TOC
REF
GLOSSARY QUIT
Section 3
Component and Assembly Issues
IPC Designer Certification Study Guide
TOC
REF
GLOSSARY QUIT
Section 3.1
Considerations for Component
Mounting
Component and Assembly Issues
TOC
REF
GLOSSARY QUIT
Component mounting and attachment
are fast becoming the most important
element of printed board design. The
issues have always been important
because of component density and
conductor routing considerations.
Considerations for
Component Mounting - 3.1
TOC
REF
GLOSSARY QUIT
However, an increase in the complexity of the
assembly process occurs due to:
2221
8.0
8.2.3
• The drive for more functions per assembly.
• Combining surface mount and through-hole
components on one printed board.
• Using both sides of the printed board to attach
the parts (impacts assembly, solder joint
integrity, reliability and testing).
The tradeoffs to be made regarding component
mounting must be considered early in design.
Considerations for
Component Mounting - 3.1
TOC
REF
GLOSSARY QUIT
Through-hole components are mostly
mounted on the side opposite to that
which comes into contact with the solder.
Automatic insertion techniques are
preferred, so rules for these conditions
should be taken into account when
arranging through-hole parts.
Considerations for
Component Mounting - 3.1
TOC
REF
GLOSSARY QUIT
These rules include appropriate
clearances for the insertion heads of the
automatic equipment, and having
sufficient clearance between the lead
diameter and the component hole used
for attachment and electrical connection.
The orientation of the components is
also important.
Considerations for
Component Mounting - 3.1
TOC
2221
8.1.2
8.1.11
8.2.3
Fig
7-1
REF
GLOSSARY QUIT
This includes the direction in which the
components are lined up electrically with
respect to the polarity of polarized
components to one another and usually
with respect to the board edges.
In addition, uniform component orientation,
i.e. all pin #1s located at the lower left,
reduces machine cycle time, thus
controlling cost during the assembly
operation.
Considerations for
Component Mounting - 3.1
TOC
REF
GLOSSARY QUIT
The edge of the board becomes the
design envelope. Except for
connectors, components should not
extend over the edge of the board or
interfere with board mounting.
Design for Assembly (DFA) principles
dictate that the designer also know how
the assembly will be manufactured.
Considerations for
Component Mounting - 3.1
TOC
REF
GLOSSARY QUIT
Automated techniques require that
standard assembly panels be used to
maximize the use and the efficiency of
the equipment. Special fixtures can
accommodate any shape, however,
these fixtures add unnecessary cost to
the assembly process.
Considerations for
Component Mounting - 3.1
TOC
2221
8.1.5
REF
GLOSSARY QUIT
Thus the board perimeter at LMC (least
material condition) should be the boundary
that no component, at MMC (maximum
material condition), extends beyond.
Assembly equipment limitations must be
recognized early in the design process.
Mounting rails for the automatic machines
may require additional clearance. All
requirements should be documented on
the assembly drawing.
Considerations for
Component Mounting - 3.1
TOC
REF
GLOSSARY QUIT
There are many other parameters that must be
considered for component mounting; component
body centering, mounting over conductive areas,
clearance between components, and physical
support are just a few.
When designing mixed assemblies that include
standard SMT parts along with through-hole
parts the designer must have close contact with
the assembly manufacturing representative to
ensure an assembly doesn’t require workarounds of the process being used by the
manufacturer.
Considerations for
Component Mounting - 3.1
TOC
2221
8.1.7
8.1.10
Fig
8-8
REF
GLOSSARY QUIT
Since many services are provided by the
infrastructure of the Electronic
Manufacturing Services Industry (EMSI), it
is preferred to select the company, or
companies, you will work with and tailor the
design to their specific process. It should
be evident that the design is done once,
whereas the board and board assembly are
produced many times. Even design
changes are easier to implement working
with a single or small group of assemblers.
Considerations for
Component Mounting - 3.1
TOC
REF
GLOSSARY QUIT
Another element of component
mounting that must be considered is the
lead clinching requirement.
Is it allowed? Is it required?
Sometimes it is left to the discretion of
the assembly manufacturer. If the
requirements are restrictive they should
be indicated on the assembly drawing.
Considerations for
Component Mounting - 3.1
TOC
REF
GLOSSARY QUIT
Some designs require that the dualinline packages (DIP) have the four
corner leads partially bent to a 30
degree angle. This requirement (and
any others) for lead clinch should be
specified.
Considerations for
Component Mounting - 3.1
TOC
2221
8.3.1
8.3.1.2
8.3.1.3
8.3.1.4
GLOSSARY QUIT
Another element that must be
considered is the electrical test
considerations. Test point lands must
be identified when completing the
component placement. In fact, the test
strategy should be established before
the design starts.
8.3.1.5
Fig
8-19
Fig
8-20
REF
Considerations for
Component Mounting - 3.1
TOC
REF
GLOSSARY QUIT
Section 3.2
Axial and Radial Lead Mounting
Differences
Component and Assembly Issues
TOC
REF
GLOSSARY QUIT
Axial leaded components are throughhole components that have the lead wire
extending from the component body, or
module body, along its longitudinal axis.
The leads normally come out in a straight
line and must be bent. They enter the
holes in the printed board perpendicular
to the body of the component.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
REF
GLOSSARY QUIT
The component is horizontally mounted
with the component body parallel to the
board surface. When bending the leads
care must be taken to avoid damaging
the seal where the lead connects to the
component body.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
2221
8.3.1.6
8.1.6
Fig
8-21
REF
GLOSSARY QUIT
When mounting axial leaded parts, the
designer should space the holes at a
sufficient distance to avoid bending the
lead too close to the body of the
component. The goal being that the body
of the component should be
approximately centered between the two
mounting holes.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
REF
GLOSSARY QUIT
The lead extension serves as a form of
stress relief and therefore, the tighter the
lead is bent to the body of the component
the greater stress that can be transferred
to the component as the board expands
when it heats up in service.
