
Electrical creepage and clearance distances are defined as follows:
(a) Clearance distance is the shortest point-to-point distance in air between un-insulated
energized parts or between an energized part and ground.
(b) Creepage distance is the shortest distance between energized parts, or between an uninsulated energized part and ground, along the surface of an insulating material. When
necessary, insulating barriers may be used to interrupt continuous electrical creepage
paths. Cemented or butted joints should not be accepted as techniques to obtain the
minimum creepage distances in Table 12-4.
Creepage and clearance distances between electrical circuits, between each electrical
circuit and ground, and across lines and between circuit elements that operate at
significantly different potential levels within each circuit should be not less than those
values shown in Table 12-4. It is emphasized that the values shown in Table 12-4
represent the minimum acceptable limits for non-arcing rigid construction based on
normal volt-ampere (product of the normal voltage applied to the circuit times the current
carried) ratings and that they take into consideration only the average degree of enclosure
and service exposure. Therefore, the designer should employ creepage and clearance
distances in excess of these minimums where-it is probable that structural features,
contaminants, lack of maintenance, environment, exposure or application overstress will
create service conditions more severe than normal.
26
12.6.1 Distance From Enclosure Exposed non-arcing current-carrying parts within
enclosures should have an air space between them and the un-insulated part of the
enclosure of not less than 0.75 inch. However, the values shown in Table 12-4 may be
applied to the creepage and clearance distances between un-insulated parts of enclosures
and exposed non-arcing current-carrying parts of devices whose mounting is sufficiently
rigid and so designed to prevent decrease of the clearance distance through a blow on, or
distortion of the enclosure.
Table 12 -4 Electrical Creepage and Clearance Distance (Note 1)
Voltage
(ac or dc)
Set (Note 2)
Clearance (Inches)
Up to 64
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
C
C
1/16
1/8
1/8
1/16
1/8
1/4
1/16
1/8
1/4
1/16
1/8
1/4
1/8
1/4
1/2
2
3
Over 64-150
Over 150-300
Over 300-600
Over 600-1000
Over 1000-3000
Over 3000-5000
Creepage (Note 3)
Open
Enclosed (Inches)
(Inches)
(Note 5)
(Note 4)
1/16
1/16
1/8
1/8
3/8
1/2
1/16
1/16
¼
1/8
¾
3/8
1/16
1/16
¼
1/8
¾
1/2
1/8
1/8
¼
1/4
¾
1/2
½
3/8
1
3/4
2
1.5
4
2
5
3
Note 1: Use of electrical parts or assemblies such as potentiometers, connectors, printed
wiring assemblies, and :similar devices having lesser creepage and clearance distances is
permissible provided these parts and assemblies conform with applicable specifications,
and their energized portions are enclosed to protect against entry of dust and moisture.
For example, wire connections to a power semiconductor may require additional
protection since the clearance distance between the semiconductor terminals may be less
than listed in Table 12-4.
Note 2: Set A - Normal operating volt-ampere rating up to 50.
Set B - Normal operating volt-ampere rating of 50 to 2000.
Set C - Normal operating volt-ampere rating over 2000.
Note 3: For top curved surfaces having a radius greater than 3 inches and for top flat
surfaces, surface creepage distance should be increased 33 percent where these surfaces
have irregularities which permit the accumulation of dust and moisture.
Note 4: Open is defined as equipment or parts with open enclosures in accordance with
MIL-STD-108.
27
Note 5: Enclosed is defined as equipment or parts with enclosures in accordance with
MIL-STD-108, except open enclosures.
12.7 Accessibility All wire and cable harness assemblies, as installed, which may require
servicing or replacement during the life of the equipment, should be readily accessible for
such actions without major disassembly of the equipment and without removing the
equipment from its foundation. Access to wire and cable harness assemblies should be
from the front of the enclosure.
12.8 Maintenance and Repair Cable and wire harness assemblies, and installation into
the next higher assembly, should be designed for ease of maintenance and repair
Whenever possible, the design should be capable of being repaired either by replacement
of defective individual parts or by utilizing commonly available bulk materials (wire,
varnish, insulation, etc.).
12.9 Smoke and Fire Hazards Wire, cable and associated installation components,
should be selected and installed in such a manner to minimize the danger of smoke and
fire hazards. Adequate protective means, both physical and electrical, should be
employed to provide reliability and safety commensurate with this expectation.
12.10 Cable and Wire Harness Installation Design of wire and cable installation
should conform to the following order of precedence:
(a)
(b)
(c)
Safety
Cost
Ease of maintenance, removal and replacement of cable and wire.
Cable and wire harnesses should be fabricated and installed to achieve the following:
(a)
(b)
(c)
Maximum reliability.
Minimum interference and coupling between systems.
Prevention of damage.
12.10.1 Arrangement and Harnessing Wiring should be neatly formed into groups
which are locked, sleeved, tied, or clamped in a manner that provides support and
prevents chafing of the wire insulation due to vibration and shock.
There should be no splices in the wire (unless such splices have been specifically
authorized for repairs), and all connections should be made at the terminals of the
devices, at terminal blocks, or at part mounting boards.
Wire groups running from hinged panels and doors should be flexible and covered with
protective material, secured in place, as appropriate.
28
Finished harness diameter should not restrict flexibility requirements where necessary.
The use of preformed cables and wiring harnesses is preferred to the point-to-point
method of wiring.
Conductors combined into a harness should be securely held together by means of lacing,
ties, or clamps, or be permanently mounted in cabling ducts. Individual conductors which
are thus combined should lie parallel to one another and should not entwine other
conductors. This is not intended to preclude the use of twisted pairs or triads where
required for electrical reasons.
The combined heating of bundled wires or proximity heating by components
should not cause maximum temperatures of harnessed wire insulation to be exceeded.
12.10.2 Bundle and Group Size As a design objective, bundles and groups within
clamps should be no more than 2 inches in diameter. Wiring to high density connectors
may be run as a single group, provided all of the wiring in the group is pertinent to a
single item, equipment or system.
The number of electrical wires in high density harnesses should be limited only by
efficient and good design. The use of wire sizes larger than 16 is discouraged unless
there are also smaller electrical wires in the same harness.
10.10.3 Dead Ending Each un-designated wire end should be dead ended with a suitable
protective cap such as an AS25274 crimp cap, or other suitable means of protection.
10.10.4 Splicing Splicing of conductors should be avoided and the use of a splice
connection may require user approval. If splicing is allowed, the splice should be made
using standard crimp and/or solder splices made for this purpose. It is recommended that
each lot of splice connections have a sample pull test performed to verify the integrity of
the splice connection.
12.10.5 Routing In addition to 12.10.1 above, wires and cables should be routed to
ensure reliability and to offer protection from the following hazards:
(a)
Chafing – Chafing is defined as repeated relative motion between wiring
system components, or between a wiring system component and structure of
equipment, which results in a rubbing action that causes wear which will
likely result in mechanical or electrical failure during the equipments specified
service life.
(b)
Use as handhold or as support for personal equipment.
(c)
Damage by personnel during use or storage.
(d)
Damage by anticipated environmental conditions, including any harsh
environmental conditions (moisture, high temperatures, fluids, etc.).
29
(e)
Damage by moving parts (e.g., cooling fans, etc.).
(f)
Crushing, kinking or stretching fiber optic cable. These cables should be
routed to avoid the application of axial and lateral loads to the cable end
terminations. When not being used, the fiber optic connectors should be
covered with a temporary protective cap to preclude damage to the
termination.
(g)
Routing through the equipment mounting base should be avoided unless
specifically required (e.g., such as when using “stuffing tubes” to facilitate
wire and cable entry from the bottom of an enclosure).
(h)
Fire hazard may require special wire or cable bundle protection, or otherwise
routing in a separate bundle such that a fire in a single bundle will not impact
wires or cables in a redundant circuit.
12.10.6 Stress Relief and Mechanical Support Wire and cable assemblies should be
routed such that they are not under tensile or compressive stress in order to prevent
damage to the terminations under a shock or vibration environment, or when the wire and
cables connect between movable sections of the equipment.
Electrical connections should be designed and provided with supports to prevent
breakage and minimize changes in performance due to vibration, inclination, or shock.
Where electrical connections are constructed of members in firm contact, such as parts
held together by bolts, the contacts should not depend on force transmitted through
plastic spacers or other deformable parts. Only metal parts should be so employed and
these electrical connections should not rely on the clamping screw, bolt or fastener
threads to carry current; however, stud type semiconductor devices may be mounted
separated from their heat sinks or other mounting surfaces by insulators when direct
metallic contact is incompatible with the circuit requirements, provided the method of
mounting conforms to the device manufacturer’s recommendations.
12.10.7. Slack in Cable and Wiring In addition to slack provided for drip loops
(See 12.10.11), slack should also be provided as noted below. In production, wire harness
fabrication provisions should be incorporated into the harness design and fabrication
process to ensure that the installed harness meets the criteria without the need for
straining, forcing or modifying the harness.
Slack should be provided so as not to impair movement or put undue stresses on the wires
or parts in those places where movement of parts may be expected. Slack should also be
provided to prevent undue-stresses on terminal connections due to shock or vibration.
Where soldering is used to connect hook-up wires to the terminals of replaceable parts, sufficient slack should be provided for at least one replacement of the part in the event
that the wires are damaged or have to be clipped at the terminals during disassembly.
30
Where solder-less lug terminals are used, sufficient slack should be provided for one
replacement of the terminals on 14 AWG and smaller wires.
12.10.7.1 Connector Termination When wiring is terminated in a connector or terminal
junction, a minimum of 0.5 inch of slack for complete connector replacement should be
provided. This slack should be between the connector and the second wiring support
clamp. The 0.5 inch slack requirement is interpreted to mean that with the connector
unmated and the first wiring support clamp loosened, the wiring will permit the front end
of the connector shell to extend 0.5 inch beyond the point normally required to properly
mate the connector. Slack for replacement of potted connectors should be, as a minimum,
the length of the potting plus one inch. Connectors terminating size 8 and larger
electrical wires, RF cables, and fiber optics cables are normally not subjected to retermination slack requirements.
12.10.7.2 Lug Termination At each end of a wire terminated by a lug, a minimum
length of slack equal to twice the barrel length of the lug should be provided. For copper
wire, size 2 and larger, and aluminum wire, size 4 and larger, the minimum length of
slack should be equal to one barrel length of the lug. The slack should be in the vicinity
of the lug and available for replacement of the lug by maintenance personnel.
12.10.7.3 Strain Protection The wiring and cable installation should be designed to
prevent strain on wires, junctions and supports.
12.10.7.4 Free Movement The wiring and cable installation should permit free
movement of shock and vibration mounted equipment.
12.10.7.5 Cable and Wire Shifting The cable and wire installation should permit
shifting of the wire, cable, and equipment to perform maintenance.
12.10.8 Inspection and Maintenance In open wiring, groups should be installed to
permit replacement of the group without removal of the bundle. High density harnesses
should be designed so that they are readily replaceable in sections.
Fiber optic cables should be installed so they are accessible for periodic inspection, and
replacement, if needed, without the need to disassembly any riveted or bonded
attachments.
Wires or cables should not be routed such that they impinge on, or otherwise impede
component safety provisions (e.g., do not block the pressure-relief plug on power can
type capacitors, etc.).
12.10.9 Protection and Support Wiring and cable should be supported to;
(a)
(b)
Prevent chafing as previously defined.
Secure wiring and cable where routed through bulkheads and structural
31
(c)
(d)
(e)
(f)
(g)
(h)
(i)
members.
Properly group, support and route wiring and cables in junction boxes,
panels and bundles.
Prevent mechanical strain or work hardening that would tend to break the
conductors and connections.
Prevent arcing or overheated wiring from causing damage to mechanical control
cables, and associated moving equipment.
Facilitate reassembly to equipment terminal boards.
Prevent interference between wiring and other equipment.
Provide support for wiring to prevent excessive movement in
areas of high vibration.
Dress the wiring at connectors and terminating devices in the
direction of the run without deformation of grommet seals.
Primary support of wiring and cable should be provided by metal cushion clamps and
plastic clamps spaced at appropriate intervals. In addition, where wiring is routed through
cutouts in any metal structure, clamps should be installed as necessary to preclude
damage, including chafing. Open wiring contained in troughs, ducts or conduits is
exempt from the need for clamps.
Clamps for harnesses other than round should be shaped to fit the contour of the harness
and should provide a snug fit. Plastic clamps should not be used to support rigid portions
of harnesses. Plastic cable straps should not be used as primary supporting devices unless
allowed by the user. The primary support of wiring should not be attached to adjacent
wiring.
Clamps should be of a size that holds the wiring and cable in place without damaging the
insulation or degrading its performance. Wire or cable bundle diameter should be
adjusted (built-up) with a suitable material such as authorized tape or sleeving so the
harness fits snug (but not overly compressed) within the clamp such that the clamp
provides the desired clamping action.
Holes in metal that are not chamfered to prevent wire or cable chafing should be provided
with a protective grommet. The grommet should be installed with the grommet seam
facing away from the cable or wire and should be selected for compatibility with the
maximum expected temperature. Normally these grommets are made from plastic
material requiring an adhesive to secure the grommet, or an epoxy coated spring type
stainless steel grommet that does not require an adhesive (See NASM22529 and
NASM21266 for acceptable types of grommets). Ring grommets per NASM3036 are
also acceptable for use. An epoxy rather than RTV material is recommended for securing
the plastic type grommets.
Harnesses can also be protected using tape suited for the purpose. One such acceptable
tape is silicone based self-adhering tape per A-A-59163.
Terminals may be protected using a terminal nipple specifically made for this purpose.
One such acceptable terminal nipple is silicone conforming to A-A-59178.
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12.10.10 Bend Radius See IPC/WHMA-A-620, paragraph 14.3.2 for recommended
minimum bend radius. However, these values are not conservative for aerospace
application where AS50881 may apply. For aerospace application, the following
minimum bend radius is recommended:
(a)
(b)
(c)
Individual wires, cables and harnesses, the recommended minimum bend
radius is 10 X the diameter of the largest wire or cable in the harness. If wires
used as shield terminators or jumpers are required to reverse direction in the
harness, the minimum bend radius should be three times the diameter at the
point of reversal providing the wire and cable are adequately supported.
For semi rigid coaxial cable the minimum diameter should be ten times the
outside diameter.
For fiber optics cable the minimum bend radius should be in accordance with
the cable manufacturer’s recommendations and be sufficient to avoid
excessive losses or damage to the cable.
12.10.11 Drip Loop When wiring is dressed to a connector, terminal block, panel or
junction box, in addition to the slack provision addressed above, a trap or drip loop
should be provided in the wiring and cable to prevent fluids or condensate from running
into the aforementioned devices. Potted connectors and connectors containing only fiber
optics do not require a drip loop.
12.10.12 Routing Near Moving Parts or Controls Wiring or cables attached to
assemblies where relative movement occurs (such as at hinges and rotating items) should
be installed or protected in a manner to prevent damage or deterioration caused by
flexing, pulling, abrasion and other effects of frequent removal and replacement of
equipment. Whenever possible, it is recommended that bundles be installed to twist
instead of bending across hinges.
Wires and cables that must be located close to operating controls should be installed in a
manner that allows for proper operation of the controls in the event of failure of any
single point of attachment.
12.10.13 Routing near Fluid Lines Wiring and cables should be routed independent and
separate from gas and fluid containing lines whenever possible. When this is not possible,
it is recommended that the routing be above, and at an angle to, rather than parallel to the
lines containing gas or fluids. Terminating devices should not be placed under any lines,
and unless otherwise required, the wiring and cables should not be tied or otherwise
supported by the lines. If wiring is installed in locations where fluids may be trapped and
the wires and cables contaminated, the wires and cables should be properly routed and
protected against fluid damage.
