TOC
REF
Section 1
Design Considerations
IPC Designer Certification Study Guide
GLOSSARY QUIT
TOC
REF
GLOSSARY QUIT
Section 1.1
Interrelated Considerations for
Design
Design Considerations
TOC
REF
GLOSSARY QUIT
The end product requirements are the
characteristics of an individual part or
assembly in its final completed state.
To ensure the part or assembly will work
as intended, the environment in which it
will operate must be known at the time
of the design.
Interrelated Considerations for
Design - 1.1
TOC
2221
3.2.1
SM782
3.5.2
3.5.3
Table
3-6
REF
GLOSSARY QUIT
Equipment environmental conditions
such as ambient temperature, heat
generated by components, ventilation,
shock, vibration, etc., necessitate
different materials, tolerances, and
final product configurations.
Interrelated Considerations for
Design - 1.1
TOC
REF
GLOSSARY QUIT
To facilitate communication between
the designer and manufacturers,
different performance classes have
been developed to reflect progressive
increases in sophistication, functional
performance, and frequency and/or
intensity of inspection or stress testing.
Interrelated Considerations for
Design - 1.1
TOC
REF
GLOSSARY QUIT
These are identified by a class designation
where:
• Class 1 is defined as General Electronic
Products
• Class 2 is defined as Dedicated Service
Electronic Products
• Class 3 is defined as High Reliability
Electronic Products
Interrelated Considerations for
Design - 1.1
TOC
2221
1.6.2
2222
1.5.1
Extra
1.1
REF
GLOSSARY QUIT
Each has its definition which can be
related to end use environments such
as computers, telecommunications,
aerospace, or automotive applications.
The user has the responsibility to
determine the class to which their
product belongs.
Interrelated Considerations for
Design - 1.1
TOC
REF
GLOSSARY QUIT
Producibility levels have also been
established to help communicate the
design complexity to the manufacturer.
These levels reflect progressive
increases in sophistication of tooling,
materials or processing and, therefore
progressive increases in fabrication
costs.
Interrelated Considerations for
Design - 1.1
TOC
REF
GLOSSARY QUIT
There are three levels of producibility:
Level A: General Design Complexity -Preferred
Level B: Moderate Design Complexity -Standard
Level C: High Design Complexity -Reduced Producibility
Interrelated Considerations for
Design - 1.1
TOC
REF
GLOSSARY QUIT
2221
1.6.3
Most printed board manufacturers can
produce Level A products at a very high
yield and, therefore, at a reasonable
cost. The number of available
manufacturers that have the precision
capability to manufacture product at the
C level drops to approximately 20%.
Interrelated Considerations
for Design - 1.1
TOC
REF
GLOSSARY QUIT
The number of manufacturers able to
accommodate designs that are state-ofthe-art and need even greater
sophistication of tooling, materials, and
processing is around 1%. State-of-the-art
technology cannot be standardized due to
the fact that every few years, the levels
shift causing that which was moderate to
become general, and so on.
Interrelated Considerations
for Design - 1.1
TOC
REF
GLOSSARY QUIT
Since the design is intended to meet all
the requirements of the product including
performance, cost, reliability, etc., all of
the issues must be discussed at the
beginning of the design process.
These discussions should include
manufacturing engineering for both the
board assembly and test.
Interrelated Considerations
for Design - 1.1
TOC
REF
GLOSSARY QUIT
If a company wishes to stay competitive
with the products they sell to the
customer, the days of tossing the design
“over the wall” are gone. The designer
must be aware if “backward compatibility”
is required. This means the design must
be able to be used in any past installation
without modifications.
Extra
1.2
Interrelated Considerations for
Design - 1.1
TOC
2221
3.2
3.5
SM782
3.4
3.4.4
REF
GLOSSARY QUIT
The physical constraints of the
installation interface are the primary
consideration when redesigning a
board for a product already in the field.
Thus, design for excellence is design
for producibility and involves all the
disciplines needed to manufacture and
maintain the product.
Interrelated Considerations for
Design - 1.1
TOC
REF
Section 1.2
Copper Clad Laminates
Design Considerations
GLOSSARY QUIT
TOC
REF
GLOSSARY QUIT
Copper clad laminates used to produce
printed boards consist of three parts:
2221
4.1
• the resin - which is a natural or synthetic
resinous material
• the reinforcement - such as different forms
of paper, matte glass,
or woven glass
• the copper foil
Copper Clad Laminates - 1.2
TOC
REF
GLOSSARY QUIT
The resin and reinforcement make up
the base material which is the insulating
material upon which a conductive
pattern may be formed. Base material
may be rigid or flexible or both. It may
be a dielectric or insulated metal sheet.
Copper Clad Laminates - 1.2
TOC
REF
GLOSSARY QUIT
Product safety may require that the
base materials withstand tests
performed by a product safety agency,
such as Underwriters Laboratories.
Tests include flame retardance (UL 94),
printed board construction (UL 796), and
base material (UL746).