The lead diameter helps to determine
where the bend begins. This is usually
specified as a minimum of one lead
diameter from the component body before
the bend radius starts.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
2221
8.1.11
Fig
8-9
Fig
8-10
REF
GLOSSARY QUIT
If the component has a weld, or other
lead configuration as the lead exits the
component, the amount of straight lead
distance should be considered after the
weld. Although the lead diameter
determines the distance prior to
bending, that dimension should never
be less than 0.75mm [.030"].
Axial and Radial Lead Mounting
Differences - 3.2
TOC
REF
GLOSSARY QUIT
Radial leaded components may have two
or more leads exiting from the body. The
leads usually come from the same surface
and bending may not be necessary as the
leads can be inserted directly into the
holes of the board. The lead spacing of
radial leaded components varies greatly,
so the hole spacing is usually predicated
by the exit of the leads from the body of
the component.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
2221
8.3.1.7
Fig
8-23
Fig
8-24
Fig
8-25
REF
GLOSSARY QUIT
The DIP is an excellent example of a
multiple radial leaded part. The leads
of the DIP need not be bent to enter the
holes because the part was specifically
designed to avoid having to perform the
pre-bend operation. However, some
small capacitors must have their leads
spread slightly in order to have
sufficient clearance between the holes.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
REF
GLOSSARY QUIT
Axial or radial leaded parts are intended
for through-hole mounting. The axial
leaded components are intended to mount
horizontally, while the radial leaded parts
mount in a vertical axis which is
perpendicular to the board.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
2221
8.2.3
8.3.1.8
Fig
8-26
REF
GLOSSARY QUIT
For some designs that have a very dense
packaging requirement, axial leaded parts
may be vertically mounted provided that
they are not too heavy (less than 14
grams), and that they do not extend too
high above the surface of the board
(approximately 15mm). This
characteristic is only appropriate for
through-hole components. Their surface
mount equivalents are always mounted
horizontally, parallel to the board surface.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
REF
GLOSSARY QUIT
Metal Electrical Face (MELF) components are
cylindrical parts that have no leads and are
always mounted so that the solder joint is
between the land on the surface and the metal
face of the part. Thus vertical mounting has
never been considered for this part, however, on
some very dense designs the MELF has been
installed in an unsupported (unplated) hole and
then soldered to the lands on the external layers.
This practice requires good thermal management
to avoid solder cracking as the board expands.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
2221
8.2.3
REF
GLOSSARY QUIT
As intermixing components continue to
prevail, innovations in the design
process become necessary in order to
package all the components and
circuitry within the design envelope.
Through-hole components are
modified for surface mounting; surface
mount leaded parts are modified for
through-hole mounting.
Axial and Radial Lead Mounting
Differences - 3.2
TOC
REF
GLOSSARY QUIT
Section 3.3
Design Differences for SMT vs.
Through-Hole
Component and Assembly Issues
TOC
REF
GLOSSARY QUIT
Surface mounting is the term given to the
method of electrical connection of
components to the surface of the conductive
pattern.
Surface Mount Technology (SMT) does not
utilize component holes.
Design Differences for SMT vs.
Through-Hole - 3.3
TOC
2221
8.1.1.3
8.3.1.9
Fig
8-27
Fig
8-28
REF
GLOSSARY QUIT
The technique is not new. In the
1960s it was called planar mounting.
It came into vogue when ceramic flat
pack components were introduced
which were hot-bar soldered to the
surface of the printed board at a time
when most designs used throughhole leaded components.
Design Differences for SMT vs.
Through-Hole - 3.3
TOC
REF
GLOSSARY QUIT
The industry quickly learned to deal with
intermixing of components that mounted to
the surface, and those that had to be
inserted in plated-though, or unsupported
holes. Because the goal has always been
to reduce the complexity of the
manufacturing and assembly processes,
many designers took components that were
intended for one or the other technique and
modified the leads to accommodate the
majority of the components on the board.
Design Differences for SMT vs.
Through-Hole - 3.3
TOC
2221
8.4.4
Fig
8-33
REF
GLOSSARY QUIT
Through-hole components had their
leads flattened, coined, or bent, so that
they could be surface mounted. And
surface mount parts had leads
configured so that they could be
inserted into holes. Lead temper and
lead length being the major
consideration for that approach.
Design Differences for SMT vs.
Through-Hole - 3.3
TOC
2221
8.1.1.2
8.1.11
8.2.1
8.3.1.9
Fig
8-27
Fig
8-28
REF
GLOSSARY QUIT
The industry, then and today, still mounts
through-hole axial, radial, and multiple leaded
components. When determining the spacing
of the lead bends, several considerations are
taken into account. These include the
distance from the body of the part the leads
can be bent, the lead stiffness, the lead
diameter (used to determine the bend radius)
and the grid system used for the board to
locate as many holes as possible on the
selected grid.
Design Differences for SMT vs.
Through-Hole - 3.3
TOC
2221
8.1.1.2
8.1.11
8.2.1
8.3.1.9
Fig
8-27
Fig
8-28
REF
GLOSSARY QUIT
Intermixing SMT and THT components will be
used within the industry for many years. The
design for manufacturing issues encourage the
designer to work closely with the assembly
process engineers in order to achieve the best
through-put of the assembly operation. The
boards are normally assembled in a panel
format, and require careful consideration as to
how the parts are positioned, oriented, and
arranged, in order to speed-up the assembly
operation.
Design Differences for SMT vs.
Through-Hole - 3.3
TOC
REF
GLOSSARY QUIT
The clearances around the parts are
determined by the maintenance required.
The heads of the insertion or pick and
place equipment play a major role in that
the designer normally leaves room to
provide sufficient clearance for the
clinching mechanisms; however, the
assembly sequence varies from
manufacturer to manufacturer, so the
emphasis is usually placed on how difficult
it is to replace a faulty component.
Design Differences for SMT vs.