Wiring and cables should not be routed through fuel tanks except where there is no
alternative. It is recommended that if wiring and cables must be routed in a fuel tank,
that the routing be via a separate dry access space to preclude fuel contact.
33
12.10.14 Ground Return The electrical power source ground terminals should be
connected to the primary metallic structure of the equipment using terminal lugs and
threaded fasteners. However, for large size ground wires (AWG-4 or larger diameter), it
is recommended that a separate ground tab sized to handle the ground current and
fastened to the equipment structure by a suitable method be used. The shortest possible
ground wire connection is recommended. Ground return wiring should not be connected
to magnesium. Where EMI is a consideration, see MIL-STD-464 for additional
guidance.
12.10.15 Shielded Wire Grounding Equipment EMI requirements may dictate the need
to shield wiring and cable. Shields should be terminated as close as practicable to
connectors, and specifically within any booted areas of breakout terminations. AS83519
shield terminations is one acceptable method for terminating tin and silver coated shields
except when the operating temperature of the equipment exceeds the part rating. It is
recommended that the un-terminated end of a shield be covered with sleeving or other
means to preclude un-wanted local shield shorts to ground or to each other. In some
applications, shields may need to float at one end since tying both ends of a shielded wire
to ground may cause un-wanted noise signals.
12.10.16 Multiple Grounds The total number of ground connections to any single
ground stud should not be greater than the stud is designed to handle. In some situations,
it may be necessary to separate the grounds depending on the circuit application.
12.10.17 Connectors Connectors should be selected so that contacts on the “live” or
“hot” side of the connection are socket type rather than pin type to minimize personnel
hazard and to prevent accidental shorting of live circuits if the connector is unmated with
power still applied.
12.10.17.1 Environment Resisting Connectors It is recommended that connectors be
sealed against the ingress of water and water vapor under all service conditions including
changes in altitude, humidity and temperature. The connectors should have an interracial
seal as well as sealing at wire ends. Environment resisting connectors having wire sealing
grommets are preferred; however, potting may be used where a grommet seal connector
would not be suitable.
12.10.17.2 Contacts Connectors using removable crimp contacts are preferred to solder
contact types. One acceptable contact is AS39029, or otherwise as specified by the base
connector specification.
Wire size should be within the crimp barrel size range as identified in the contact
specification.
Crimp tools used by the manufacturer should be such as to meet specified performance in
accordance with the applicable connector specification and the crimp connections should
be replaceable in the field using standard crimp tools available in the field.
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12.10.17.3 Spare Contacts In some applications, the user may require a certain number
(e.g., 10%) of spare contacts to be provided within a connector for future growth. These
spare contacts may require sealing grommets be installed if the connector is
environmentally sealed. In some cases, unused contacts may need to be terminated with
a dead-ended pigtail lead that can be connected to in the future without damaging the
connector. In other applications, the connector insert may be provided with unused holes
such that the contacts can be added later if needed. However, if the connector
manufacturer recommends that all un-used insert holes be filled with contacts or
otherwise where this is needed for shock and vibration considerations, then this is
preferred over leaving the holes un-filled.
12.10.17.3 Connector Installation It is recommended that connectors be used to join
harnesses to equipment or other harnesses when frequent disconnection is required to
remove or service equipment, components or wiring. Connectors should be located and
installed so that they will not provide hand holds or foot rests to operating and
maintenance personnel, or be damaged. Fasteners should be used in all holes of flange
mounted connectors. Both plug and receptacle should be visible for engagement and
orientation of polarizing key(s). Mated plugs should not be strained by the attached
wiring.
Connectors used to provide separation of or connections to multiple electric circuits in
the same location should be installed so that it will be impossible to mate the wrong
connector in another mating unit. It is preferred that wiring be routed and supported such
that an improper connection cannot be made.
12.10.17.3.1 Circular Connector Installation Adequate space should be provided for
mating and un-mating connectors without the use of tools. A one inch minimum spacing
for removal of the connectors by hand is recommended.
Unless otherwise specified, circular connectors, when installed with the axis in a
horizontal direction, should be positioned so that the master keyway is located at the top.
When Installed with the axis in a vertical direction the master keyway should be located
forward in relation to the equipment.
12.10.17.3.2 Rectangular Connector Installation Since rectangular connectors are
normally provided with jack screws in lieu of a circular locking ring; much less hand
space is needed for removing the connectors. Therefore, the rectangular connectors can
be installed closer to each other. However, sufficient access for use of connector removal
and re-installation tools is needed.
12.10.17.4 Potting Unless otherwise specified, potting is an acceptable method of
providing sealing or stress relief protection for electrical connectors. The type of potting
material should be compatible with the worst case equipment operating temperature in its
worst case environmental temperature. Potting materials should also be chosen for
compatibility with the various materials they will come in contact with. In some
applications, the user may specifically prohibit potting of connectors. Potting compound
35
should be given a sufficient cure cycle (time and temperature) to preclude out-gassing
after delivery.
12.10.17.5 Safety Wiring Non-self-locking threaded coupled connectors located in areas
of high vibration, and in areas which are normally inaccessible for periodic maintenance
inspection, should have the coupling nut safety-wired or otherwise mechanically locked
to prevent opening of the connector due to vibration.
12.10.17.6 Dust Protection During production manufacturing, un-mated connectors
should be suitably covered with a protective material to preclude the entry of dust. This
is of particular importance for fiber optic connectors. The protective material should
remain installed until the wire or cable harness is installed in its next higher assembly.
Plastic caps may be suitable as a protective material.
12.10.17.7 Connector Accessories Connectors should be provided with strain relief
accessories when necessary to preclude damage to the wire or cable conductors. It is
recommended that these accessories not be used for ground wire connections unless the
accessories were specifically designed for this purpose.
12.10.18 Splices The use of wire or cable splices should be avoided whenever possible,
unless their use has been specifically authorized by the contract documentation. When
splices are allowed for use, the following use criteria is recommended:
a. Limit the total number of splices to one per conductor.
b. Install splices such that they do increase the size of the bundle so as to prevent its
fitting in its designated space or which would adversely affect maintenance.
c. Do not use splices to salvage scrap lengths of wire or cable.
d. Don’t use splices within 12 inches of a termination device except when otherwise
specifically approved.
e. The engineering documentation should identify all allowable use of splices.
f. Splices may be used for repairs when authorized.
g. Splices should not be used for critical circuits such as firing or control circuits
associated with ordnance or explosive sub-systems.
h. Spliced wires in current carrying circuits should be of a size adequately protected by
the circuit protection devices.
Splices installed for assembly or subassemblies should be contained in splice areas
identified as such on the applicable drawings. Splice areas should be selected so that they
are readily accessible for maintenance and inspection including splices contained in the
center of the bundle.
12.10.19 Terminal Lugs Terminal lugs should be used to connect wiring to terminal
board studs, equipment terminal studs and ground studs. The maximum number of
terminal lugs used on any single terminal should not exceed the maximum allowable
number based on the stud design and need to maintain full engagement of the stud
36
fasteners. Normally, this is four total (three lugs and one bus strip connection, or four
lugs and no bus strip connection). When the terminal lugs attached to a stud vary in
diameter, the greatest diameter should be placed on the bottom and the smallest
diameter on top.
Terminal lugs should be selected with a stud hole diameter which matches the diameter
of the stud.
Tightening terminal connections should not deform the terminal lugs or the studs.
Whenever possible, it is recommended that straight terminal lugs not be bent for
installation. However, bending of the lug tongue up to 90 degrees is acceptable as long
as; the bend radius is not less than twice the thickness of the lug tongue; the distance
from the tip of the tongue to the beginning of the bend is not less than the diameter across
the lug; bending is not required to remove the fastening screw or nut; and the bend does
not cause a crack or other damage to the lug.
The position of the terminal lug should be such that movement of the lug will tend to
tighten the fastening screw or nut.
Spacers or washers should normally not be sandwiched between the tongues of terminal
lugs.
12.10.20 Terminal Boards and Terminal Junction Modules Terminal boards or
terminal junction modules should be used for junctions of wiring requiring infrequent
disconnection or for joining two or more wires to a common point.
Terminal boards and their associated terminals should be assigned a reference
designation as shown on the electrical schematic or wiring diagram. For example, TB112A designates terminal board # 1, terminal 12, side A, if the terminal board contains two
rows of terminals. The identification should be marked on the assembly adjacent to the
terminal board such that it can be easily read; left to right, or top to bottom. Removal of
the terminal board should leave the identification intact.
Terminal boards should be secured with machine screws so they can easily be replaced if
needed.
12.10.21 Wiring Mockup Consideration should be given to manufacturing a wiring
mockup when complex harness assemblies are involved. This would facilitate clear
understanding of how the harnesses must be installed within an assembly to establish
necessary clearances between other components, to preclude potential chafing issues, to
establish minimum bend radius for harnesses, and to address periodic inspection for
tightness of electrical connection to terminal boards, and other factors. This mockup
could also be used to determine the most cost effective manufacturing methods and to
facilitate review and buy-in by the user when required.
37
12.10.22 Screw Thread Standards for Fastening Devices Screw threads for all
threaded fastening devices should be in accordance with ANSI B1.1. The threads
should be the coarse-thread series, unified form, class 2A/2B unless the component
design indicates a necessity for the use of the fine thread series.
12.10.22.1 Fastening of Harnesses and Associated Parts Through bolting should be
used wherever practicable. For electrical panels and other applications where frequent
disassembly is required, blind nuts and captive fasteners should be used when practical.
Similarly, these types of fasteners should be used when practical to prevent a loose
fastener from dropping into electrical equipment.
12.10.22.2 Threads in Aluminum Threads in aluminum or aluminum alloys should
be avoided, where practicable, by use of through bolting. Where through bolting
is not practicable, and screws are removed for routine equipment maintenance
or where high stress in the screw is needed for alignment of a vital part, metal inserts for
the fastenings should be cast or screwed into the aluminum or aluminum
alloy.
Inserts should be given a corrosion-resistant treatment, except where
bushing type inserts of corrosion-resisting steel are cast into the aluminum or
aluminum alloy. Inserts need not be provided for securing identification plates,
terminal boards or other items that are removed only when the equipment is
overhauled or modified.
12.10.22.3 Threads in Plastic. Metal inserts should be used where threads in
plastic are used.
12.10.22.4 Inserts Metal inserts, where required in aluminum alloys or plastics, should
be the bushing type, or the helical-coil. See MS22076 through MS122115 and
NASM21209 for examples of the helical-coil inserts.
Care should be exercised when installing helical-coil inserts, since if they are not
properly installed, the inserts can easily back out of the hole when the mating fastener is
removed. These inserts also contain a tang which is normally removed after installation.
Attention should be given to ensure the inserts are installed in the correct orientation and
that non-locking inserts are not inadvertently installed when a locking insert is required.
Visual inspection should confirm that the insert has been installed since one can mistake
a tapped hole as containing an insert when it in fact may not.
The bushing type is recommended. The use of helical-coil type inserts should be limited
to applications where the threaded hole permits full engagement of the insert. Bushing
type inserts should be the cast-in, molded-in, or screwed-in types. Screwed-in types
should be pin, key-, swage-, or ring-locked to prevent backing out.
12.10.22.5 Thread Projection Except for threading into blind holes or in
38
thick material, bolts, and machine screws should be of such length that when
tightened, at least one thread and preferably not more than four threads should
project beyond the outer face of the nut or bolted part. With plastic insert
self-locking nuts, the thread projection should be measured from the crown of the
plastic insert.
12.10.22.6 Bolt and Screw Thread Engagement For materials having similar
mechanical properties, the full thread engagement should be no less than one major
diameter (ID).
For materials having dissimilar mechanical properties, the minimum thread engagement
should be in accordance with FED-STD-H28; part, 1, appendix 5, using the maximum
tensile strength of the stud material and minimum specified tensile strength of the body
material, plus one thread; but in no instance less than the root diameter.
Where helical-coil type threaded inserts are used, the length of the thread engagement
should be not less than 1-1/2 times the major diameter (nominal) of the bolt thread.
12.10.22.7 Thread Locking of Mechanical Assemblies Bolts, nuts, and screws,
used for mechanical connections, where the specified operation under all
anticipated conditions, including shock, vibration, and heating, depends upon
maintaining tight connection of parts, or where a holding screw, bolt, nut, or
fastened part may fall into the equipment, should be secured by one of the
following means:
(a) Lock Washer.
(b) Lock Nut.
(c) Castellated nut with cotter pin or safety wiring.
(d) Self-locking screws. This method should only be used where removal
for maintenance is very infrequent.
(e) Deformation of screw or bolt threads projecting from nut or secured,
part. This method should only be used in cases where disassembly is
never required for maintenance or repair.
(f) Locking wire for use to lock bolts when only bolt heads are
available to apply a locking device.
(g) Self-locking nut.
(h) Blind nuts and captive fasteners.
12.10.22.8 Flexible Wiring Where flexible wiring is required by hinged doors, panels or
sliding, subassemblies, abrasion and chafing should be minimized by use of flexible
plastic sheaths on wiring.
Wire groups running from hinged panels or doors should be formed and clamped so that
sharp bends do not occur with the panel or door in either the open or closed position; and,
if more than three wires are contained in the group and the panel or door is required to be
39
removable, a terminal block (or the receptacle portion of a multi-pin connector, where
permitted) mounted on a stationary part of the structure within the enclosure or on
& the hinged panel or door should be used for connections. Flexible harnesses should
be broken down into individual bundles.
12.10.22.9 Wire Connections and Terminals The ends of each conductor (except for
conductors requiring solder connections to a terminal or stud) should be connected to
terminals on the part or to terminal boards by means of solder-less lug terminals, or by
forming the conductor around a part terminal and retaining the-loop in a cup or crimped
washer.
If a wire loop is used, strands of the conductor should be secured together by soldering.
No more than three connections (or three connections with a bus strip) should be made to
each terminal unless the terminal is specifically designed to accommodate additional
conductors.
Pins or conductors should not be paralleled for the purpose of increasing current capacity
except where capacity above 220 amperes is required or where specifically allowed by
the user.
Nuts, bolts, studs, and screws used for electrical connections should be secured by lock
washers, except lock washers may not be required when certain types of terminal boards,
having a barrel-nut locking capability are used.
External tooth flat lock washers are recommended for electrical connections, where
practical.
12.10.22.10 Spare Terminals Terminal boards or cable connectors should have not less
than 10 percent unused terminals when used for connections in the equipment and when
used for the connection of assemblies with enclosures. There should be not less than two
such terminals, except that no spares are required where a total of six, or less, active
terminals are involved. Spare terminals in connectors should be in the outermost row of
terminals.
Where connectors or terminal boards are used only for primary power connections, no
spare terminals need be provided. If more than one terminal board or connector is needed
at a common place, only 10 percent of the total number of terminals at this place are
required as spare terminals.
12.10.22.11 Manufacturing Processes Consult the applicable sections of this handbook
and/or IPC/WHMA-A-620 for manufacturing processes such as crimping, soldering,
weld connections, wire wrap connections, wire marking, etc.
12.11 Bonding As used herein, bonding refers to the process by which a low impedance
path for the flow of an electric current is established between two metallic objects. Other
40
types of bonding which involve simply the physical attachment of one substance or
object to another through various mechanical or chemical means are not discussed herein.