Copper Clad Laminates - 1.2
TOC
2221
4.1.1
4.2.1.2
4.3
2222
4.3
Table
4-1
REF
GLOSSARY QUIT
Copper forms the cladding and is
available in two types; rolled annealed,
or electrodeposited (ED). Unclad base
material can also be used when
producing printed boards using additive
technology, where copper is deposited
only where required.
Copper Clad Laminates - 1.2
TOC
REF
GLOSSARY QUIT
The most popular laminate resin
system is epoxy. The most common
thicknesses for laminate for rigid
base material are 0.75mm (.030
inches), 1.5mm (.060 inches), and
02.4mm (.090 inches); however,
minimum thicknesses for rigid base
material is 0.05mm (.002 inches).
Copper Clad Laminates - 1.2
TOC
2222
4.3
Table
4-1
REF
GLOSSARY QUIT
There are various improvements that
have been made to epoxy resins over
time. These include difunctional
epoxy, multifunctional epoxy, BT
epoxy and others. All of the
improvements are intended to
provide better and more consistent
dimensional stability and minimal
thermal expansion characteristics.
Copper Clad Laminates - 1.2
TOC
REF
GLOSSARY QUIT
The most popular reinforcement is
woven glass. It provides structural
strength to the resin and comes in
various thicknesses to accommodate
the various thicknesses of the sheets
used to produce the laminate. A sheet
of woven glass that has been coated
with resin is referred to as prepreg or
preimpregnated reinforcement.
Copper Clad Laminates - 1.2
TOC
2221
4.2.1.2
4.3
REF
GLOSSARY QUIT
This material is also known as “B” stage
since it is at a partial stage of cure of the
resin.
B stage material can be handled,
combined with other sheets, and then
laminated under heat and pressure to
form the base material.
Extra
1.3
Copper Clad Laminates - 1.2
TOC
REF
GLOSSARY QUIT
Copper foil thickness is defined in
ounces. The origin of the practice comes
from the days when a copper foil was
used to cover roofs. Therefore, half
ounce copper is defined as the weight of
a square foot of copper foil that is 17
micrometers thick (0.0007 inches).
Copper Clad Laminates - 1.2
TOC
2221
4.4.9.1
Table
4-5
REF
GLOSSARY QUIT
Subtractive boards can be made by the
process that starts with copper foil which is
then plated and etched where the
unwanted copper is removed.
Printed boards can also be additively
produced. This is where the copper is
patterned on a bare laminate in an
electroless process (no electricity involved
in moving the atoms of copper to the
surface of the board).
Copper Clad Laminates - 1.2
TOC
REF
GLOSSARY QUIT
New resins are appearing on the scene.
These include such products as
cyanate ester, polyimide, or PTFE
(teflon); however, epoxy resin is still the
most popular resin used in the United
States.
Copper Clad Laminates - 1.2
TOC
REF
GLOSSARY QUIT
Another polymer material used with the
Extra
printed board is an epoxy permanent
1.4
polymer coating known as solder mask.
2222
The fact that the polymers are very
4.3.1
similar to the laminate permits good
2222 adhesion of the two systems to one
Fig 4.1
another.
2222
4.3.2
Copper Clad Laminates - 1.2
TOC
Copper Clad Laminates - 1.2
REF
GLOSSARY QUIT
TOC
REF
GLOSSARY QUIT
Section 1.3
Thermal Management Techniques
for Printed Boards
Design Considerations
TOC
REF
GLOSSARY QUIT
Material selection for the printed board is
an essential element of the design to
accomplish the structural strength
properties needed to:
• support the electronic components
• handle any vibrational requirements
• dissipate the heat from the conductors
and the components
Thermal Management Techniques
for Printed Boards - 1.3
TOC
2221
4.1
4.1.1
7.3.1
Extra
1.5
Table
7-4
2222
Table
4-1
REF
GLOSSARY QUIT
The characteristics that must be
understood by the designer are the
safe continuous operating temperature
of the materials and the coefficients of
thermal expansion
-1,1,(CTE). Some of
these properties are provided by the
reinforcement.
Fig
7-2
Thermal Management Techniques
for Printed Boards - 1.3
TOC
REF
GLOSSARY QUIT
Copper is a relatively good conductor
of heat as well as being the main
material used to conduct electricity.
This capability permits the use of large
planes of copper to perform the heat
sinking function necessary to keep the
board and the board assembly cool.
Thermal Management Techniques
for Printed Boards - 1.3
TOC
REF
GLOSSARY QUIT
Depending on the amount of current being
drawn through the circuit conductors, they
can also become a heat generator.
Therefore, proper conductor width and
thickness are characteristics that must be
checked to ensure that the wattage being
passed through the conductor does not
raise the temperature of the copper above
a safe temperature, which may increase the
failure rate of a printed board.
Thermal Management Techniques
for Printed Boards - 1.3
TOC
REF
GLOSSARY QUIT
Charts are usually used to determine
the original heat management goals
for the conductors. These are later
verified when the whole assembly
comes together and the total heat
generated by the components and
the conductors are assessed.