Through-Hole - 3.3
TOC
2221
8.2.1
REF
GLOSSARY QUIT
With that parameter taken into
consideration there is usually sufficient
clearance for the component placement
equipment. Some companies provide this
clearance as a standard around the body
and land pattern of each component,
however, there is no general consensus on
what the clearance should be. Much
depends on the density of the design and
whether the component is repairable or
disposable.
Design Differences for SMT vs.
Through-Hole - 3.3
TOC
REF
GLOSSARY QUIT
Section 3.4
Differences Between Automatic &
Manual Insertion
Component and Assembly Issues
TOC
REF
GLOSSARY QUIT
There are many things that must be taken
into account when determining the method
of through-hole insertion. Manual insertion
techniques, although they have greater
flexibility for placing components very
densely or close together, can be error
prone. To reduce the possibility of placing a
part in the wrong location has necessitated
a great variety of manufacturing assembly
aids.
Differences Between Automatic & Manual
Insertion - 3.4
TOC
REF
GLOSSARY QUIT
These aids or systems help to conveyorize
the process so that each operator on an
assembly line has only a few functions to
manage. Components are kitted into
groups which, when kept to a minimum
number of different or similar parts, are
easy to manage.
Differences Between Automatic & Manual
Insertion - 3.4
TOC
REF
GLOSSARY QUIT
Automatic component insertion is the act
or operation of assembling discrete
components to the printed board by
means of electronically controlled
equipment.
The orientation of the components, the
clearance between components, the
sequence in which the parts are to be
assembled, plus many more factors all
become issues that the designer must
address.
Differences Between Automatic & Manual
Insertion - 3.4
TOC
2221
8.1.1
8.1.1.3
REF
GLOSSARY QUIT
The size of the board is also important
since many assembly companies want to
treat the boards in a panel format to ease
board handling. The relationship
between the board fabrication panel and
the assembly panel, plus the amount of
room needed for tooling, or conveyor
guides, are issues that impact the design
methodology and approval of the final
layout.
Differences Between Automatic & Manual
Insertion - 3.4
TOC
REF
GLOSSARY QUIT
The days when printed boards consisted only
of through-hole parts are past. Today’s
designs have intermixed assemblies that
mount both through-hole and surface mount
parts. The parts that were only on one side of
the board are now on both sides, thus the
design process establishing parameters for
component positioning must take into account
the placement, insertion, and attachment
processes used to develop the final assembly.
Differences Between Automatic & Manual
Insertion - 3.4
TOC
REF
GLOSSARY QUIT
Some design facilities pause after initial
component placement in the CAD system
to send a preliminary arrangement to the
assembly company, thus creating a true
concurrent engineering environment.
Differences Between Automatic & Manual
Insertion - 3.4
TOC
REF
GLOSSARY QUIT
Lead clinching is another important
consideration to be taken into account
when deciding if a part is to be manually
or automatically inserted. Most automatic
equipment heads have the feature
capability to clinch leads. They are
trimmed to size and then swaged,
clinched, or partially clinched to retain the
parts during the solder operation.
Differences Between Automatic & Manual
Insertion - 3.4
TOC
2221
8.2.1
8.2.2
8.3.1.2
8.3.1.4
Fig
7-1
REF
GLOSSARY QUIT
It becomes an important parameter for
the designer to understand the exact
methods for part retention in that the
land size or electrical clearance of
adjacent conductors can be affected.
Manual assembly is less precise and
requirements of the design should not
exceed the physical dexterity of the
operator.
Fig
8-19
Differences Between Automatic & Manual
Insertion - 3.4
TOC
REF
GLOSSARY QUIT
Last, but not least, is the relationship
between the hole size and the lead
diameter. In general, automated assembly
requires a slightly larger hole than manual
techniques. The larger hole is intended to
account for the differences in machine
accuracy versus printed board hole location
accuracy.
Differences Between Automatic & Manual
Insertion - 3.4
TOC
2221
8.3.1
2222
9.2.2
Fig
9-3
REF
GLOSSARY QUIT
Some companies provide additional targets
called fiducials to compensate for hole to
machine location mismatch. Other systems
require that the hole is oversize from what it
could be with manual insertion. But, it is
important that the hole diameter should not
exceed the lead diameter too much. If this
occurs the solder might not stay in the
plated-through, or unsupported hole.
The maximum for automated attachment is
usually 0.7mm [.028"] larger than the lead.
Differences Between Automatic & Manual
Insertion - 3.4
TOC
REF
GLOSSARY QUIT
Section 3.5
Manual vs. Pick-and-Place SMT
Placement
Component and Assembly Issues
TOC
REF
GLOSSARY QUIT
Surface mounting (SMT) is the process
of electrically connecting components
to the surface of a conductive pattern
that does not utilize component holes.
The process requires placing the
components on the pattern and
attaching them using solder.
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
2221
8.4
REF
GLOSSARY QUIT
The attachment process can take a
variety of forms but fall into two distinct
groups; those where solder is added to
the joint and those where existing
solder (tin and lead) is reflowed. The
differences between manual and
automatic placement depends a great
deal on the method of attachment.
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
REF
GLOSSARY QUIT
Manual placement has its limitations in
speed and accuracy (even though some
operators have excellent skills in
positioning and hand soldering very small
parts or leads to the surface of the
printed board). To get started in SMT
many companies first experimented with
manual techniques. They positioned
components into a location and added
the solder with a fine tip soldering iron.
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
2221
8.4
REF
GLOSSARY QUIT
As the parts became more exotic or wave
soldered, they used a small dot of adhesive
to secure the part before soldering. So the
process was one of positioning and
soldering. If the solder was already on the
land in the form of solder paste or a solid
solder dot, the manual technique was to use
a hot air device which reflowed the solder.
This hot air technique is currently used in
many assembly operations to remove and
replace defective parts.