12.11.1 Purposes of Bonding In any realistic electronic system, whether it be only one
piece of equipment or an entire facility, numerous interconnections between metallic
objects are made in order to provide electric power, minimize electric shock hazards,
provide lightning protection, establish references for electronic signals, etc. Ideally, each
of these interconnections should be made so that the mechanical and electrical properties
of the path are determined by the connected members and not by the interconnection
junction. Further, the joint should maintain its properties over an extended period of time
in order to prevent progressive degradation of the degree of performance initially
established by the interconnection. Bonding is concerned with those techniques and
procedures necessary to achieve a mechanically strong, low impedance interconnection
between metal objects and to prevent the path thus established from subsequent
deterioration through corrosion or mechanical looseness.
In terms of the results to be achieved, bonding is necessary for the:
a. protection of equipment and personnel from the hazards of lightning discharges,
b. establishment of fault current return paths,
c. establishment of homogeneous and stable paths for signal currents,
d. minimization of rf potentials on enclosures and housings,
e. protection of personnel from shock hazards arising from accidental power grounds, and
f. prevention of static charge accumulation.
With proper design and implementation, bonds minimize differences in potential between
points within the fault protection, signal reference, shielding, and lightning protection
networks of an electronic system. Poor bonds, however, lead to a variety of hazardous
and interference-producing situations. For example, loose connections in ac power lines
can produce unacceptable voltage drops at the load, and the heat generated by the load
current through the increased resistance of the poor joint can be sufficient to damage the
insulation of the wires which may produce a power line fault or develop a fire hazard or
both. Loose or high impedance joints in signal lines are particularly annoying because of
intermittent signal behavior such as decreases in signal amplitude, increases in noise
level, or both. Poor joints in lightning protection networks can be particularly dangerous.
The high current of a lightning discharge may generate several thousand volts across a
poor joint. Arcs produced thereby present both a fire and explosion hazard and may
possibly be a source of interference to equipment. The additional voltage developed
across the joint also increases the likelihood of flashover occurring to objects in the
vicinity of the discharge path.
Degradation in system performance from high noise levels is frequently traceable to
poorly bonded joints in circuit returns and signal referencing networks. As noted
previously, the reference network provides low impedance paths for potentially
incompatible signals. Poor connections between elements of the reference network
increase the resistance of the current paths. The voltages developed by the currents
41
flowing through these resistances prevent circuit and equipment signal references from
being at the same reference potential. When such circuits and equipments are
interconnected, the voltage differential represents an unwanted signal within the system.
Bonding is also important to the performance of other interference control measures. For
example, adequate bonding of connector shells to equipment enclosures is essential to the
maintenance of the integrity of cable shields and to the retention of the low loss
transmission properties of the cables. The careful bonding of seams and joints in
electromagnetic shields is essential to the achievement of a high degree of shielding
effectiveness. Interference reduction components and devices also must be well bonded
for optimum performance. Consider a typical power line filter like that shown in Figure
12-1. If the return side of the filter (usually the housing) is inadequately bonded to the
ground reference plane (typically the equipment case or rack), the bond impedance
ZB may be high enough to impair the filter’s performance. The filter as shown is a low
pass filter intended to remove high frequency interference components from the power
lines of equipment. The filter achieves its goal in part by the fact that the reactance, Xc,
of the shunt capacitors is low at the frequency of the interference. Interfering signals
present on the ac line are shunted to ground along Path 1 and thus do not reach the load.
If ZB is high relative to Xc, however, interference currents will follow Path 2 to the load
and the effectiveness of the filter is compromised.
Figure 12-1 Effects of Poor Bonding on the Performance of a Power Line Filter
If a joint in a current path is not securely made or works loose through vibration, it can
behave like a set of intermittent contacts. Even if the current through the joint is at dc or
at the ac power frequency, the sparking which occurs may generate interference signals
with frequency components up to several hundred megahertz.
Poor bonds in the presence of high level rf fields, such as those in the immediate vicinity
of high powered transmitters, can produce a particularly troublesome type of interference.
42
Poorly bonded joints have been shown to generate cross modulation and other mix
products when irradiated by two or more high level signals. Some metal oxides are
semiconductors and behave as nonlinear devices to provide the mixing action between
the incident signals. Interference thus generated can couple into nearby susceptible
equipment.
12.11.2 Resistance Criteria A primary requirement for effective bonding is that a low
resistance path be established between the two joined objects. The resistance of this path
must remain low with use and with time. The limiting value of resistance at a particular
junction is a function of the current (actual or anticipated) through the path. For example,
where the bond serves only to prevent static charge buildup, a very high resistance, i.e.,
50 kilohms or higher, is acceptable. Where lightning discharge or heavy fault currents are
involved, the path resistance must be very low to minimize heating effects.
Noise minimization requires that path resistances of less than 50 milliohms be achieved.
However, noise control rarely ever requires resistances as low as those necessary for fault
and lightning currents. Bond resistance based strictly on noise minimization requires
information on what magnitude of voltage constitutes an interference threat and the
magnitude of the current through the junction. These two factors will be different for
every situation.
A bonding resistance of 1 milliohm is considered to indicate that a high quality junction
has been achieved. Experience shows that 1 milliohm can be reasonably achieved if
surfaces are properly cleaned and adequate pressure is maintained between the mating
surfaces. A much lower resistance could provide greater protection against very high
currents but could be more difficult to achieve at many common types of bonds such as at
connector shells, between pipe sections, etc. However, there is little need to strive for a
junction resistance that is appreciably less than the intrinsic resistance of the conductors
being joined.
Higher values of resistance tend to relax the bond preparation and assembly
requirements. These requirements should be adhered to in the interest of long term
reliability. Thus, the imposition of an achievable, yet low, value of 1 milliohm bond
resistance ensures that impurities are removed and that sufficient surface contact
area is provided to minimize future degradation due to corrosion.
A similarly low value of resistance between widely separated points on a ground
reference plane or network ensures that all junctions are well made and that reasonably
adequate quantities of conductors are provided throughout the plane or network. In this
way, resistive voltage drops are minimized which helps with noise control. In addition,
the low value of resistance tends to force the use of reasonably sized conductors which
helps minimize path inductance.
It should be recognized that a low dc bond resistance is not a reliable indicator of the
performance of the bond at higher frequencies. Inherent conductor inductance and stray
capacitance, along with the associated standing wave effects and path resonances, will
43
determine the impedance of the bond. Thus, in rf bonds these factors should be
considered along with the dc resistance.
12.11.3 Direct Bonds Direct bonding is the establishment of the desired electrical path
between the interconnected members without the use of an auxiliary conductor. Specific
portions of the surface areas of the members are placed in direct contact. Electrical
continuity is obtained by establishing a fused metal bridge across the junction by
welding, brazing, or soldering or by maintaining a high pressure contact between the
mating surfaces with bolts, rivets, or clamps. Examples of direct bonds are the splices
between bus bar sections, the connections between lightning down conductors and the
earth electrode subsystem, the mating of equipment front panels to equipment racks, and
the mounting of connector shells to equipment panels.
Properly constructed direct bonds exhibit a low dc resistance and provide an rf
impedance as low as the configuration of the bond members will permit. Direct bonding
is always preferred; however, it can be used only when the two members can be
connected together and can remain so without relative movement. The establishment of
electrical continuity across joints, seams, hinges, or fixed objects that must be spatially
separated requires indirect bonding with straps, jumpers, or other auxiliary conductors.
Current flow through two configurations of a direct bond is illustrated in Figure 12-2.
The resistance, Rc, of the path through the conductors on either side of the bond is given
by
Rc = r l/A
Where r is the resistivity of the conductor materials, l is the total path length of the
current through conductors, and A is the cross-sectional area of the conductors (assumed
equal). Any bond resistance at the junction will increase the total path resistance.
Therefore, the objective in bonding is to reduce the bond resistance to a value negligible
in comparison to the conductor resistance so that the total path resistance is primarily
determined by the resistance of the conductors.
Metal flow processes such as welding, brazing, and silver soldering provide the lowest
values of bond resistance. With such processes, the resistance of the joint is determined
by the resistivity of the weld or filler metal which can approach that of the metals being
joined. The bond members are raised to temperatures sufficient to form a continuous
metal bridge across the junction.
For reasons of economy, future accessibility, or functional requirements, metal flow
processes are not always the most appropriate bonding techniques. It may then be more
appropriate to bring the mating surfaces together under high pressure. Auxiliary fasteners
such as bolts, screws, rivets or clamps are employed to apply and maintain the pressure
on the surfaces. The resistance of these bonds is determined by the kinds of metals
involved, the surface conditions within the bond area, the contact pressure at the surfaces,
and the cross-sectional area of the mating surfaces.
44
IPC ACTION TO CREATE FIGURE 12-2 FROM MIL-HDBK-419A, FIGURE 7-2,
PAGE 7-5 AND INSERT IT HERE
12.11.3.1 Contact Resistance No metallic surface is perfectly smooth. In fact, surfaces
consist of many peaks and valleys. Even the smoothest commercial surfaces exhibit an
RMS roughness of 0.5 to 1 millionth of an inch; the roughness of most electrical bonding
surfaces will be several orders of magnitude greater. When two such surfaces are placed
in contact, they touch only at the tips of the peaks - so called asperities. Thus the actual
area of contact for current flow is much smaller than the apparent area of metallic
contact.
An exaggerated side view of the actual contact surfaces at a bond interface is shown in
Figure 12-3. Theoretically, two infinitely hard surfaces would touch at only three
asperities. Typically, however, under pressure, elastic deformation and plasticity allows
other asperities to come into contact. Current passes between the surfaces only at those
points where the asperities have been crushed and deformed to establish true metal
contact. The actual area of electrical contact is equal to the sum of the individual areas
of contacting asperities. This actual area of contact can be as little as one millionth of the
apparent (gross surface) contact area.
IPC ACTION TO CREATE FIGURE 12-3 FROM MIL-HDBK-419A, FIGURE 7-3,
PAGE 7-6 AND INSERT IT HERE
12.11.3.1.1 Surface Contaminants Surface films will be present on practically every
bond surface. The more active metals such as iron and aluminum readily oxidize to form
surface films while the noble metals such as gold, silver, and nickel are less affected by
oxide films. Of all metals, gold is the least affected by oxide films. Although silver does
not oxidize severely, silver sulfide forms readily in the presence of sulfur compounds.
If the surface films are much softer than the contact material, they can be squeezed from
between the asperities to establish a quasi-metallic contact. Harder films, however, may
support all or part of the applied load, thus reducing or eliminating the conductive contact
area. If such films are present on the bond surfaces, they should be removed through
some thermal, mechanical, or chemical means before joining the bond members. Even
when metal flow processes are used in bonding, these surface films should be removed or
penetrated to permit a homogeneous metal path to be established.
Foreign particulate matter on the bond surfaces will further impair bonding. Dirt and
other solid matter such as high resistance metal particles or residue from abrasives can act
as stops to prevent metallic contact. Therefore, all such materials must be thoroughly
removed from the surfaces prior to joining the bond members.
12.11.3.1.2 Surface Hardness The hardness of the bond surfaces also affects the contact
resistance. Under a given load, the asperities of softer metals will undergo greater plastic
deformation and establish greater metallic contact. Likewise, at a junction between a soft
45
and a hard material, the softer material will tend to conform to the surface contours of the
harder material and will provide a lower resistance contact than would be afforded by two
hard materials. Table 12-5 shows how the resistance of 6.45 square cm (1 square inch)
bonds varies with the type of metals being joined.
Table 12-5 DC Resistance of Direct Bonds Between Selected Metals
Bond Composition
Resistance (Micro-ohms)
Brass-Brass
6
Aluminum-Aluminum
25
Brass-Aluminum
50
Brass-Steel
150
Aluminum-Steel
300
Steel-Steel
1500
Notes: Bond Area: 1 in2(6.45 cm2) Fastener Torque: 100 in-lb
12.11.3.1.3 Contact Pressure The influence of mechanical load on bond resistance is
illustrated by Figure 12-4. This figure shows the resistance variation of a 6.45 square cm
(1 square inch) bond held in place with a 1/4-20 steel bolt as a function of the torque
applied to the bolt. The resistance variation for brass is lowest due to its relative softness
and the absence of insulating oxide films. Even though aluminum is relatively soft, the
insulating properties of aluminum oxide cause the bond resistance to be highly dependent
upon fastener torque up to approximately 40 in. -lb torque (which corresponds to a
contact pressure of about 1200 psi). Steel, being harder and also susceptible to oxide
formations, exhibits a resistance that is dependent upon load below 80 in.-lb or about
1500 psi (for mild steel). Above these pressures, no significant improvement in contact
resistance can be expected.
IPC ACTION TO CREATE FIGURE 12-4 FROM MIL-HDBK-419A, FIGURE 7-4,
PAGE 7-9 AND INSERT IT HERE
12.11.3.1.4 Bond Area Smaller bond areas with the same loadings would produce higher
contact pressure which would decrease the resistance. However, as shown in Figure 12-4,
an increase in pressure over 1500 psi for steel and 1200 psi for aluminum produces
relatively slight changes in bond resistance. Further, the improvement in resistance due to
increased pressure is offset by the smaller overall bond area. In a similar fashion, a larger
bond area (with no change in fastener size) under the same torque results in a lowered
pressure at the bond surfaces. The reduced pressure would be counterbalanced to some
extent by the increased bond area, but the net effect can be expected to be an increase in
bond resistance. Thus, when larger bond areas are used, larger bolts at correspondingly
higher torques should be used for fastening.
Bond mating surfaces with areas as large as practical are desirable for several reasons.
Large surface areas maximize the cross-sectional area of the path for current and
correspondingly maximizes the total number of true metallic contacts between the
surfaces. In addition to the obvious advantage of decreased bond resistance, the current
crowding which can occur during power fault conditions or under a severe lightning
46
discharge is lessened. Such current crowding produces a higher effective bond resistance
than is present during low current flow. The increased bond resistance raises the voltage
drop across the junction to even higher values and adds to the heat generated at the
junction by the heavy current flow. Large bond areas not only lessen the factors which
contribute to heat generation, they also distribute the heat over a larger metallic area
which facilitates its removal. A further advantage of a large bond is that it will probably
provide greater mechanical strength and will be less susceptible to long term erosion by
corrosive products because only a small portion of the total bond area is exposed to the
environment.
12.11.4 Direct Bonding Techniques Direct bonds may be either permanent or semipermanent in nature. Permanent bonds may be defined as those intended to remain in
place for the expected life of the installation and not required to be disassembled for
inspection, maintenance, or system modifications. Joints which are inaccessible by virtue
of their location should be permanently bonded and appropriate steps taken to protect the
bond against deterioration.
Many bonded junctions must retain the capability of being disconnected without
destroying or significantly altering the bonded members. Junctions which should not be
permanently bonded include those which may be broken for system modifications, for
network noise measurements, for resistance measurements, and for other related reasons.
In addition, many joints cannot be permanently bonded for cost reasons. Not
permanently joined bonds are defined as semi-permanent bonds. Semi-permanent bonds
include bolts, screws, rivets, clamps and other auxiliary devices for fasteners.
12.11.4.1 Welding In terms of electrical performance, welding is the ideal method of
bonding. The intense heat (in excess of 4000° F) involved is sufficient to boil away
contaminating films and foreign substances. A continuous metallic bridge is formed
across the joint: the conductivity of this bridge typically approximates that of the bond
members. The net resistance of the bond is essentially zero because the bridge is very
short relative to the length of the bond members. The mechanical strength of the bond is
high: the strength of a welded bond can approach or exceed the strength of the bond
members themselves. Since no moisture or contaminants can penetrate the weld, bond
corrosion is minimized. The erosion rate of the metallic bridge should be comparable
to that of the base members; therefore, the lifetime of the bond should be as great as that
of the bond members.
Welds should be utilized whenever practical for permanently joined bonds. Although
welding may be a more expensive method of bonding, the reliability of the joint makes it
very attractive for bonds which will be inaccessible once construction is completed. Most
metals which will be encountered in normal construction can be welded with one of the
standard welding techniques such as gas, electric are, Heliare and exothermic.
Conventional welding should be performed only by appropriately trained and qualified
personnel. Consequently, increased labor costs can be expected. In many instances, also,
the welding of bonds can be much slower than the installation of fasteners such as bolts
47
or rivets. In such cases, the added costs of welding may force the use of alternate bonding
techniques.
An effective welding technique for many bonding applications is the exothermic
mixture of aluminum, copper oxide, and other powders is held in place around the
joint with a graphite mold. The mixture is ignited and the bent generated (in excess of
4000° F) reduces the copper oxide to provide a homogeneous copper blanket around the
junction. Because of the high temperatures involved, copper materials can be bonded to
steel or iron as well as to other copper materials.
This process is advantageous for welding cables together, for welding cables to rods, or
for welding cables to I-beams and other structural members. It is particularly attractive
for the bonding of interconnecting cables to ground rods where the use of conventional
welding techniques might be awkward or where experienced welders are not available.
Because of the cost of the molds (a separate mold is necessary for each different bond
configuration), this process is most economical when there are several bonds of the same
configuration to be made.
When using this process, the manufacturer’s directions should be followed closely. The
mold should be dried or baked out as specified, particularly when the mold has not been
used for several hours and may have absorbed moisture. The metals to be bonded should
be cleaned of dirt and debris and should have the excess water dried off. Water, dirt and
other foreign materials cause voids in the weld which may weaken it or may prevent a
low resistance joint from being achieved. The mold size should match the cable or
conductor cross sections; otherwise, the molten metal will not be confined to the bond
region.
12.11.4.2 Brazing Brazing, to include silver soldering, is another metal flow process for
permanent bonding. In brazing, surfaces are heated to a temperature above 800° F but
below the melting point of the bond members. Metal with an appropriate flux is applied
to the heated members which wets the bond surfaces to provide intimate contact between
the brazing solder and the bond surfaces. As with higher temperature welds, the
resistance of the brazed joint is essentially zero. However, since brazing frequently
involves the use of metal different from the primary bond members, additional
precautions must be taken to protect the bond from deterioration through corrosion.
12.11.4.3 Soft Solder Soft soldering is an attractive metal flow bonding process because
of the ease with which it can be applied. Relatively low temperatures are involved and it
can be readily employed with several of the high conductivity metals such as copper, tin
and cadmium. With appropriate fluxes, aluminum and other metals can be soldered.
Properly applied to compatible materials, the bond provided by solder is nearly as low in
resistance as one formed by welding or brazing. Because of its low melting point,
however, soft solder should not be used as the primary bonding material where high
currents may be present. For this reason, soldered connections are not permitted by the
National Electrical Code, and some military specifications, in grounding circuits for fault
protection. Similarly, soft solder is not permitted for interconnections between elements
48
of lightning protection networks by either the Military Standard, the National Fire
Protection Association’s Lightning Protection Code or the Underwriter's Master Labeled
System. In addition to its temperature limitation, soft solder exhibits low
mechanical strength and tends to crystallize if the bond members move while the solder is
cooling. Therefore, soft solder should not be used if the joint must withstand mechanical
loading. The tendency toward crystallization should also be recognized and proper
precautions observed when applying soft solder.
Soft solder can be used effectively in a number of ways, however. For example, it can be
used to tin surfaces prior to assembly to assist in corrosion control. Soft solder can be
used effectively for the bonding of seams in shields and for the joining of circuit
components together and to the signal reference subsystem associated with the circuit.
Soft solder is often combined with mechanical fasteners in sweated joints. By heating the
joint hot enough to melt the solder, a low resistance filler metal is provided which
augments the path established by the other fasteners; in addition, the solder provides a
barrier to keep moisture and contaminants from reaching the mating surfaces.
12.11.4.4 Bolts In many applications, permanent bonds are not desired. For example,
equipment that must be removed from enclosures or moved to other locations which
require that ground leads and other connections be broken. Often, equipment covers must
be removable to facilitate adjustments and repairs. Under such circumstances, a
permanently joined connection could be highly inconvenient to break and would limit the
operational flexibility of the system. Besides offering greater flexibility, less permanent
bonds may be easier to implement, require less operator training, and require less
specialized tools.
The most common semi-permanent bond is the bolted connection (or one held in place
with machine screws, lag bolts, or other threaded fasteners) because this type bond
provides the flexibility and accessibility that is frequently required. The bolt (or screw)
should serve only as a fastener to provide the necessary force to maintain the 1200-1500
psi pressure required between the contact surfaces for satisfactory bonding. Except for
the fact that metals are generally necessary to provide tensile strength, the fastener does
not have to be conductive. Although the bolt or screw threads may provide an auxiliary
current path through the bond, the primary current path should be established across the
metallic interface. Because of the poor reliability of screw thread bonds, self-tapping
screws should not be used for bonding purposes. Likewise, Tinnernman nuts (a.k.a.,
spring type nut-plates), because of their tendency to vibrate loose, should not be used for
securing screws or bolts intended to perform a bonding function.
The size, number and spacing of the fasteners should be sufficient to establish the
required bonding pressure over the entire joint area. The pressure exerted by a bolt is
concentrated in the immediate vicinity of the bolt head. However, large, stiff washers can
be placed under the bolt head to increase the effective contact area. Because the load is
distributed over a larger area, the tensile load on the bolt should be raised by increasing
the torque. The monograph of Figure 12-5 may be used to calculate the necessary torque
for the size bolts to be used. Where the area of the mating surfaces is so large that
49
unreasonably high bolt torques are required, more than one bolt should be used. For very
large mating areas, rigid backing plates should be used to distribute the force of the bolts
over the entire area.
IPC ACTION TO CREATE FIGURE 12-5 FROM MIL-HDBK-419A, FIGURE 7-7,
PAGE 7-15 AND INSERT IT HERE
12.11.4.5 Rivets Riveted bonds are less desirable than bolted connections or joints
bridged by metal flow processes. Rivets lack the flexibility of bolts without offering the
degree of protection against corrosion of the bond surface that is achieved by welding,
brazing or soldering. The chief advantage of rivets is that they can be rapidly and
uniformly installed with automatic tools.
The bonding path established by a rivet is illustrated in Figure 12-6. The current path
through a rivet is theorized to be through the interface between the bond members and the
rivet body. This theory is justified by experience which shows that the fit between the
rivet and the bond members is more important than the state of the mating surfaces
between the bond members. Therefore, the hole for the rivet must be a size that provides
a close fit to the rivet after installation. The sides of the hole through the bond members
must be free of paint, corrosion products, or other non-conducting material. For riveted
joints in shields, the maximum spacing between rivets is recommended to be
approximately 2 cm (3/4 inch) or less. In relatively thin sheet metal, rivets can cause
bowing of the stock between the rivets as shown by Figure 12-7. In the bowed or warped
regions, metal-to-metal contact may be slight or nonexistent. These open regions allow rf
energy to leak through and can be a major cause of poor rf shield performance. By
spacing the rivets close together, warping and bowing are minimized. For maximum rf
shielding, the seam should be provided with a gasket with some form of wire mesh or
conductive epoxy to supplement the bond path of the rivets.
IPC ACTION TO CREATE FIGURE 12-6 FROM MIL-HDBK-419A, FIGURE 7-8,
PAGE 7-17 AND INSERT IT HERE
IPC ACTION TO CREATE FIGURE 12-7 FROM MIL-HDBK-419A, FIGURE 7-9,
PAGE 7-17 AND INSERT IT HERE
12.11.4.6 Conductive Adhesive Conductive adhesive is a silver-filled, two-component,
thermosetting epoxy resin which when cured produces an electrically conductive
material. It can be used between mating surfaces to provide low resistance bonds. It
offers the advantage of providing a direct bond without the application of heat as is
required by metal flow processes. In many locations, the heat necessary for metal flow
bonding may pose a fire or explosion threat. When used in conjunction with bolts,
conductive adhesive provides an effective metal-like bridge with high corrosion
resistance along with high mechanical strength. In its cured state, the resistance of the
adhesive may increase through time. It also tends to adhere tightly to the mating surfaces
and thus an epoxy-bolt bond is less convenient to disassemble than a simple bolted bond.
50
In some applications, the advantages of conductive adhesive may outweigh this
inconvenience.
12.11.5 Indirect Bonds The preferred method of bonding is to connect the objects
together with no intervening conductor. Unfortunately, operational requirements or
equipment locations often preclude direct bonding. When physical separation is
necessary between the elements of an equipment complex or between the complex and its
reference plane, auxiliary conductors should be incorporated as bonding straps or
jumpers. Such straps are commonly used for the bonding of shock mounted equipment to
the structural ground reference. They are also used for by-passing structural elements,
such as the hinges on distribution box covers or on equipment covers, to eliminate the
wideband noise generated by these elements when illuminated by intense radiated fields
or when carrying high level currents. Bond straps or cables are also used to prevent static
charge buildup and to connect metal objects to lightning down conductors to prevent
flashover.
12.11.5.1 Resistance. The resistance of an indirect bond is equal to the sum of the
intrinsic resistance of the bonding conductor and the resistances of the metal-to-metal
contacts at each end. The resistance of the strap is determined by the resistivity of the
material used and the dimensions of the strap. With typical straps, the dc bond resistance
is small. For example with a resistivity of 1.72 x 10-6 ohm-cm, (6.77 x 10-7 ohm-inches),
a copper conductor 2.5 cm, (1 inch) wide, 40 mils thick, and 0.3 meters (1 foot) long has
a resistance of 0.2 milliohms. To this resistance will be added the sum of the dc
resistances of the direct bonds at the ends of the strap. With aluminum, copper, or brass
straps, these resistances should be less than 0.1 milliohm with properly made
connections. If long straps are required, however, the resistance of the conductor can be
significant.
12.11.5.2 Frequency Effects
12.11.5.2.1 Skin Effect Because high conductivity materials attenuate radio frequencies
rapidly, high frequency currents do not penetrate into conductors very far, i.e., they tend
to stay near the surface. At frequencies where this effect becomes significant the ac
resistance of the bond strap can differ significantly from its dc value.
12.11.5.2.2 Bond Reactance The geometrical configuration of the bonding conductor
and the physical relationship between objects being bonded introduce reactive
components into the impedance of the bond. The strap itself exhibits an inductance
that is related to its dimensions. For a straight, flat strap of nonmagnetic metal, the
inductance in micro henries is given by
L = 0.002l {2.303 log 2l/ (b+c) + 0.5 + 0.2235 (b+c)/l} µH
where l= length in cm,
b = width of the strap in cm
c = thickness of the strap in cm
51
Even at relatively low frequencies, the reactance of the inductive component of the bond
impedance becomes much larger than the resistance. Thus, in the application of bonding
straps, the inductive properties as well as the resistance of the strap must be considered.
The physical size of the bonding strap is important because of its effect on the rf
impedance. As the length of the strap is increased its impedance increases nonlinearly for
a given width; however, as the width increases, there is a nonlinear decrease in strap
impedance. Because of this reduction in reactance, bonding straps which are expected to
provide a path for rf currents are frequently recommended to maintain a length-to-width
ratio of 5 to 1 or less, with a ratio of 3 to 1 preferred.
In many applications, braided straps are preferred over solid straps because they offer
greater flexibility. Because the strands are exposed they are more susceptible to
corrosion; braided straps may be undesirable for use in some locations for these reasons.
Fine braided straps also are generally not recommended because of higher impedances at
the higher frequencies as well as lower current carrying capacities.
12.11.5.2.3 Stray Capacitance A certain amount of stray capacitance is inherently
present between the bonding jumper and the objects being bonded as well as between the
bonded objects themselves.
The combination of the inductance and capacitance can result in a resonant circuit
condition at certain frequencies. These resonances can occur at surprisingly low
frequencies -- as low as 10 to 15 MHz in typical configurations. In the vicinity of these
resonances, bonding path impedances of several hundred ohms are common. Because of
such high impedances, the strap is not effective. In fact, in these high impedance regions,
the bonded system may act as an effective antenna system which increases the pickup of
the same signals which bond straps are intended to reduce. The bond effectiveness
indicates the amount of voltage reduction achieved by the addition of the bonding strap.
Positive values of bonding effectiveness indicate a lowering of the induced voltage. At
frequencies near the network resonances, the induced voltages are higher with the
bonding straps than without the straps.
At low frequencies where the reactance of the strap is low, bonding straps will provide
effective bonding;
At frequencies where parallel resonances exist in the bonding network, straps may
severely enhance the pickup of unwanted signals.
Above the parallel resonant frequency, bonding straps do not contribute to the pickup of
radiated signals either positively or negatively.
In conclusion, bonding straps should be designed and used with care with special note
taken to ensure that unexpected interference conditions are not generated by the use of
such straps.
52
12.11.6 Surface Preparation To achieve an effective and reliable bond, the mating
surfaces should be free of any foreign materials, e.g., dirt, filings, preservatives, etc., and
non-conducting films such as paint, anodizing, and oxides and other metallic films.
Various mechanical and chemical means can be used to remove the different substances
which may be present on the bond surfaces. After cleaning, the bond should be assembled
or joined as soon as possible to minimize recontamination of the surfaces. After
completion of the joining process the bond region should be sealed with appropriate
protective agents to prevent bond deterioration through corrosion of the mating surfaces.
12.11.6.1 Solid Materials Solid material such as dust, dirt, filings, lint, sawdust and
packing materials impede metallic contact by providing mechanical stops between the
surfaces. They can affect the reliability of the connection by fostering corrosion. Dust,
dirt, and lint will absorb moisture and will tend to retain it on the surface. They may even
promote the growth of molds, fungi, and bacteriological organisms which give off
corrosive products. Filings of foreign metals can establish tiny electrolytic cells which
will greatly accelerate the deterioration of the surfaces.
The bond surface should be cleaned of all such solid materials. Mechanical means such
as brushing or wiping are generally sufficient. Care should be exercised to see that all
materials in grooves or crevices are removed. If a source of compressed air is available,
air blasting is an effective technique for removing solid particles if they are dry enough to
be dislodged.
12.11.6.2 Organic Compounds Paints, varnishes, lacquers, and other protective
compounds along with oils, greases and other lubricants are non-conductive and in
general, should be removed. Commercial paint removers can be used effectively.
Lacquer thinner works well with oil-based paints, varnish, and lacquer. If chemical
solvents cannot be used effectively, mechanical removal with scrapers, wire brushes,
power sanders, sandpaper, or blasters should be employed. When using mechanical
techniques, care should be exercised to avoid removing excess material from the surfaces.
Final cleaning should be done with a fine, such as 400-grit, sandpaper or steel wool. After
all of the organic material is removed, abrasive grit or steel wool filaments should be
brushed or blown away. A final wipe down with denatured alcohol, dry cleaning fluid or
lacquer thinner should be accomplished to remove any remaining oil or moisture films.
WARNING
Many paint solvents such as lacquer thinner and acetone are highly flammable and toxic
in nature. They should never be used around open flames and adequate ventilation should
be present. Inhalation of the fumes must be prevented.
Oils, greases, and other petroleum compounds should be wiped with a clean cloth or
scraped off. Residual films should be dissolved away with an appropriate solvent. Hot
soapy water can be used effectively for removing any remaining oil or grease. If water is
used, however, the surfaces must be thoroughly dried before completing the bond. For
small or intricate parts, vapor degreasing is an effective cleaning method. Parts to be
cleaned are exposed to vapors of cleaning solvents until the surfaces reach the
53
temperature of the vapor. In extreme cases, further cleaning by agitation in a bath of dry
chromic acid, 2 lbs per gallon of water, and sulfuric acid, 4 oz per gallon of water, may
be necessary. The average dip time should be restricted to less than 30 seconds because
prolonged submersion of parts in this bath may produce severe etching and cause loss of
dimension. This bath should be followed by a thorough rinse with cold water and then a
hot water rinse to facilitate drying.