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
REF
GLOSSARY QUIT
It is important to know the methods of
attachment in order to take full advantage of
the placement technology. Adding solder is
done manually with solder paste dispensers
(solder dot), or solder wire that contains the
flux. Solder is added automatically by:
•Sending the board through a wave solder
machinewhere a rotating solder wave or
dual waves comes into contact with the
components and the board.
Extra
1.11b
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
REF
GLOSSARY QUIT
•Drag solderingwhere the parts are brought into
contact with a stagnant pool of hot solder.
•Reflow solderingwhich requires that solder is on
the land prior to component placement, and that
following component placement, the board must be
exposed to a heat source in order to get the solder
to liquefy or reflow. Techniques include hot air,
vapor phase, infra-red, or a combination of these.
The reflow may be accomplished in a normal
environment or inert environment where a blanket
of nitrogen gas improves the reflow process.
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
REF
GLOSSARY QUIT
Automatic component placement is
performed by machines that pick up
parts from reels of tape, trays, or
cassettes, hence the name pick-andplace. The placement is very fast and in
most instances, very accurate provided
that the board or panel is properly
registered to the machine origin.
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
REF
GLOSSARY QUIT
The fastest machines are those that
place discrete resistors and capacitors.
These are called “chip shooters.” They
place the parts into solder paste that is
already on the board, or glue them on
the side intended to go through the
wave.
Extra
1.12c
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
REF
GLOSSARY QUIT
Most of these machines use a vacuum
pick-up so the heads do not require any
special added clearance. However,
some components require electrical test
before they are placed and the pick-up
tool must make contact with the
electrodes of the chip in order to test the
part.
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
REF
GLOSSARY QUIT
More complex IC parts that are packaged
in a variety of configurations are also
placed automatically, though not as fast
as the discrete chips. Most require that
they are reflowed using solder paste or
forms of solid solder already on the land.
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
REF
GLOSSARY QUIT
A few components, for example SOT’s
and SOIC’s, can tolerate the temperature
of wave or drag soldering, and may be
reflowed or have the solder added.
These components are attached with
adhesive to position them.
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
2221
8.4
REF
GLOSSARY QUIT
Very complex parts with many leads and
small lead pitch require that the machine
reorient itself to the exact placement
locations. This is accomplished using small
marks, usually dots, known as fiducials.
Typically, machines require 2 fiducials in
opposite corners. They are positioned near
8.1.1.3 the fine-pitch part, and an optical TV camera
senses their location and repositions the
Fig machine’s memory board and part location.
8-1
Manual vs. Pick-and-Place SMT
Placement - 3.5
TOC
REF
GLOSSARY QUIT
Section 3.6
DIP and SIP Components
Component and Assembly Issues
TOC
REF
GLOSSARY QUIT
Dual-inline packaged (DIP) components
are basically rectangular component
packages that have a row of leads
extending from each of the longer sides
of the component body that form, at
right angles, to a plane which is parallel
to the base of the body.
Single-inline packages (SIP’s), are
component packages with one straight
row of pins or wire leads.
DIP and SIP Components - 3.6
TOC
2221
8.3.1.5
Fig
8-20
REF
GLOSSARY QUIT
The typical lead spacing used for
DIP and SIP packages is 2.54mm
(0.100 inch).
Spacing between the rows of leads
is typically 7.62mm (0.300 inch) for
14 and 16 leaded components.
DIP and SIP Components - 3.6
TOC
REF
GLOSSARY QUIT
DIPs were designed primarily to be
mounted as through-hole parts.
Their leads are usually ribbon leads.
The ribbon leads are positioned in a
plated-through hole that is large
enough to accommodate the width of
the lead.
DIP and SIP Components - 3.6
TOC
CM770
10.3.1
10.3.1.1
10.3.2
Fig
10-6
REF
GLOSSARY QUIT
The four corner leads are usually bent
slightly to retain the part during wave
soldering. One reason not all leads
are clinched is to facilitate part
removal; one need only heat the
solder slightly, and remove the part
without the concern of lead
unclinching (further discussion about
clinched and unclinched leads in a
following section).
DIP and SIP Components - 3.6
TOC
10.3.2
CM770
10.3.1.2
Fig
10-7
REF
GLOSSARY QUIT
Both DIPs and SIP’s have surface mount
capability. The requirements being that the
leads must be formed on a mandrel with
specific tooling to ensure proper bending of
the lead into a surface mount gull wing
configuration.
One caution is that the plastic of the DIP and
SIP are not necessarily designed to withstand
the heat of reflow soldering, therefore, special
attention is required to make certain that the
body of the part is compatible with the
assembly process.
DIP and SIP Components - 3.6
TOC
REF
Section 3.7
DIP and Chip Carrier
Sockets
Component and Assembly Issues
GLOSSARY QUIT
TOC
REF
GLOSSARY QUIT
Some designs require the use of a
socket to interconnect the part to the
board. Sockets are usually used if the
part to be connected frequently needs
replacing or the part cannot tolerate
exposure to high soldering temperatures.
Most discrete components are not in this
category since they were designed to
withstand assembly processes.
DIP and Chip Carrier
Sockets - 3.7
TOC
REF
GLOSSARY QUIT
However, occasionally a part must be
selected at the time of assembly testing
to fine tune the circuit. When this is
Extra
1.13 required, and it is not desired to solder
the “as required part” in place, sockets
2221 may be used for the discrete part. In this
8.2.5.2
instance they are a low-profile part, of a
grip-type device, that are like miniature
connectors with a single metal contact
for each of the leads of the component.
DIP and Chip Carrier
Sockets - 3.7
TOC
REF
GLOSSARY QUIT
The use of sockets for semiconductor
devices is much more prevalent
especially in the frequent up-grading of
certain equipment.
The ability to easily remove a part and
replace it with one that provides
additional capability has been a
practice for many years.
DIP and Chip Carrier
Sockets - 3.7
TOC
REF
GLOSSARY QUIT
When sockets are used, their pin numbers
are assigned according to the device
inserted in the socket. The DIP socket is
soldered into the board at the same time
the other components are wave soldered.