12.11.6.3 Plating and Inorganic Finish Many metals are plated or coated with other
metals or are treated to produce surface films to achieve improved wear ability or provide
corrosion resistance. Metal plating such as gold, silver, nickel, cadmium, tin, and
rhodium should have all foreign materials removed by brushing or scraping and all
organic materials removed with an appropriate solvent. Since such plating are usually
very thin, acids and other strong etchants should not be used. Once the foreign substances
are removed, the bond surfaces should be burnished to a bright shiny condition with fine
steel wool or fine grit sandpaper. Care should be exercised to see that excessive metal
is not removed. Finally, the surfaces should be wiped with a cloth dampened in a
denatured alcohol or other appropriate solvent and allowed to dry before completing the
bond.
Chromate coatings such as iridite 14, iridite 18P, oadkite 36, and alodine 1000 offer low
resistance as well as provide corrosion resistance. These coatings should not be removed.
In general, any chromate coatings that have been applied in conformance with the
applicable coating specification should be left in place.
Many aluminum products are anodized for appearance and corrosion resistance. Since
these anodic films are excellent insulators, they should be removed prior to bonding.
Those aluminum parts to be electrically bonded either should not be anodized or the
anodic coating should be removed from the bond area.
12.11.6.4 Corrosion By-Products Oxides, sulfides, sulfates, and other corrosion byproducts should be removed because they restrict or prevent metallic contact. Soft
products such as iron oxide and copper sulfate can be removed with a stiff wire brush,
steel wool, or other abrasives. Removal down to a bright metal finish is generally
adequate. When pitting has occurred, refinishing of the surface by grinding or milling
may be necessary to achieve a smooth, even contact surface. Some sulfides are difficult
to remove mechanically and chemical cleaning and polishing may be necessary. Oxides
of aluminum are clear and thus the appearance of the surface cannot be relied upon as an
indication of the need for cleaning. Although the oxides are hard, they are brittle and
roughening of the surface with a file or coarse abrasive is an effective way to prepare
aluminum surfaces for bonding.
12.11.7 Completion of the Bond After cleaning of the mating surfaces, the bond
members should be assembled or attached as soon as possible. Assembly should be
completed within 30 minutes if at all possible. If more than 2 hours is required between
cleaning and assembly, a temporary protective coating should be applied. Of course, this
coating must also be removed before completing the bond.
54
The bond surfaces should be kept free of moisture before assembly and the completed
bond should be sealed against the entrance of moisture into the mating region. Acceptable
sealants are paint, silicone rubber, grease, and polysulfates. Where paint has been
removed prior to bonding, the completed bond should be repainted to match the original
finish. Excessively thinned paint should be avoided; otherwise, the paint may seep under
the edges of the bonded components and impair the quality of the connection.
Compression bonds between copper conductors or between compatible aluminum alloys
located in readily accessible areas not subject to weather exposure, corrosive fumes, or
excessive dust do not require sealing. This is subject to user agreement.
12.11.8 Bond Corrosion Corrosion is the deterioration of a substance (usually a metal)
because of a reaction with its environment. Most environments are corrosive to some
degree. Those containing salt sprays and industrial contaminants are particularly
destructive. Bonds exposed to these and other environments should be protected to
prevent deterioration of the bonding surfaces to the point where the required low
resistance connection is destroyed.
12.11.8.1 Chemical Basis of Corrosion The basic diagram of the corrosion process for
metals is shown in Figure 12-8. The requirements for this process to take place are that
(1) an anode and a cathode are present to form an electrochemical cell and (2) a complete
path for the flow of direct current exists. These conditions occur readily in many
environments. On the surface of a single piece of metal anodic and cathodic regions are
present because of impurities, grain boundaries and grain orientations, or localized
stresses. These anodic and cathodic regions are in electrical contact through the body of
metal. The presence of an electrolyte or conducting fluid completes the circuit and allows
the current to flow from the anode to the cathode of the cell.
IPC ACTION TO CREATE FIGURE 12-8 FROM MIL-HDBK-419A, FIGURE 717, PAGE 7-30 AND INSERT IT HERE
Anything that prevents the existence of either of the above conditions will prevent
corrosion. For example, in pure water, hydrogen gas will accumulate on the cathode to
provide an insulating blanket to stop current flow. Most water, however, contains
dissolved oxygen which combines with the hydrogen to form additional molecules of
water. The removal of the hydrogen permits corrosion to proceed. This principle of
insulation is employed in the use of paint as a corrosion preventive. Paint prevents
moisture from reaching the metal and thus prevents the necessary electrolytic path from
being established.
12.11.8.1.1 Electrochemical Series The oxidation of metal involves the transfer of
electrons from the metal to the oxidizing agent. In this process of oxidation, an
electromotive force (EMF) is established between the metal and the solution containing
the oxidizing agent. A metal in contact with an oxidizing solution containing its own
metal ions establishes a fixed potential difference with respect to every other metal in the
same condition. The set of potentials determined under a standardized set of conditions,
55
including temperature and ion concentration in the solution, is known as the EMF (or
electrochemical) series. The EMF series (with hydrogen as the referenced potential of 0
volts) for the more common metals is given in Table 12-6. The importance of the EMF
series is that it shows the relative tendencies of metals to corrode. Metals high in the
series react more readily and are thus more prone to corrosion. The series also indicates
the magnitude of the potential established when two metals are coupled to form a cell.
The farther apart the metals are in the series, the higher the voltage between them. The
metal higher in the series will act as the anode and the one lower will act as the cathode.
When the two metals are in contact, loss of metal at the anode will occur through
oxidation to supply the electrons to support current flow. This type of corrosion is
defined as galvanic corrosion. The greater the potential difference of the cell, i.e., the
greater the dissimilarity of the metals the greater the rate of corrosion of the anode.
Table 12-6 Standard Electromotive Series
Metal
Electrode Potential (Volts) (Note: 1)
Magnesium
2.37
Aluminum
1.66
Zinc
0.763
Iron
0.440
Cacmium
0.403
Nickel
0.250
Tin
0.136
Lead
0.126
Copper
-0.337
Silver
-0.799
Palladium
-0.987
Gold
-1.50
Note: 1 Signs of potential are those employed by the American Chemical Society
12.11.8.1.2 Galvanic Series The EMF series is based on metals in their pure state -- free
of oxides and other films -- in contact with a standardized solution. Of greater interest in
practice, however, is the relative ranking of metals in a typical environment with the
effects of surface films included. This ranking is referred to as the galvanic series. The
most commonly referenced galvanic series is listed in Table 12-7. This series is based on
tests performed in sea water and should be used only as an indicator where other
environments are of concern.
Galvanic corrosion in the atmosphere is dependent largely on the type and amount of
moisture present. For example, corrosion will be more severe near the seashore and in
polluted industrial environments than in dry rural settings. Condensate near the seashore
or in industrial environments is more conductive even under equal humidity and
temperature conditions due to increased concentration of sulfur and chlorine compounds,
The higher conductivity means that the rate of corrosion is increased.
56
Table 12-7 Galvanic Series of Common Metals and Alloys in Seawater
(ANODIC OR ACTIVE END)
Magnesium
Magnesium Alloys
Zinc
Galvanized Steel or iron
1100 Aluminum
Cadmium
2024 Aluminum
Mild Steel or Wrought Iron
Cast Iron
Chromium Steel (active)
Ni-Resist (high-Ni cast iron)
18-8 Stainless Steel (active)
18-8 Mo Stainless Steel (active)
Lead-Tin Solders
Lead
Tin
Nickel(active)
Inconel (active)
Hastelloy B
Manganese Bronze
Brasses
Aluminum Bronze
Copper
Silicon Bronze
Monel
Silver Solder
Nickel
Inconel
Chromium Steel
18-8 Stainless Steel
18-8 Mo Stainless Steel
Hastelloy C
Chlorimet 3
Silver
Titanium
Graphite
Gold
Platinum
(CATHODIC OR MOST NOBLE END)
12.11.8.1.3 Relative Area of Anodic Member When joints between dissimilar metals
are unavoidable, the anodic member of the pair should be the larger of the two. For a
given current flow in a galvanic cell, the current density is greater for a small electrode
than for a larger one. The greater the current density of the current leaving an anode, the
greater is the rate of corrosion. As an example, if a copper strap or cable is bonded to a
steel column, the rate of corrosion of the steel will be low because of the large anodic
area. On the other hand, a steel strap or bolt fastener in contact with a copper plate will
corrode rapidly because of the relatively small area of the anode of the cell.
12.11.8.1.4 Protective Coatings Paint or metallic plating used for the purpose of
excluding moisture or to provide a third metal compatible with both bond members
should be applied with caution. When they are used, both members should be covered.
57
Covering the anode alone should be avoided. If only the anode is covered then at
imperfections and breaks in the coating, corrosion will be severe because of the relatively
small anode area. All such coatings should be maintained in good condition.
12.11.9 Workmanship Whichever bonding method is determined to be the best for a
given situation, the mating surfaces should be cleaned of all foreign material and
substances which would preclude the establishment of a low resistance connection. Next,
the bond members must be carefully joined employing techniques appropriate to the
specific method of bonding. Finally, the joint should be finished with a protective coating
to ensure continued integrity of the bond. The quality of the junction depends upon the
thoroughness and care with which these three steps are performed. In other words, the
effectiveness of the bond is influenced greatly by the skill and conscientiousness of the
individual making the connection. Therefore, this individual should be aware of the
importance of electrical bonds and should have the necessary expertise to correctly
implement the method of bonding chosen for the job. Those individuals charged with
making bonds should be carefully trained in the techniques and procedures required.
Where bonds are to be welded, for example, work should be performed only by qualified
welders. No additional training should be necessary because standard welding techniques
appropriate for construction purposes are generally sufficient for establishing electrical
bonds. Qualified personnel should also be used where brazed connections are to be made.
Exothermic welding can be effectively accomplished by personnel not specifically
trained as welders. Every individual doing exothermic welding should become familiar
with the procedural details and with the precautions required with these processes.
Contact the manufacturers of the materials for such processes for assistance in their use.
By taking reasonable care to see that the bond areas are clean and free of water and that
the molds are dry and properly positioned reliable low resistance connections can be
readily achieved.
Pressure bonds utilizing bolts, screws, or clamps should be given special attention. Usual
construction practices do not require the surface preparation and bolt tightening necessary
for an effective and reliable electrical bond. Therefore, emphasis beyond what would be
required for strictly mechanical strength is necessary. Bonds of this type should be
checked rigorously to see that the mating surfaces are carefully cleaned, that the bond
members are properly joined, and that the completed bond is adequately protected against
corrosion.
12.12 Electrostatic Discharge Control (ESD) Program Wire and cable harness
assemblies are not subject to damage due to electrostatic fields unless they are attached to
assemblies containing ESD sensitive components such as microcircuits and
semiconductors, or otherwise to ESD sensitive components. However, when the
assemblies are installed into their next higher assembly, they may be attached to printed
wiring assemblies or other assemblies that contain static sensitive components. In such
instances, the manufacturer should establish, implement and maintain an ESD Control
Program conforming to ANSI/ESD S20.20.
58
If cable and wire harness connectors are connected to ESD sensitive components or
assemblies, the connectors or terminals should be covered with an ESD protective cap or
ESD protective material until they are installed for use to protect against ESD damage.
HDBK_620_Section_12_GLB.doc
59
13
Verification and/or Validation of Cable and Wire Harness Assemblies
13.1
Scope
IPC/WHMA-A-620 provides accept/reject criteria and process information for
manufacture of various types of cable and wire harness assemblies.
Manufacturers who implement the manufacturing and process criteria specified in
IPC/WHMA-A-620, by practicing due diligence, and with an effective process control
program in place, as supplemented by sound verification and validation practices, should
be able to consistently produce quality cable and wire harness assemblies conforming to
IPC/WHMA-A-620, and end-item performance requirements.
It is recognized that people sometimes make unintended mistakes, some of which could
have a minimal or otherwise significant impact on the cable and wire harnesses produced.
To ensure that the workforce is capable of detecting, and subsequently correcting such
mistakes prior to product shipment, it is imperative that a means of verification and
validation of cable and wire harness acceptability be implemented, and that the results of
verification and validation be continuously analyzed for individual defects that can have
an impact on the end-item system performance requirements, and for repetitive defects
that are indicative of an adverse product quality trend. Verification and validation is one
tool that is available to identify the need to drive corrective action and continuous process
improvements.
Verification and validation as referred to herein is the series of in-process inspections that
may be referred to within the individual sections of IPC/WHMA-A-620, and the specific
electrical and mechanical testing methodologies included in Section 19 of IPC/WHMAA-620.
Normally, the user (the ultimate customer) should have identified in the contract
documentation, the extent of verification and validation required to ensure that cable and
wire harness assemblies will perform their intended function over the expected life-time
of the end item in its specified worst case environmental conditions. However, if the
contract does not specifically identify the verification and validation methods, then
Section 19 of IPC/WHMA-A-620 may be used for verification and validation methods, to
the extent, as agreed to between the user and the supplier (AABUS). Supplier as referred
to herein means the organization that ultimately delivers the completed cable and wire
harness assemblies to the customer, and this supplier is responsible for proper flow-down
of all applicable user defined contract requirements to lower-tier suppliers and/or
manufacturers.
Section 19 of IPC/WHMA-A-620 provides a verification and validation methodology
that may be implemented, AABUS. However, nothing in Section 19 is considered
mandatory for implementation unless, AABUS. For example, if the user did not
specifically require performance of a dielectric strength test, then the fact that this test
methodology is included within IPC/WHMA-A-620 does not mandate performance of
1
this testing, AABUS. Additionally, if the user specified the methodology and
accept/reject criteria for the testing, as example, dielectric strength testing, then the user
specified criteria applies in lieu of the criteria of IPC/WHMA-A-620, unless, AABUS.
Data obtained from the results of verification and validation methods, if properly
analyzed and used to drive corrective actions and/or preventive actions when warranted,
is an effective tool that should be used by the manufacturer to provide a quality product
on a continuing basis. Management should periodically review the data and provide for
the tools and resources to enable continuous process improvement.
Although verification and validation, if effectively implemented, is capable of detecting
and correcting defects prior to product shipment, it is not intended to replace the need for
an effective process control program. One cannot inspect and/or test the quality into
completed cable and wire harness assemblies. Quality can only be assured by
implementing an effective process control program.
There are many processes involved in the manufacture of cable and wire harness
assemblies (e.g., stripping, crimping, soldering, etc.). Each of these processes has
important process parameters that, if not properly controlled, can produce defects. For
example, failure to have a crimp tool properly calibrated and/or verified prior to use can
result in an improper crimp connection which could cause an open or intermittent circuit
in the end-item. It is imperative that an effective process control methodology be
implemented, and that Management establishes process performance metrics which are
periodically reviewed and the results of these reviews are used to drive continuous
improvement. The use of process metrics to drive continuous improvement is a
mandatory requirement for organizations that intend to conform to the quality program
requirements such as, but not limited to, the requirements of ISO-9001.
This section of the Handbook provides a high level overview of the various verification
and validation methods that are available as one of the many tools used to ensure a
quality wire and cable harness assembly is delivered to the ultimate customer.
Verification and validation as addressed herein means the methodology applied to
production cable and wire harness assemblies. It does not include the numerous
verification and validation methods that may have been used for qualification of cable
and wire harness assemblies for the harsh environments that the assemblies may be
subjected to when installed in the end item. For example, the end item may need to
survive in a shipboard or aerospace environment where shock, vibration or high humidity
conditions apply. Verification and validation methods to qualify cable and wire
harnesses to survive in the end item environment such as, but not limited to a high
humidity, shock and vibration environment are not included herein or within
IPC/WHMA-A-620.
Users of this Handbook and IPC/WHMA-A-620 are cautioned that any verification and
validation method that may be employed in conjunction with the manufacture of cable
and wire harness must be carefully chosen to preclude damage to the completed cable and
2
wire harness assemblies. For example, selection of a test voltage for dielectric strength
test must not be high enough to cause permanent damage to the assemblies.
Some of the information herein is based on common test methodologies used for military
applications (e.g., MIL-STD-202; however, it also has relevance for non-military
applications as well).
13.2 Visual Inspection. Performing visual inspection of cable and wire harness
assemblies is one of the easiest forms for verification that contract requirements have
been met.
The inspection frequency (100 percent or sample inspection) specified in the contract
applies. If defects or nonconforming conditions are discovered during sample inspection,
the balance of the items submitted for inspection may need to be 100 percent inspected
for the noted condition. If inspection results reflect a condition that may have gone
undetected in shipped equipment, this should be made known to all affected customers.
Magnification aids should be used, as appropriate, depending on the physical size of the
area being inspected.
Automated Optical Inspection (AOI) may be used as appropriate; however, this is
normally used for inspection of printed circuit boards.
Although visual inspection alone cannot determine whether the item is functionally
acceptable (e.g., meets DC resistance limits, insulation resistance limits, will not break
down under dielectric strength testing, etc.), visual inspection can detect the types of
inspectable characteristics noted below.
•
•
•
•
•
•
•
•
•
•
•
Parts used conform to the contract drawing parts list or bill of materials (BOM).
No evidence of damage (insulation, conductor, parts, part plating, etc.).
Workmanship (e.g., soldering, crimping, lacing, taping, etc.) conforms to
applicable requirements.
Overall length and length/location of breakouts conform to contract document
requirements.
Sleeving or other protective material provided (when required).
No evidence of unauthorized repairs (e.g., wire splicing).
Wire markers contain correct information and are installed in the correct location.
Other markings (when such marking is required) (e.g., part number, serial
number, national stock number, drawing revision level, reference designations,
etc.) are correct.
Marking is legible and permanent (simple test for permanency is to lightly rub the
marking several times with your finger. If it smudges, it is not permanent).
Any items to be shipped with the item (e.g., separately bagged and tagged) are
correct and are provided with the assembly.
Manufacturing, inspection and test records for the item reflect completion of all
required manufacturing operations, inspections and tests. Any identified non-
3
•
•
•
conforming conditions have been cleared, including customer approval of any
“use-as-is” conditions (when required).
Connectors with removable contacts have the correct number of contacts installed
at the correct position in the connector insert, and are fully seated within the
insert.
The shipping paperwork reflects the item being shipped (correct part number,
quantity, and drawing revision level – when required) and contains all required
information, including proper completion of Government shipment forms (DD250
and/or DD1149, when applicable.
Packaging, packing and marking for shipment conforms to applicable contract
requirements, including any special markings (see IPC J-STD-609, etc).
13.3 Electrical Testing. Electrical testing of the types noted below can determine the
functional acceptability of cable and wire harness assemblies. It is important that all
testing be performed verbatim compliance to applicable test procedures (approved by the
user when required). Test instruments requiring periodic calibration should be verified as
being within current calibration. Test fixtures, calibrated when required, should be as
specified in the test procedure. Data should be formally documented in the test records as
the test results are obtained. It is not good practice to document results after the test is
completed. Test data should be subjected to independent review when required, and
should confirm that all test data entries have been completed, and that the data reflects
acceptable test results.
Test voltages/current can cause damage to the item being testing in event the limits
identified in the test procedure are not adhered to. Some testing requires the application
of voltages/currents that may be lethal to personnel; therefore implement appropriate
safety precautions as required by the organization doing the testing.
13.3.1 Continuity Test. Continuity testing verifies that the point-to-point electrical
connections conform to the assembly drawing, wire list, wiring diagram, or electrical
schematic. Normally, continuity of an electrical connection assures that the DC
resistance is low (e.g., 2 ohms or less – not counting the resistance of the wire).
However, in some cases, a maximum resistance value in ohms is specified.
Continuity testing is performed using various types of test equipment, from a simple
“light bulb/LED indicator” type test; a standard digital ohmmeter, or an automated
continuity tester dedicated to performing continuity testing. If a specific DC resistance
value is required, then the simple “light bulb/LED indicator type testing is not appropriate
unless a test circuit is designed to only illuminate when the correct resistance exists.
Continuity testing can be performed as an in-process test, or otherwise as part of final
assembly acceptance testing. Testing is sometimes performed while the assembly is still
mounted to a “harness board”. It is good practice to combine the continuity test with the
visual examination for correct wire marking since point-to-point connections are being
verified and should align with the appropriate wire markers.
4
The continuity test usually precedes the higher voltage tests such as the dielectric
withstanding voltage test and the insulation resistance test.
13.3.2 Shorts Test. The shorts test is similar in concept to the continuity test, except that
in lieu of looking for low DC resistance to verify a point-to-point connection, the shorts
test verifies that there is no unwanted connection (i.e., an open circuit). Low voltage is
used for this test; in some cases, limits pertaining to voltage, current and ohms may
apply. Failure to adhere to the low voltage/current specified in the test procedure may
cause product damage.
Unless otherwise specified by the user, the shorts test is not required when the insulation
resistance or dielectric strength test is performed since these tests normally verify proper
isolation between circuits/conductors that are intended to be isolated (i.e., no unintended
connections).
13.3.3 Dielectric Withstanding Voltage (DWV) Test.
13.3.3.1
Purpose. The dielectric withstanding voltage test (also called highpotential, over potential, voltage break down, or dielectric-strength test) consists of the
application of a voltage higher than rated voltage for a specific time between mutually
insulated portions of a component part or between insulated portions and ground. This is
used to prove that the component part can operate safely at its rated voltage and
withstand momentary over potentials due to switching, surges, and other similar
phenomena. Although this test is often called a voltage breakdown or dielectric-strength
test, it is not intended that this test cause insulation breakdown or that it be used for
detecting corona; rather, it serves to determine whether insulating materials and spacing
in the component part are adequate. When a component part is faulty in these respects,
application of the test voltage will result in either disruptive discharge or deterioration.
Disruptive discharge is evidenced by flashover (surface discharge), spark over (air
discharge), or breakdown (puncture discharge). Deterioration due to excessive leakage
currents may change electrical parameters or physical characteristics.
13.3.3.2
Precautions. The dielectric withstanding voltage test should be used with
caution, as even an over potential less than the breakdown voltage may injure the
insulation and thereby reduce its safety factor. Therefore, repeated application of the test
voltage on the same specimen is not recommended. In cases when subsequent application
of the test potential is specified in the test routine, it is recommended that the succeeding
tests be made at reduced potential. When either alternating-current (ac) or direct current
(dc) test voltages are used, care should be taken to be certain that the test voltage is free
of recurring transients or high peaks. Direct potentials are considered less damaging than
alternating potentials which are equivalent in ability to detect flaws in design and
construction. However, the latter are usually specified because high alternating potentials
are more readily obtainable. Suitable precautions must be taken to protect test personnel
and apparatus because of the high potentials used.
5
13.3.3.3
Factors affecting use. Dielectric behavior of gases, oils, and solids is
affected in various degrees by many factors, such as atmospheric temperature, moisture,
and pressure; condition and form of electrodes; frequency, waveform, rate of application,
and duration of test voltage; geometry of the specimen; position of the specimen
(particularly oil-filled components); mechanical stresses; and previous test history.
Unless these factors are properly selected as required by the type of dielectric, or suitable
correction factors can be applied, comparison of the results of individual dielectric
withstanding voltage tests may be extremely difficult.
13.3.3.4
High voltage test source. The nature of the potential (ac or dc) used for
the test is identified in the test procedure. When an alternating potential is specified, the
test voltage provided by the high voltage source is nominally 60 hertz in frequency and
approximates, as closely as possible, a true sine wave in form. Other commercial power
frequencies may be used when specified. All alternating potentials shall be expressed as
root-mean square (RMS) values, unless otherwise specified. The kilovolt-ampere (KVA)
rating and impedance of the source are selected such as to permit operation at all testing
loads without serious distortion of the waveform and without serious change in voltage
for any setting. When the test specimen demands substantial test source power capacity,
the regulation of the source is specified. The test procedure may specify a minimum
KVA rating is required. When a direct potential is specified, the ripple content is
normally controlled so as not to exceed 5 percent RMS of the test potential. If high
surge currents can damage the item under test a suitable current-limiting device is used to
limit current surges to the value specified.
13.3.3.5
Voltage and current measuring devices. In some cases, a maximum
leakage current requirement may be specified. It is important to select test instruments
that have the requisite accuracy to measure applied test voltage and leakage current
(when required). A five percent tolerance on voltage and/or current is recommended
unless otherwise specified.
13.3.3.6
Fault indicator. Suitable means to indicate the occurrence of disruptive
discharge and leakage current is recommended in case it is not visually evident in the test
specimen. The voltage and current instruments noted above, or an appropriate indicator
light or an overload protective device may be used for this purpose.
13.3.3.7
Test performance using the approved (when required) test procedure.
13.3.3.7.1 Preparation. The test procedure may specify special preparations or
conditions such as special test fixtures, reconnections, grounding, isolation, or immersion
in water.
13.3.3.7.2 Test voltage and points of application. Specimens are subjected to a test
voltage of the magnitude and nature (ac or dc) specified in the test procedure and the test
voltage is applied between mutually insulated portions of the specimen or between
insulated portions and ground as specified. The method of connection of the test voltage
to the specimen may need to be specified when it is a significant factor.
6
13.3.3.7.3 Rate and duration of application of test voltage. The test voltage is
normally raised from zero to the specified value as uniformly as possible, at a rate of
approximately 500 volts (RMS or DC) per second, unless otherwise specified, or it may
be applied instantaneously.
Unless otherwise specified in the test procedure, the test voltage is maintained at the
specified value for a period of 60 seconds for qualification testing, or at reduced voltage
for production testing. Upon completion of the test, the test voltage is gradually reduced
to avoid surges, or may be removed instantaneously in event surges are not a concern.
During the dielectric withstanding voltage test, the fault indicator is monitored for
evidence of disruptive discharge and leakage current, followed by a visual examination to
determine the effect of the dielectric withstanding voltage test on specific operating
characteristics, when specified.
13.3.3.7.4 Examination and measurement of specimen. During the dielectric
withstanding voltage test, the fault indicator is monitored for evidence of disruptive
discharge and leakage current. Following this, the specimen is examined and
measurements performed to determine the effect of the dielectric withstanding voltage
test on specific operating characteristics, when specified. Normally, the insulation
resistance test follows the dielectric withstanding voltage test, while using the same
isolated circuit connections used for the dielectric withstanding voltage test. The
operating test, if required, normally follows the insulation resistance test.
Normally the test procedure includes criteria for the following;
a. Special preparations or conditions, if required.
b. Magnitude and type (AC or DC) of test voltage and duration of voltage application.
c. Points of application of test voltage.
d. Method of connection of test voltage to specimen, if significant.
g. Test power source voltage/current regulation, when applicable.
13.3.4 Insulation resistance (IR) test.
13.3.4.1
Purpose. This test measures the resistance offered by the insulating
members of a component part to an impressed direct voltage tending to produce a leakage
of current through or on the surface of these members. Knowledge of insulation
resistance is important, even when the values are comparatively high, as these values may
be limiting factors in the design of high-impedance circuits. Low insulation resistances,
by permitting the flow of large leakage currents, can disturb the operation of circuits
intended to be isolated, for example, by forming feedback loops. Excessive leakage
currents can eventually lead to deterioration of the insulation by heating or by direct
current electrolysis. Insulation resistance measurements should not be considered the
equivalent of dielectric withstanding voltage or electric breakdown tests. A clean, dry
insulation may have a high insulation resistance, and yet possess a mechanical fault that
would cause failure in the dielectric withstanding voltage test. Conversely, a dirty,
7
deteriorated insulation with a low insulation resistance might not break down under a
high potential. Since insulating members composed of different materials or
combinations of materials may have inherently different insulation resistances, the
numerical value of measured insulation resistance cannot properly be taken as a direct
measure of the degree of cleanliness or absence of deterioration. The test is especially
helpful in determining the extent to which insulating properties are affected by
deteriorative influences, such as heat, moisture, dirt, oxidation, or loss of volatile
materials.
13.3.4.2
Factors affecting use. Factors affecting insulation resistance
measurements include temperature, humidity, residual charges, charging currents of time
constant of instrument and measured circuit, test voltage, previous conditioning, and
duration of uninterrupted test voltage application (electrification time). In connection
with this last-named factor, it is characteristic of certain components (for example,
capacitors and cables) for the current to usually fall from an instantaneous high value to a
steady lower value at a rate of decay which depends on such factors as test voltage,
temperature, insulating materials, capacitance, and external circuit resistance.
Consequently, the
measured insulation resistance will increase for an appreciable time as test voltage is
applied uninterruptedly. Because of this phenomenon, it may take many minutes to
approach maximum insulation resistance readings, but specifications usually require that
readings be made after a specified time, such as 1 or 2 minutes. This shortens the
testing time considerably while still permitting significant test results, provided the
insulation resistance is reasonably close to steady-state value, the current versus time
curve is known, or suitable correction factors are applied to these measurements. For
certain components, a steady instrument reading may be obtained in a matter of seconds.
When insulation resistance measurements are made before and after a test, both
measurements should be made under the same conditions.
13.3.4.3. Test apparatus. Insulation resistance measurements are made on an apparatus
suitable for the characteristics of the component to be measured such as a megohm
bridge, megohm-meter, insulation resistance test set, or other suitable apparatus. The
direct potential applied to the specimen depends on the circuit voltage and is as specified
in the test procedure. Unless otherwise specified, the measurement error at the
insulation resistance value required is within 10 percent. Proper guarding techniques may
be required to prevent erroneous readings due to leakage along undesired paths.
.
13.3.5 Voltage standing wave ratio (VSWR), insertion loss, and reflection coefficient
tests1.
13.3.5.1
Overview. VSWR, Insertion Loss and Reflection are parameters that are
interrelated with each other and are important parameters, in particular for cable and wire
harness assemblies that must transmit signals/power at high frequencies (i.e., RF).
Testing to verify the aforementioned parameters is performed using specialized test
equipment. One such equipment is the network analyzer. In RF applications we are most
8
concerned with getting signals from one point to another with maximum efficiency and
with minimum distortion. Testing using network analyzer instruments enables one to
characterize the signal received at the load versus the known signal input at the source.
There are two different types of network analyzers; a scalar type, based on a diode
detection scheme, or a vector type, based on a tuned-receiver (narrowband) techniques.
The scalar type is the least expensive; however, it is not as accurate as the vector type.
The type of network analyzer used should by based on the desired accuracy required for
the application.
A network analyzer must provide a source for stimulus, signal-separation devices,
receivers for signal detection, and display/processing circuitry for reviewing results. The
source is usually a built-in-phase-locked-voltage-controlled oscillator. Signal-separation
hardware allows measurements of any portion of the incident signal to provide a
reference for ratio measurements, and it separates the incident (forward) and reflected
(reverse) signals present at the input to the item being tested.
13.3.5.2
Signal propagation in DC circuits versus RF systems. It is important to
understand the difference between how signals propagate in DC circuits versus RF
systems. In DC circuits or circuits where the propagating signal has low frequencies, the
voltage of the signal at different points on a cable in the signal path varies minimally.