The sockets are low-cost, and can be
assembled at high-density, since they are
mainly an electro-mechanical part. DIP
sockets are generally designed to resist
solder wicking into the socket or cavity.
DIP and Chip Carrier
Sockets - 3.7
TOC
REF
GLOSSARY QUIT
When surface mounting first became popular the
leadless ceramic chip carrier led the way to
show the benefit of very high density circuit
designs. Unfortunately, the coefficient of thermal
expansion (CTE) of the part did not match that of
the printed board. Many articles appeared in
technical journals indicating a common problem
with cracked solder joints when the CTE
mismatch between part and board exceeded the
limits of the component to survive the thermal
cycles to which it was subjected.
DIP and Chip Carrier
Sockets - 3.7
TOC
REF
GLOSSARY QUIT
As the industry learned about surface
mounting many companies switched to
leaded parts; others analyzed the amount of
solder required and made sure that the
solder volume was sufficient to account for
the CTE mismatch. Solder gave some of
the same compliancy characteristics that
were provided by the leads.
DIP and Chip Carrier
Sockets - 3.7
TOC
Extra
1.14
REF
GLOSSARY QUIT
A third approach was to stay with the
leadless chip carrier and provide a
socket that could function to
accomplish several things. One was
to permit ease of installation or
replacement of the chip carrier, the
other was to eliminate the surface
mount CTE mismatch problem.
DIP and Chip Carrier
Sockets - 3.7
TOC
REF
GLOSSARY QUIT
The chip carrier sockets were
designed to be through-hole mounted
or surface mounted depending on the
termination configuration of the socket
contacts. When through-hole
mounted, the lead spacing for the chip
carrier socket was normally that of the
standard 2.54mm [.100"] centers
configuration.
DIP and Chip Carrier
Sockets - 3.7
TOC
REF
GLOSSARY QUIT
The surface mount version of the chip
carrier socket usually had the additional
feature of a mechanical interface. This
prevented the stress from different
expansion characteristics from getting
to the solder joints.
DIP and Chip Carrier
Sockets - 3.7
TOC
REF
GLOSSARY QUIT
In fact some sockets had only a
mechanical hold-down and relied on the
mechanical contact pressure to interface
to the board. The advantage was that
the sockets were easy to replace and
the board was not damaged due to
soldering and desoldering. The
disadvantage, however, is that there was
now a second mechanical interface in
the circuit path.
DIP and Chip Carrier
Sockets - 3.7
TOC
REF
GLOSSARY QUIT
Due to the added cost of the socket
and the assembly operation most
companies have re-thought the
solutions and options available for ease
of part exchange or curing CTE
mismatch problems.
DIP and Chip Carrier
Sockets - 3.7
TOC
Section 3.8
Clinched and
Unclinched Leads
Component and Assembly Issues
REF
GLOSSARY QUIT
TOC
REF
GLOSSARY QUIT
Clinched leads are leads of through-hole
components where the lead material is of
a temper that permits bending. The lead
is inserted into a hole in a printed board
and when it exits is bent down to the
surface of the land 75 to 90 degrees from
a vertical line perpendicular to the board.
Clinched and
Unclinched Leads - 3.8
TOC
REF
GLOSSARY QUIT
This technique is used when maximum
mechanical retention of a lead or
terminal is required by the design.
The component holes into which the lead
is inserted can be plated-through holes,
unsupported holes, or eyeleted holes.
Clinched and
Unclinched Leads - 3.8
TOC
2221
8.3.1.2
8.3.1.3
8.3.1.4
REF
GLOSSARY QUIT
In some designs leads are clinched to
adjacent lands where the lead comes
through an unsupported hole.
The purpose for this design parameter is
that leads can be individually unsoldered
and bent straight to allow for easy
removal of multiple leaded parts.
Clinched and
Unclinched Leads - 3.8
TOC
Fig
8-19
REF
GLOSSARY QUIT
Unclinched leads are those that go straight
through the board or are partially bent for
retention during soldering. If no clinching
requirements are specified on the assembly
drawing, then unclinched lead termination
requirements apply in most instances.
Leads of parts that have hard tempered leads
or that of connectors are usually straight
through leads. They extend out from the
board range of 0.25-2.0mm [.010-.080 inches].
Clinched and
Unclinched Leads - 3.8
TOC
2221
8.3.1.3
8.3.1.5
Fig
8-20
REF
GLOSSARY QUIT
Clinched leads are not applicable for
DIPs. Bends are limited to 30 degrees
maximum outward from the axis of the
hole with a maximum of 4 leads per
component and not more than 2 leads
on one side.
Clinched and
Unclinched Leads - 3.8
TOC
REF
GLOSSARY QUIT
For component mounting requirements in
nonplated-through holes (unsupported
holes), the leads must be clinched. This is
in order to ensure proper solder attachment
to the lands of a single sided board. For
tempered leads that cannot be clinched, or
for leads greater than 1.3mm (.05 inches) in
diameter, the hole to lead ratio must be kept
as tight as possible in order to ensure a
proper solder joint.
Clinched and
Unclinched Leads - 3.8
TOC
REF
Section 3.9
Edge Board Connectors
Component and Assembly Issues
GLOSSARY QUIT
TOC
Fig
8-11
REF
GLOSSARY QUIT
Edge-board connectors are used to
allow the printed board to be
“plugged into” the equipment for
which it is intended. An edge-board
connector is used specifically for
making nonpermanent
interconnections with the edge-board
contacts on a printed board.
Edge Board Connectors - 3.9
TOC
2221
8.2.5.3
Fig
8-11
Fig
8-12
Fig
8-13
REF
GLOSSARY QUIT
These board contacts are near any
edge of a board that is designed to
mate with the connector. Thus, the
configuration uses the board as the
plug of the connector system with the
plated conductors being the male
contacts.