This is not so in the case of RF or high-frequency signals where wavelength of the signal
is considerably small in comparison to the length of the cable allowing multiple cycles of
the signal to propagate through the cable at the same time.
Consider an example where two waves (signals) of different frequencies are made to
propagate through a 1 m coaxial cable. The frequency of the first signal is 1 MHz while
that of the second signal is 1 GHZ. To calculate their wavelengths, we will use the
following formula:
λ = VF 3 X 108 m
f
In the above formula, λ is the wavelength of the signal, f is its frequency, and VF is the
velocity factor of the cable. Let us assume that the coaxial cable being used to route both
signals is of type RG8, which is known to have a velocity factor of 0.66.
Then for Signal 1 where f = 1MHz: λ1 = (0.66) 3 X 108 m = 198m
1 X 106
9
In the case of Signal 1, the length of the coaxial cable is considerably small in
comparison to the wavelength of the signal propagating through it. Therefore, as can be
seen in the above Figure , the variation in potential of the signal at different points in the
cable is negligible.
For Signal 2 where f = 1GHZ: λ2 = (0.66) 3 X 108 m = 0.198m
1 X 109
In the case of Signal 2, the length of the coaxial cable is much larger (almost 5X) than the
wavelength of the signal propagating through it. Therefore, at any given time, multiple
cycles of the signal will travel through the cable simultaneously. Because of their small
wavelengths, high-frequency signals travel through cables in waves. Such signals
therefore suffer reflections and power loss when traveling between varying media (wave
theory). In the case of electrical circuits, this variation in medium takes place when the
signal (wave) is made to pass through system components that have varying characteristic
impedances. Therefore, to minimize reflections and power loss, RF systems must be
constructed using suitable components with matched impedances. As a rule of thumb,
signal degradation due to power loss and reflections that occur in the transmission line
become significant once the length of the cable exceeds 0.01 of the wavelength of the
signal it is being used to route.
10
13.3.5.3.1 Characteristic impedance. Characteristic impedance is a transmission line
parameter that is determined by the physical structure of the line. It also helps
determine how propagating signals are transmitted or reflected in the line.
Impedance of RF components is not a DC resistance and, in the case of a
transmission line, can be calculated using the following formula:
In the above formula:
Z0 = Characteristic impedance
L = Inductance per unit length of the RF transmission line caused due to magnetic fields
that are formed around the wires when current flows through them.
C = Capacitance per unit length of the RF transmission line. This is also the capacitance
that exists between two conductors.
R = DC resistance per unit length of the RF transmission line.
G = The dielectric conductance per length.
ω = Frequency (radians/s).
Because an ideal cable has no resistance or dielectric leakage, its characteristic
impedance can be calculated using the above formula as:
Z0 = [L/C]1/2
Because all components in an RF system have to be impedance matched to minimize
signal losses and reflections, component manufacturers specifically design their
equipment to have a characteristic impedance of either, 50 or 75 Ω. 50 Ω systems make
up the bulk of the RF market and include most communications systems. 75 Ω RF
systems are smaller in number and are prevalent mainly in video RF systems. It is crucial
that engineers ensure that parts such as cables and connectors, in addition to other
instruments that may reside in the test system, are all impedance matched.
13.3.5.4
Insertion loss. Significant power loss in the signal occurs if the length of
the transmission line it is made to propagate through is greater than 0.01 of its own
wavelength. The “insertion loss” specification of a cable assembly is a measure of this
power loss and signal attenuation. Insertion loss of a cable assembly at a particular
11
frequency can be used to calculate the power loss or voltage attenuation caused by the
cable assembly on a signal at that frequency.
Formula for calculating power loss: Insertion Loss (dB) = 10 log10(Pout/Pin)
Formula for calculating voltage attenuation: Insertion Loss (dB) = 20 log10(Vout/Vin)
To understand the concept of insertion loss, think of a cable assembly as a low pass filter.
Every cable assembly in the real world has some parasitic capacitance, resistance, and
conductance. These parasitic components combine to attenuate and degrade the signal
the cable assembly is being used to route. The power loss and voltage attenuation caused
by these components varies with the frequency of the input signal and can be quantified
by the insertion loss specification of the cable assembly at that frequency. It is therefore
critical to ensure that the insertion loss of the cable assembly is acceptable at the
bandwidth requirement of the application.
13.3.5.5
Voltage standing-wave ratio (VSWR). VSWR is the ratio of reflectedto-transmitted waves. As mentioned earlier, at higher frequencies, signals take the form
and shape of waves when passing through a transmission line or cable. For this reason,
just as in the case of sound and light waves, reflections occur when the signal traverses
over different media (such as components with unmatched impedances). In a cable
assembly, this mismatch can be between the characteristic impedance of the connector,
the cable itself, and the connected load. Because VSWR is a measure of the power of the
reflected wave, it can also be used to measure the amount of power loss in the
transmission line. The reflected wave, when summed with the input signal either
increases or decreases its net amplitude, depending on whether the reflection is in phase
or out of phase with the input signal. The ratio of the maximum (when reflected wave is
in phase) to minimum (when reflected wave is out of phase) voltages in the “standing
wave” pattern is known as VSWR. To understand how to calculate VSWR and return
loss in an RF system, let us consider the RF transmission in the figure below.
In the above circuit, the impedance of the load (40.5 Ω) is not equal to that of the source
and the transmission line (50 Ω). For this reason, some portion of the signal propagating
through the transmission line is reflected back from the load. We can measure this
reflection using the following formula: Insertion Loss (dB) = 10 log10(Preflected/Pin).
12
As you can see, return loss is a measure of the power of the reflected signal. It is also a
subset of insertion loss. The higher the return loss (or reflections) in an RF system, the
higher the insertion loss.
VSWR is another way of measuring signal reflections. It can be calculated as:
VSWR = 1 + (Г)
1 – (Г)
In the above formula, Г is the reflection coefficient and can be calculated using the
following formula:
Г = ZL – Z0
ZL + Z0
From the circuit in the above figure, we calculate VSWR to be:
VSWR = 1 + ZL – Z0
ZL + Z0
_______________
1 - ZL – Z0
ZL + Z0
Inserting the numbers we get: 1 + 0.1/1-0.1 = 1.22.
To visualize what is happening in this example, let us imagine that the signal being
sourced in the RF system is a 1 Vpp sine wave. Because the reflection coefficient for the
system is 0.1, we can determine that the magnitude of the reflected is 0.1 X 1 = 0.1 V or
100 mv. The following figures display the maximum and minimum amplitudes of the
resultant signal which occurs when the reflected wave is in phase and 180 deg out of
phase with the input signal, respectively.
13
As stated earlier, VSWR is the ratio of maximum voltage to minimum voltage in the
standing wave pattern. Using this definition, we can calculate VSWR from the above
figures to be:
VSWR = 1.1/0.9 = 1.22
13.3.6 User Defined Electrical Tests. The specific tests listed in IPC/WHMA-A-620,
Section 19 are recommended default tests for use in event the user (customer) has not
invoked any test requirements in the contract. Normally, the user will invoke a standard
test suite that, if not the same as specified in IPC/WHMA-A-620, is similar in scope.
However, specific test equipment, test parameters, and accept/reject criteria may be
different. Additionally, the user may specify different tests. In all instances, unless,
AABUS, the user identified testing applies.
13.4 Mechanical Testing. Various types of mechanical testing are normally
performed to verify the mechanical integrity of the cable assembly such as adequacy of a
crimp connection, insertion of a contact into a connector, etc. The following types of
mechanical tests are normally performed:
13.4.1 Crimp testing (Crimp height, pull force/tensile testing, crimp force
monitoring)2, 3. In order to better understand the various types of crimp testing
that are used to verify the acceptability of crimp connections, it is important to
understand some of the more important aspects of the crimping process/tooling.
The following discussion provides an oversight of the crimping process and the
crimp tooling used.
Quality, cost, and throughput are associated with specific measurements and linked to
process variables. Crimp height, pull test values, leads per hour, and crimp symmetry are
some of the measures used to monitor production termination processes. Many variables
affect the process such as wire and terminal quality, machine repeatability, setup
parameters, and operator skill.
Crimp tooling is a significant contributor to the overall crimp termination process. The
condition of crimp tooling is constantly monitored in production by various means. These
14
means are often indirect measures. Crimp quality monitors and crimp cross sections are
methodologies that infer the condition of the crimp tooling. Visual inspection of the
crimp tooling can be used to check for gross failures such as tool breakage or tooling
deformation which occurred as a result of a machine crash. Continuous monitoring of
production will help determine when the process needs to be adjusted and the
replacement of crimp tooling can be one of the adjustments that are made.
Crimp tooling can a have positive effect on the quality, cost, and throughput of the
termination process. High quality crimp tooling can produce high quality crimps with less
in-process variation over a greater number of terminations. It is difficult to distinguish
critical tooling attributes with visual inspection only. Some attributes cannot be inspected
even by running crimp samples. The following discussion provides the reader with
information that identifies key crimp tooling attributes and the effect of those attributes
on the crimping process.
13.4.1.1
Key crimp tooling characteristics. There are four key characteristics for
crimp tooling. These are:
• Geometry and associated tolerances
• Materials
• Surface condition
• Surface treatment
Each of these categories contributes to the overall performance of the production
termination process.
13.4.1.2
Geometry and associated tolerances. Terminals are designed to perform
to specification only when the final crimp form is within a narrow range of dimensions.
Controlling critical crimp dimensions is influenced by many factors including:
• Wire size and material variation
• Terminal size and material variation
• Equipment condition
The final quality and consistency of a crimp can never be any better than the quality and
consistency of the tooling that is used. If other variations could be eliminated, tooling can
and should be able to produce crimp forms that are well within specified tolerances. In
addition, variation from one tooling set to another should be held to a minimum. Crimp
tooling features that are well controlled and exhibit excellent consistency from tooling set
to tooling set can result in shorter setup time as well as more consistent production
results.
Some critical crimp characteristics are directly defined by the tooling form and are
obvious. These include:
15
• Crimp width
• Crimp length
Other critical crimp characteristics can be related to several tooling form features and/or
other system factors. These may be less obvious and include:
• Flash
• Roll, twist, and side-to-side bend
• Up/down bend
• Crimp symmetry
• Bellmouth
The following discussion focuses on two characteristics, crimp width and flash, as
examples of how tooling can affect crimp form. Similar arguments can be applied to the
others.
13.4.1.2.1 Crimp width. Crimp width is a good example of a feature that should be
consistent and in control between different crimpers of the same part number. The reason
for this is quite straightforward. For a given terminal and wire combination, it is
necessary to achieve an area index, AI, which is determined by the terminal designer for
optimal mechanical and electrical performance. Crimp height, CH, and crimp width,CW,
directly affect achieving proper AI. Area index, AI(as a percentage), is defined as:
AI = [At/(AW + AB)] X 100
16
where At is the total area of the wire and barrel after crimping. AW and AB are,
respectively, the initial cross-sectional areas of the wire and barrel before crimping.
A typical design point for AI is 80%. In order to maintain the same AI, the crimp height,
CH, needs to change inversely to the change of crimp width, CW, in approximately the
same proportion. Thus, if the CW increases +2%, the CH needs to change approximately
-2% in order to achieve the same AI design point. At first glance that may not seem
significant, but in reality it can be very significant. Using another general industry design
rule of the ratio of CH to CW of approximately 65%, a typical set of dimensions used as
an example may be:
CW = 0.110 in
CH = 0.068 in
Therefore, varying the CW by 2% would result in a CH variation of 2%, or 0.0014 in. At
a CH tolerance of ± 0.002 in, 35% of the total CH tolerance would be used by a 2%
variation in CW. Thus, the importance of crimp width control is obvious when tooling is
changed during a production run.
Cross Sections Showing Minimum
(a) and Maximum (b) Area Index per
Terminal Specification – a variation
of ± 3.5%
(a)
(b)
17
13.4.1.3. Flash. Most crimp terminations have a requirement to limit flash. Flash is
defined as the material which protrudes to the sides of the terminal down and along the
anvil. Flash is normal in the crimping process but excessive flash is very undesirable.
Controlling flash requires a balance of several geometric factors. Other factors
influencing flash are related to surface finish and friction, which will be discussed later.
A dominant factor in controlling flash is controlling the clearance between the crimper
and anvil during the crimp process. Defining the ideal clearance could in itself be a
simple matter were it not for two facts:
• In order to minimize terminals’ sticking in the crimper, the sides of the terminal are
tapered. Thus the clearance between the anvil and crimper varies throughout the stroke.
• Crimper and anvil sets are typically designed to terminate two to four wire sizes. This
creates multiple crimp heights. Since the sides of the crimper are tapered to minimize
terminal sticking,
the maximum clearance permitted without creating flash must be assigned to the
maximum crimp height specified for the tooling set. In addition, a minimal clearance
must be maintained for the smallest crimp height specified by the tooling set to prohibit
contact between the anvil and crimper.
Crimper to anvil clearance is thus a combination of crimp width, crimper leg taper, anvil
width, and crimp height. The critical design point is at the largest crimp height. This
contribution to the gap is directly dependent on dimensional control. The following is
offered as an example:
Nominal condition:
CH = 0.073 in
CW = 0.110 in
18
Crimper leg taper = 3.0 degree
Anvil Width = 0.109 in
Nominal anvil to crimper total clearance = 0.005 in
The clearance can grow rapidly with small changes to the nominal dimensions:
CH remains unchanged = 0.073 in
Increase in crimp width, CW, = 0.0008 in
Increase in crimper leg taper = 0.8 degree
Decrease in anvil width = 0.0008 in
The total increase in total clearance is this case = 0.0026 in.
This more than a 50% increase in the nominal design clearance, which can result in
unacceptable flash (see below). Dimensional control is clearly critical.
In the photographs below, significant flash can be generated with excessive anvil to
crimper distance, as shown by the nominal design condition (a) and +0.003 in over
nominal condition (b).
(a)
(b)
13.4.1.4
Material system. The material selection for tooling is critical. The
material must be able to meet the in-service demands placed on the tooling components.
The two critical tooling components to be reviewed are the wire crimper and the anvil.
The wire crimper and the anvil have different functional demands. Both have the need to
withstand high loads and moderate shock. However, the wire crimper is in fact an
aggressive forming tool. It must withstand high shear loading that is a result of frictional
19
loads generated as the terminal barrel slides along the crimper surfaces in the forming
process, and then as the barrel terminal is plastically deformed and extruded to complete
the termination. The anvil experiences some of the same conditions but to a much lower
level of severity.
The wire crimper and the anvil can be likened to a punch and die in the world of
metalworking. The materials used in punch and die applications have been well
documented, along with the material selection process. The added severity of the
aggressive forming and the terminal and wire extrusion during crimping add complexity
to the material selection. The material selection process involves:
• Strength of materials with emphasis on toughness needed to withstand the moderate
shocks generated during crimping
• Wear resistance to maintain form
In addition to the above design considerations, there exists another phenomenon that
occurs during crimping that can significantly shorten the useable life of a wire crimper.
Material can be transferred from the terminal barrel to the wire crimper. This material
buildup can result in unacceptable terminations. The crimped terminal surfaces can
actually be deformed by the indentations of the deposited material. Crimp deformation
may result due to increased friction. Tooling wear can be accelerated due to higher crimp
forces. Surface treatments that minimize this material transfer are critical to extended
tooling life.