8.2.5.6
Fig
8-15
Edge Board Connectors - 3.9
TOC
REF
GLOSSARY QUIT
Plating on the copper of the board
contact varies depending on the amount
of insertions to which the connector
system must be subjected.
The most popular technique is to plate
the copper contact with a nickel plate
followed by an over-plate of gold.
Edge Board Connectors - 3.9
TOC
REF
GLOSSARY QUIT
The nickel plating serves a dual function.
It provides an anvil effect under the gold
adding an essential extra hardness to the
gold, and the nickel is an effective barrier
layer preventing diffusion of copper into
the gold.
Thicknesses of the nickel and gold vary
depending on the application and the
number of insertions required.
Edge Board Connectors - 3.9
TOC
2221
4.4.4
4.4.5
4.4.8
Table
4-3
REF
GLOSSARY QUIT
When the use of gold became too
expensive companies looked for
alternatives and came up with many
other plating possibilities such as
rhodium, tin-nickel, palladium-nickel,
bright acid tin, gold dot, etc.; however,
nickel-gold is the most preferred plating
to date.
Table
4-4
Edge Board Connectors - 3.9
TOC
REF
GLOSSARY QUIT
Designing the board for edge-board
mating requires important considerations
on how the board is dimensioned and
configured. The width of the printed board
edge (tang) must be dimensioned so that
when the width of the tang reaches its
maximum material condition (MMC) the
tang will be no greater that the throat of
the mating connector.
Edge Board Connectors - 3.9
TOC
Fig
8-12
REF
GLOSSARY QUIT
There is also special board processing
involved to permit ease of insertion and
thus prevent undue wear or damage of
the board. This process consists of
beveling (chamfering) the leading edge
and corners of the board tang.
Edge Board Connectors - 3.9
TOC
2221
8.2.5.1
8.2.5.3
Fig
8-11
Fig
8-12
Fig
8-13
REF
GLOSSARY QUIT
In addition, some designs require a
keying slot to prevent mating the board
in the wrong connector. The keying slot
must be dimensioned so as not to
interfere with the throat of the connector
at the time the board keying slot mates
with the connector keying pin.
Edge Board Connectors - 3.9
TOC
REF
GLOSSARY QUIT
With an edge-board connector system
the responsibility for proper mating and
unmating is shared between the board
manufacturer and the connector
supplier. At times this can be risky if
the system is to be exposed to any
severe environmental exposures.
Edge Board Connectors - 3.9
TOC
2221
8.2.5.4
8.2.5.5
REF
GLOSSARY QUIT
One solution is to use a two piece
connector system where both portions
of the connector interface are made by
the connector manufacturer. In this
manner the connector system may be
proven through stress testing to
ascertain that the system will meet the
end-use conditions.
Edge Board Connectors - 3.9
TOC
REF
GLOSSARY QUIT
Mounting of the female portion of the
two part system is similar to that of
mounting a card-edge connector.
These are usually mounted on a
backplane or mother board with the pins
of the connector being soldered or
press-fit into plated-through holes of the
board.
Edge Board Connectors - 3.9
TOC
2221
8.2.5.4
8.2.5.5
Fig
8-14
REF
GLOSSARY QUIT
The connector on the plug end is
usually positioned where a right angle
pin comes out of the connector body at
90 degrees to the board surface.
The right angle pins are soldered into
plated-though holes on the daughter
board.
Edge Board Connectors - 3.9
TOC
REF
GLOSSARY QUIT
Mounting of the mother board or backplane
and its relationship to the card guides, used
to bring the daughter boards into correct
alignment with the connector cavities on the
motherboard, is critical. A detailed tolerance
analysis is required to make certain that the
parts align properly. This is done to ensure
that the insertion force required to mate the
connecting parts is not so great as to
damage any of the parts if someone
attempts to force the connection.
Edge Board Connectors - 3.9
TOC
REF
GLOSSARY QUIT
Section 3.10
Bus Bar Mounting Characteristics
Component and Assembly Issues
TOC
REF
GLOSSARY QUIT
A bus is considered to be one or more
conductors used for transmitting data
signals or power. In printed board design
a bus can also be configured as a plane,
such as might be included in a multilayer
construction. Therefore, a voltage plane is
a conductor layer, or portion of a layer, that
serves as a common voltage source at
other than ground potential for an
electrical circuit, shielding, or heat sinking.
Bus Bar Mounting Characteristics 3.10
TOC
REF
GLOSSARY QUIT
This definition is almost identical for a
ground plane with the only difference
being that the plane serves as a
common reference for circuit returns.
Thus a bus bar is thought of as a
conduit. The conduit can be a
component or a single conductor on a
printed board that is used for distributing
electrical energy.
Bus Bar Mounting Characteristics 3.10
TOC
2221
8.2.13
CM770
Fig
17-10
REF
GLOSSARY QUIT
Bus bars are usually preformed
components. They are part of the
assembly, just as any other component,
and serve the function to provide most,
if not all, of the power and ground
distribution.
CM770
Fig
17-11
Bus Bar Mounting Characteristics 3.10
TOC
REF
GLOSSARY QUIT
The primary reason for using a
separate bus bar as opposed to
having a plane or printed conductor
on the printed board, is that it
conserves the real estate of the
board for more signal routing.
Bus Bar Mounting Characteristics 3.10
TOC
2221
8.2.13
CM770
Fig
17-10
Fig
17-11
REF
GLOSSARY QUIT
Many electrical engineering views
are that to have a substantial
voltage and ground, copper or other
metals are necessary (i.e. bus bar).
By using the separately
manufactured bus bar component it
minimizes the use of board circuitry
for power and/or ground distribution.
Bus Bar Mounting Characteristics 3.10
TOC
REF
GLOSSARY QUIT
The number of conductor levels in the
bus bar depends on the application. In
fact, the application controls many of
the physical characteristics of the bus
bar itself, or the bus bar system.