13.4.1.4.1 Strength of materials. Crimpers and anvils are designed to be able to
withstand stresses that are typically encountered during crimping. The basic design of
tooling with reference to size and geometry has been well analyzed and generally stresses
generated during crimping are able to be accommodated. However, there are always
demanding applications that will tax the design to its stress limits. In those cases,
geometry and material may depart from the standard design. These exceptions are
dealt with on a one-by-one basis and will not be discussed here.
It is the unique requirement of stress and shock that needs to be discussed. Peak crimp
loads go from zero to maximum in less than 40 ms. Tooling needs to withstand this load
cycle at a rate of greater than once per second. Several classes of tool steels are suitable
and are well described in the material handbooks. It is the processing of these materials
that can make a significant performance difference.
In order to withstand the rapid loading to a high stress on a repeated basis, the surface of
the material must minimize cracks and imperfections that may be generated during the
machining and/or heat treat operations. It is important that grain structure be controlled
in size and orientation to achieve maximum and consistent service life.
Decarburization of the surface during heat treating must be controlled. Heat treating
process controls are critical to reproducing the optimal surface. Machining processes
20
must also be controlled to avoid surface cracking due to excessive heat generation during
overly aggressive material removal. Likewise, localized tempering may occur, which can
soften material beyond the effective range. These variations in final material and surface
conditions are not readily detectable with a visual inspection. They can manifest
themselves during service and result in unacceptable tooling performance.
13.4.1.4.2 Wear resistance. Wear is generally described as the gradual deterioration of a
surface through use. Several types of wear exist and include adhesive, abrasive, and
pitting. By design, the tooling is able to withstand normal surface loads. Thus, pitting is
typically not an issue. Abrasion can occur depending on terminal surfaces. If a terminal
is plated with an abrasive substance, the tooling could suffer from abrasive wear. This
would be an atypical condition and would be handled by special design.
The primary wear mode experienced by crimp tooling is adhesive wear. Adhesive wear
occurs as two surfaces slide across each other. Under load, adhesion, sometimes referred
to as cold welding, can occur. Wear takes place at the localized points of adhesion due to
shear and deformation. Adhesion is highest at the peaks of surface finish because that is
where the load is greatest. During crimping, the ideal conditions exist for adhesive wear.
That is,
• High loading due to crimp force
• Sliding surfaces due to crimp formation, and terminal and wire extrusion
Wear will generally manifest itself more significantly at edges of a surface. However,
adhesive wear is often observed over substantial areas of the tooling. It is important to
note here that the wire crimper is the component most susceptible to adhesive wear.
Generally, adhesive wear will be directly related to load and to the amount of relative
movement between the two materials. Although the anvil may have equal loading, the
amount of relative movement between the terminal and tooling is many times more at the
crimper than at the anvil. The insulation crimper typically experiences lower adhesive
wear because the load is reduced compared to the wire crimp and the relative movement
is less than that of the wire crimper, since there is no terminal and wire extrusion at the
insulation crimp.
Adhesive wear can be controlled in the selection of the material. Different alloys exhibit
better or worse wear properties. These properties can be measured and are well
documented. Adhesive wear is inversely proportional to the hardness of the material.
Thus, the harder the material, the less adhesive wear. In crimp tooling, there is often a
tradeoff that is made. In order to achieve higher wear resistance, the material often
exhibits lower toughness by composition, hardness, or both. The final material selection
is often based on years of experience. One material may have high wear characteristics
and lower toughness, and be suitable for a small terminal since the margin of safety on
stress is high. Another terminal may be large and the toughness could be of more
importance due a lower stress design margin. The ability to design and manufacture
21
crimpers from several materials will enable optimal material selection for a specific
application.
The final property that affects adhesive wear is surface finish. As stated earlier, adhesion
is highest at the peaks of the surface. Thus, the smoother the finish, the less significant
the peaks and the less significant the adhesion. Adhesive wear can be reduced with a
lower surface finish. Surface finish affects other crimping performance parameters.
These are discussed in the next section.
13.4.1.5
Surface condition. Surface condition can affect the performance of the
crimp tooling as well as the longevity of service. As noted in the previous section, a hard,
smooth surface has improved adhesive wear properties and, thus, longer service life. The
other attribute that needs to be considered is friction.
Friction is a contributing factor in determining the final crimp form and process
characteristics. Low tooling friction results in lower crimping force and thus can
influence crimp form as well as tooling life. Consistent frictional characteristics between
tooling sets will result in reduced process variation.
The Figure above shows the typical effect of friction on crimp force.
Friction of the crimp tooling surfaces is influenced by factors similar to those that
influence adhesive wear—hardness and surface finish. Generally, harder materials exhibit
lower coefficients for sliding friction. Friction coefficients have also been shown to be
related to surface finish. Manufacturing processes need to produce consistent results
22
such that when tooling sets need to be changed in production, minimum disruption in
crimp quality is achieved. It has been found that maintaining surface hardness above Rc
55 as well as keeping surface finishes to 8 micro-inches or less is desirable to obtain
consistent crimp results and minimize adhesive wear.
13.4.1.6
Surface treatment. Surface condition can affect the performance of the
crimp tooling as well as the longevity of service. As noted in the previous section, a hard,
smooth surface has improved adhesive wear properties and, thus, longer service life. The
other attribute that needs to be considered is friction.
A commonly accepted approach to improved crimp tooling performance and life has been
to apply a surface treatment to the crimp area. The wire crimper has been defined in
previous discussions as tooling component that is subjected to the severest duty cycle.
Thus, applying an appropriate surface treatment to the wire crimper will have the most
benefit to crimp performance and tooling life. These treatments can include hard metal
plating or ceramic coating.
An example of a treatment that has been successful in achieving significant level of
performance and life improvements is hard chromium plating. There are several valid
reasons for this success.
First, chromium plating has a very low coefficient of friction. As noted, friction has a
significant effect on crimp form. The static and sliding coefficients of friction for steel on
steel are typically 0.30 and 0.20 respectively. Chromium plated steel on steel can reduce
the static and sliding coefficients to 0.17 and 0.16. In addition, chromium plating can be
highly polished so that there is no loss of surface finish characteristics due to the plating
process.
The second area that is greatly improved with chromium plating is wear resistance.
Adhesive wear resistance is improved as surface hardness improves. Chromium plating
typically exhibits hardness Rc 65+. This hardness level greatly enhances resistance to
adhesive wear. Also, this now frees up the designer to consider more base metal options.
A base material of reduced wear resistance but greater toughness can be selected and its
wear resistance improved with chromium plating. Thus, chromium plating can enable a
better tooling solution for the crimp production process.
Third, and perhaps one of the most significant effects of chromium plating, is its
resistance to adhesion and cold welding. A side effect of adhesive wear is the transfer of
material from the terminal to the wire crimper. By definition, adhesive wear is caused by
material adhering to localized points on the surface. Some of the adhesion results in the
surface material being worn away and some in the transfer of material from the terminal
to the crimper. As more cycles occur, more material is transferred. Thus there a resultant
buildup of terminal material on the crimper. This buildup will result in two potentially
catastrophic failures:
23
• The built-up material will create deformations in the terminal surface, resulting in
unacceptable crimps.
• Crimps will be greatly distorted due to significant changes in the friction factor and
result in the terminals not conforming to the desired form. Unacceptable crimp forms,
such as unsymmetrical cross-section, excessive flash, and open barrels can result.
Chromium plating has the ability to be applied uniformly and consistently and exhibits
excellent adhesion to the base metals. The unique benefits of chromium plating, such as
ease of application, consistency of plating, adhesion to base metal, extremely low
coefficient of friction, very high hardness, and resistance to adhesion, make it truly
difficult to match in crimp performance and durability. However many alternative
coatings are being attempted, and some show excellent promise in specific applications.
13.4.1.7
Crimp height. When a connector design engineer begins to design a
crimp, several things are considered, including the size and composition—
solid/stranded/aluminum/copper, etc.—of the wires to be crimped, and the required
electrical and mechanical properties. After optimizing to obtain the best results in both
these properties, the engineer determines the best crimp profile and the appropriate
crimp height range and width, to achieve the desired reduction in the area of the wire.
Deviations from crimp height standards can result in degradation of either mechanical or
electrical performance.
A loose crimp will result in poor mechanical qualities, and likely, poor or noisy electrical
conduction. Too tight a crimp may improve the electrical properties up to a point, but
mechanical properties may suffer as a result. Individual wire strands may get cut or the
wire may start to undergo excessive plastic flow, leading to a reduction in crimp tensile
strength or vibration resistance. The Figure below illustrates the tradeoffs in optimizing
crimp design and how crimp height is an accurate measure that combines both electrical
and mechanical performance.
24
13.4.1.7.1 Comparing crimp pull-out force, crimp force, and crimp height quality
measurement technologies. It is common to measure pull-out force as a basic criterion
for crimp quality. Indeed, it checks the mechanical properties of the crimp. However, as
discussed above, too tight a crimp can be as bad as one that is not tight enough. Pull-out
force may not be sufficiently sensitive to detect over-crimping in cases where the process
has not gone so far as to break the wire or terminal. A crimped terminal that passes the
pull-out test may nonetheless have a reduced lifetime or resistance to subsequent damage
from handling, installation, or vibration. Of course, since it can be a destructive test, it
may be performed on a sampling basis, only. This sampling still provides an auditable
quality record.
Crimp force has also been employed. Strain gauges or other force sensors appropriately
mounted can acquire force data. Specifically, force during the crimp and the peak force
are evaluated in relation to average and standard deviation specifications and, as soon as
a crimp cycle occurs in which the value of either exceeds a preset multiple of the standard
deviation, the termination is evaluated for faults. This test does allow 100%, nondestructive testing, but does not validate the crimp to the original terminal design intent.
Crimp height, as illustrated in the above Figure, is also a common measurement. Crimp
height can be used to validate crimp to original design intent but by itself, crimp height
can not discern process variables such as wire and terminal variation. As a standard, this
measurement can be performed with a vernier caliper fitted with appropriate jaw
modifications. As such, of course, it is useful for standardizing but is too slow for 100%
production inspection.
25
The best method to validate crimp quality is to use a combination of crimp force and
crimp height. This results in being able to confirm the achievement of the terminal design
intent as well as discerning wire and terminal variables that can result in poor
terminations.
13.4.1 Contact Retention. The purpose of this test is to impose axial forces on the
connector contacts to determine the ability of the connector to withstand forces
that tend to displace contacts from their proper location within the connector
insert and resist contact pullout. These forces may be the result of:
a.
Loads on wire connected to the contact.
b.
Forces required to restrict contact “push-through” during assembly of
removable type contacts into connector inserts.
c.
Forces produced by mating contacts during connector mating.
d.
Dynamic forces produced by vibration and shock during normal use of the
connectors.
e.
Forces relating to bundling strains on the wire.
13.4.2.1
Test equipment. Test equipment required to perform this test normally
consists of the following:
a.
Force gages, of suitable range for the contact size under test, so that readings
lie in the middle 50 percent of the scale, where practical, with a nominal full
scale accuracy of ± 2 percent.
b.
Dial indicator gages or other suitable instruments of such range for the
contacts under test that the readings lie in the middle 50percent of the scale,
with a nominal full scale accuracy of ± 2 percent.
c.
Contact removal and insertion tools, as required.
d.
Suitable compression device.
e.
Steel test probes, to adapt the force gage plunger to the particular contact (pin,
socket, etc.) front or wiring end under test.
f.
The actual test sample – The plug or receptacle with suitable contacts in place.
Normally 20 percent of the contact complement, but not less than 3 contacts
of each size are tested unless otherwise specified.
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13.4.2.2
Preparation. All back shell hardware and compression rings, if any, are
removed. If testing on the wire side of the connector is required (destructive test), the
contacts have the wires cut off flush or the contacts are replaced. All contacts are in
place in the connector. Simulated contacts which duplicate the retention feature
geometry are sometimes used in lieu of actual contacts to facilitate testing.
The unmated connector is mounted in a position of axial alignment of the contacts with
the plunger of the test gage. Normally, a minimum space of ¼ inch is required on the
opposite side under test to permit any “push-through” that may occur.
13.4.2.3
Test procedure. One method of performing the contact retention test
consists of the following steps:
a.
Determine the direction (axially) in which the test is conducted. Apply a
sufficient seating load (“push” force) to take up any slack of the contact in its
retention system. Avoid any sudden or excessive loads.
b.
Establish the reference (zero displacement) position of the contact. The
contact may be lightly preloaded (3 pounds, maximum) to assure proper
seating.
c.
Apply an axial load to the contact at the rate of approximately one pound per
second, until the specified force has been reached. Maintain the specified
force for five to 10 seconds during which measurement of displacement is
made or the load is removed and displacement measured.
d.
If the test is required in two directions, repeat the aforementioned steps.
13.4.2.4
Typical axial loads and acceptance criteria for contact retention test.
The following example was extracted from the MIL-DTL-5015 connector specification:
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The maximum axial displacement of a contact under the load conditions in the above
table is 0.025 inches.
13.4.3 Coaxial shield pull test. This test is performed to verify the mechanical integrity
of the cable or wire shield to its termination (e.g., connector). It is similar to the pull
force/tensile testing done for crimp connections. The following test methods are
destructive and as such, the test specimen is not suitable for use after testing.
a.
Pull and break – Increasing axial force is applied to the connection until either
the connector and shield separate or the shield breaks.
b.
Pull and return – A specified force is applied to the connection. Once the
specified force is achieved, the force is removed.
c.
Pull and hold – A specified force is applied to the connection and held without
maintaining that peak value for a specified period of time; then the force is
decreased to zero.
d.
Pull, hold and break – The connection is pulled to a specified force and held
for a specified period of time; then the connection is pulled until either the
terminal or contact is separated from the wire or the wire breaks.
In performing the test, the user and/or the manufacturer specifies the required pull force,
the pull rate, and the type of test required.
13.4.4 Torsion Test. The torsion (twist) test may be performed on un-terminated wire or
on completed cable or wire harness assemblies. The test is normally performed as
follows:
a.
The item to be tested is clamped in position in a test apparatus using suitable
clamping heads that allow the test item to be subjected to the applied load
without causing damage to the test specimen.
b.
Select a test sample about 8 inches long and straighten the sample. There
should be no nicks, scratches or other types of deformation in the test sample
that could give false indications during testing. For example, a knick in the
test sample may cause the sample to fail at the knick location; which would
not represent the true strength of the test sample. A slight tension load may be
applied to maintain the test item straight during application of the rotation
cycles.
c.
With the test specimen in the test machine, rotate one end of the specimen at a
constant speed until fracture occurs, while recording the total number of
rotations until fracture. The recommended tensile forces to be applied and the
minimum number of rotations to be applied should be as specified.
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d.
There are many published standards for performing this test. One such
standard is ASTM A 938, Standard Test Method for Torsion Testing of Wire.
13.4.5 User defined mechanical tests. The specific tests listed in IPC/WHMA-A-620,
Section 19 are recommended default tests for use in event the user (customer) has not
invoked any test requirements in the contract. Normally, the user will invoke a standard
test suite that, if not the same as specified in IPC/WHMA-A-620, is similar in scope.
However, specific test equipment, test parameters, and accept/reject criteria may be
different. Additionally, the user may specify different tests. In all instances, unless
otherwise agreed to between the user and the manufacturer, the user identified testing
applies.
References. This document contains some copyright material that is included with
specific documented permission from the sources identified below.
1.
National Instruments Tutorial Entitled Chapter 1: Understanding Key RF Switch
Specifications dated June 20, 2007
2.
Tyco Electronics – AMP, Paper Entitled Crimp Tooling –Where Form Meets
Function
3.
Tyco Electronics – AMP, Paper Entitled Crimp Height – Employing the Most
Effective Crimp Quality metric for Meeting Contemporary Quality Standards
HDBK_620_Test_Nov_26_2007.doc
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