Bus Bar Mounting Characteristics 3.10
TOC
2221
8.2.13
CM770
Fig
17-10
Fig
17-11
REF
GLOSSARY QUIT
In normal printed board component
mounting requirements one of the rules is
to never have more than one lead in a
plated-through hole. Bus bars are the only
physical components permitted to violate
those rules. Some bus bar systems have
the leads of the bus bar share the same
holes as the integrated circuit and in
several instances have been placed under
ICs in DIP configurations.
Bus Bar Mounting Characteristics 3.10
TOC
REF
GLOSSARY QUIT
The number of terminals, the size and finish
of the terminals, and the dielectric strength
of the insulation around the bus bar or
between bus bar conductor levels is
determined as part of the electronic
packaging strategy.
The interface between the bus bar and the
printed board is usually at the platedthrough hole. In addition, this interface
should be in a uniform termination pattern.
Bus Bar Mounting Characteristics 3.10
TOC
2221
8.2.13
CM770
Fig
17-10
Fig
17-11
REF
GLOSSARY QUIT
All of these parameters should be
defined in the procurement document
that defines the characteristics of the bus
bar, and the assembly drawing that
specifies the sequence of assembly and
the use of automated or manual
techniques required to complete the
assembly operation.
Bus Bar Mounting Characteristics 3.10
TOC
Section 3.11
Purpose of Eyelets
Component and Assembly Issues
REF
GLOSSARY QUIT
TOC
REF
GLOSSARY QUIT
An eyelet is a short metallic tube the
ends of which can be formed outward in
order to fasten it within a hole in material
such as a printed board. Eyelets have
been used for many years. Their primary
purpose is to provide mechanical support
to the printed board.
Extra
1.15
Purpose of Eyelets - 3.11
TOC
REF
GLOSSARY QUIT
They are accepted in most designs when
they perform a mechanical function.
Examples of their use are as
reinforcement for card puller holes where
a metal hook is inserted in the eyeleted
hole so that the board assembly can be
dislodged from the connector, and
reinforcement for mechanical hardware
hold down (screws, studs, etc.).
Purpose of Eyelets - 3.11
TOC
REF
GLOSSARY QUIT
In the early days of printed circuit
technology eyelets were also used to
provide electrical interconnection from one
side of a printed board to another. Under
these design rules eyelets were treated
exactly like terminals and were soldered to
the land patterns. But, if both sides were
not soldered simultaneously the possibility
existed for developing cold solder joints.
Purpose of Eyelets - 3.11
TOC
REF
GLOSSARY QUIT
A cold solder joint is defined as a solder
connection that exhibits poor wetting, and is
characterized by a grayish, porous appearance.
If the surface to be soldered is clean, cold
solder joints are primarily caused by insufficient
heat during the soldering process. When one
side of an eyelet is soldered in place the joint is
good; soldering the opposite side re-melts the
first side causing the joint to reform or resolidify
without sufficient heat being applied.
Purpose of Eyelets - 3.11
TOC
REF
GLOSSARY QUIT
When designers decided that it was
acceptable to solder a lead into the
eyelet the concerns over bad solder
joints was magnified. If care was taken
to heat both the eyelet and the lead
uniformly, the resultant joint was reliable.
However, there were no guarantees that
the interfacial connection could be
trusted.
Purpose of Eyelets - 3.11
TOC
2221
8.2.10
2222
9.2.1.3
Table
9-3
REF
GLOSSARY QUIT
With the advent of plated-through holes,
eyelets went out of favor as a method for
making interfacial connections. They are
certainly good reinforcement for leads that
must be soldered and unsoldered many
times. Thus, for a single sided board they
are perfectly permissible. They are also
permissible for a double sided board,
provided they are not used to make
interconnections between the two outer
surfaces.
Purpose of Eyelets - 3.11
TOC
2221
8.2.10
2222
9.2.1.3
Table
9-3
REF
GLOSSARY QUIT
The relationship between the outer barrel of
the eyelet to the inner diameter of the drilled
hole is critical. There should not be excessive
clearance between the two surfaces otherwise
the flaring of the eyelet will not seat properly.
If there is too much room the eyelet barrel
may collapse to one of the side walls of the
hole.
The requirement is that the difference
between the hole diameter and the outside
diameter of the eyelet barrel shall not exceed
0.15mm [.006"].
Purpose of Eyelets - 3.11
TOC
REF
GLOSSARY QUIT
Section 3.12
Point-to-Point (Jumper) Wires
Component and Assembly Issues
TOC
REF
GLOSSARY QUIT
Wires are put on a board to facilitate
interconnections between various lands.
A point-to-point, or jumper wire is a discrete
electrical connection that is part of the basic
conductive pattern formed on a printed
board. This is opposed to a haywire which
is a discrete electrical connection that is
added to a printed board to modify the basic
conductive pattern formed on the board.
Point-to-Point (Jumper) Wires - 3.12
TOC
Extra
1.16
REF
GLOSSARY QUIT
Thus, jumper wires that are a part of the
assembly must be clearly defined on the
assembly drawing. The wire is not considered
part of the printed board but is a part of the
assembly process. Adequate provisions for
electrical testing should be made where
jumper wires are included in the circuit. This is
because jumper wires are attached
components meaning no electrical connection
is present when bare board testing is done
(reflected as a discontinuity in the circuit).
Point-to-Point (Jumper) Wires - 3.12
TOC
REF
GLOSSARY QUIT
Jumper wires are usually of the following type:
• Bare bus wireconsists of a single strand of wire
with sufficient cross section to be compatible with
the electrical requirements of the circuit.
• Sleeve bus wireconsists of a single strand of bare
bus wire that is covered by insulation tubing.
• Insulated bus wireconsists of a single strand of
wire that has its own insulation when purchased,
either a plastic or varnish coating
• An insulated stranded wireconsists of multiple
strands of wire that is purchased with a plastic
coating. Note: See telephone
jumpers JP1,JP2,JP3
Point-to-Point (Jumper) Wires - 3.12
TOC
REF
GLOSSARY QUIT
They are considered components and should
therefore be permanently fixed to the printed
board at intervals not to exceed 25.4mm [1.0”].
2221
8.2.11.1
When bare bus wires are used, they should be
no longer than 25.4mm [1.0”] and should
never cross over conductors of the bare
board.
The shortest XY path for jumper wires should
be used unless the board design
considerations dictate otherwise.
Point-to-Point (Jumper) Wires - 3.12
TOC
REF
GLOSSARY QUIT
If the jumper is to be inserted into a
through-hole the lead bending
requirements should conform to
normal component bend radii. The
bending feature is predicated by the
value of the lead diameter. Jumpers
with lead diameters up to 0.75mm
[.030"] should have a minimum bend
radius of the jumper wire diameter.
Point-to-Point (Jumper) Wires - 3.12
TOC
2221
8.1.11
8.2.11.3
Fig
8-9
REF
GLOSSARY QUIT
Thus, the radius is equal to one
diameter. If the jumper wire diameter
is larger, the minimum bend radius
should be increased.
As an example, for diameters 0.75 to
1.2mm [.030" to .047"] the bend
radius is 1.5 diameters, and for larger
wires the minimum bend radius is 2
diameters.
Point-to-Point (Jumper) Wires - 3.12
TOC
Extra
3.12
REF
GLOSSARY QUIT
Sometimes jumpers are attached to
terminals. There is a large variety of
terminal types. These include single/double-ended, or single-/multi-sectioned
turret solder terminals, as well as bifurcated
terminals. When using a turret terminal the
jumper is attached by wrapping the lead
around the terminal prior to soldering. The
leads are wrapped around the terminal a
minimum of 180 degrees. If there is more
than one lead the leads must not overlap.
Point-to-Point (Jumper) Wires - 3.12
TOC
2221
8.2.9
8.2.9.1
Fig
8-17
8.2.9.2
Fig
8-18
REF
GLOSSARY QUIT
When using bifurcated terminals the jumper
wire may be laid straight between the tines of
two terminals (though some experts prefer a
positive positioning and require that the wire
be bent to the side when attached). The
details of jumper wire dressing should be
documented on the assembly drawing.
The requirements are specified in sufficient
detail in the soldering document identified as
J-STD-001. This standard was jointly
developed by the IPC and the EIA committees.
Point-to-Point (Jumper) Wires - 3.12
TOC
REF
GLOSSARY QUIT
Jumpers may also be surface mounted.
Some designers have used small zero ohm
resistors as jumpers. This technique is
quite effective since the parts are added at
the same time that other resistors and
capacitors are attached. The rules for land
patterns and solder joint formation are
identical for resistors performing an
electrical function as that of the zero ohm
resistor.
Point-to-Point (Jumper) Wires - 3.12
TOC
REF
GLOSSARY QUIT
Also, a wire may be soldered to a surface
land as a jumper. When this technique is
used it is usually a good idea to stake the
wire in position with a dot of adhesive. Glue
dots can be added to the assembly
automatically. If the wire is in the right
orientation it can be soldered to the surface
land pattern during the wave solder or
reflow solder process.
Point-to-Point (Jumper) Wires - 3.12
TOC
REF
GLOSSARY QUIT
Round wires can be coined or flattened if
they are to be surface mounted. Although
this feature is not mandatory, it is good
practice so that the surface to surface
area between the wire and the land is
maximized.
Point-to-Point (Jumper) Wires - 3.12
TOC
REF
GLOSSARY QUIT
• Which side of the board should through-hole
components be mounted on, if the board is to
be machine (wave) soldered?
–
–
–
–
either side of the printed board
both sides of the printed board
the side that is in contact with the solder
the side opposite that which is in contact with the
solder
Answer: the side opposite that which is in contact with the solder
Quiz 3
TOC
REF
GLOSSARY QUIT
• What is the basic difference in part mounting
techniques of SMT vs. through-hole
components?
–
–
–
–
through-holes require plating
SMT devices require no holes
SMT lands are smaller in area
solder joints are formed differently
Answer: SMT devices require no holes
Quiz 3
TOC
REF
GLOSSARY QUIT
• What is the maximum lead protrusion from
the surface of the plating or the foil for
standard leaded components attached to the
board with straight through leads?
–
–
–
–
1.0mm [0.040”]
1.5mm [0.060”]
2.0mm [0.080”]
2.3mm [0.092”]
Answer: 1.5mm [0.060”]
Quiz 3
TOC
REF
GLOSSARY QUIT
• What is the purpose of the key or keying slot
in a two-piece or card-edge connector?
–
–
–
–
to restrict the use of connector types
to validate the need for two-piece connectors
to prevent improper insertion into the connector
to differentiate between printed board connector
types
Answer: to prevent improper insertion into the connector
Quiz 3
TOC
REF
GLOSSARY QUIT
• Which aspects of a component data sheet are
the most important to facilitate component
layout?
–
–
–
–
the electrical properties
the maximum power consumption
the physical description of the component
the thermal heat distribution characteristics
Answer: the physical description of the component
Quiz 3
TOC
REF
GLOSSARY QUIT
• How many component leads can be mounted
in any one hole used for attaching the
component to the circuit pattern?
–
–
–
–
one lead
no more than two leads
no more than three leads
no more than four leads
Answer: one lead
Quiz 3
TOC
REF
GLOSSARY QUIT
• What is the primary purpose for breaking up
large conductive areas on external layers?
–
–
–
–
to prevent copper blistering
to enhance copper adhesion
to prevent solder mask blistering
to enhance component soldering
Answer: to prevent solder mask blistering
Quiz 3
TOC
REF
GLOSSARY QUIT
• What is the most probable cause of a
through-hole multilayer board assembly
repeatedly containing defective solder joints
on the power and ground connections?
–
–
–
–
the solder dwell is too short
component leads are too large
the board orientation is incorrect
component through-holes are not thermally
relieved
Answer: component through-holes are not thermally relieved
Quiz 3