2221
Fig 6.4
Thermal Management Techniques
for Printed Boards - 1.3
TOC
REF
GLOSSARY QUIT
The resin system responds to heat and
temperature cycling by expanding. The
higher the resin content, the greater the
expansion rate. This expansion causes
a strain on the barrels of the plated
through-holes. Insufficient copper in the
hole, or having copper that is not ductile
are factors that influence the ability to
withstand the thermal strain.
Thermal Management Techniques
for Printed Boards - 1.3
TOC
REF
GLOSSARY QUIT
The hole size also is a contributor to the
equation since the circumference of the
barrel provides more copper volume in the
larger holes. The combination of
reinforcement and resin together provide
the expansion model for the thickness or Z
axis of the board. The industry defines the
expansion rate in parts per million (ppm/ C
or a percentage of the thickness.
Thermal Management Techniques
for Printed Boards - 1.3
TOC
2222
Table
4-1
REF
GLOSSARY QUIT
All the laminate constructions have a very
uniform rate of Z axis expansion until they
reach a particular point.
2221
7.3.1
Fig
7-2
Table
7-4
At that temperature the expansion rate
increases dramatically and the most
damage can occur. This point is known as
the glass transition temperature, or Tg.
Many laminate structures are sold by their
Tg capability.
Thermal Management Techniques
for Printed Boards - 1.3
TOC
REF
GLOSSARY QUIT
The reinforcement accounts for the
dimensional characteristics of expansion
in the X and Y axis. Some designers look
for product that has high dimensional
stability characteristics. In this regard,
they are trying to achieve a low CTE of
the material in the X and Y axis to reduce
the thermal mismatch between the board
and the components.
Thermal Management Techniques
for Printed Boards - 1.3
TOC
REF
GLOSSARY QUIT
Another way to accomplish the same
properties is to include a special plane
in the board. These constraining core
planes are also very effective in
removing the heat from hot spots or hot
components.
Thermal Management Techniques
for Printed Boards - 1.3
TOC
2221
5.2.6
7.3.1
Fig
5-2
Fig
5-3ab
Fig
7-2
REF
GLOSSARY QUIT
Heat is transferred to the plane through
plated through-holes that are filled with a
conductive material. Known as thermal vias
they conduct heat away from the
component, or other hot areas, and move
the heat to the cooler planes. Planes are
then connected to the frame of the housing
containing the board. Thus heat is moved
by conduction to the cooler surface.
Extra
1.6
Thermal Management Techniques
for Printed Boards - 1.3
TOC
REF
GLOSSARY QUIT
Section 1.4
Thermal Management Techniques
for Assemblies
Design Considerations
TOC
REF
GLOSSARY QUIT
Temperature management is one of the
most important aspects of printed board
assembly design. As electrons pass
through components and board circuitry,
heat is generated. The component
manufacturer usually specifies the amount
of heat that each component generates
based on the way it is used in the circuit.
Thermal Management Techniques
for Assemblies - 1.4
TOC
REF
GLOSSARY QUIT
Heat is generated mostly by complex
integrated circuits, power transistors,
transformers, and any component that
draws a great amount of current; however,
even resistors or capacitors can create
heat if they are not compatible with the
rated current that they must manage.
Thermal Management Techniques
for Assemblies - 1.4
TOC
2221
7.0
REF
GLOSSARY QUIT
Most of these conditions relate to the
number of watts that a component needs
to manage. Thus passive components
for the same value will be manufactured
to handle different current capabilities.
They are then specified as 1/2 watt, 1
watt, 2 watt, etc.
Thermal Management Techniques
for Assemblies - 1.4
TOC
REF
GLOSSARY QUIT
Heat and thermal cycling is the enemy
of board performance and long term
reliability. Thus, the components
should be distributed as evenly as
possible across the board and oriented
in a position which allows the best
airflow over the components.
Thermal Management Techniques
for Assemblies - 1.4
TOC
REF
GLOSSARY QUIT
Components that run too hot affect their
neighbors; change value or even fail.
They can also damage the printed board
mounting substrate, and cause problems
in the solder joints that attach the
components to the circuitry. Part of this
problem is created by the difference in
the coefficient of thermal expansion
(CTE).
Thermal Management Techniques
for Assemblies - 1.4
TOC
2221
7.3.2
7.3.3
Fig
7-2
REF
GLOSSARY QUIT
The definition of CTE is that it represents
the linear dimensional change of a material
per unit change in temperature. The
variation can create a thermal expansion
mismatch between the component and the
printed board, which places a mechanical
stress on the solder joint. The problem is
not too severe with through-hole
components, however, it can be very
detrimental with surface mounted parts.
Thermal Management Techniques
for Assemblies - 1.4
TOC
REF
GLOSSARY QUIT
Solder in sufficient volume provides a
certain amount of relief. Solder, when
heated, becomes plastic in nature, thus
the mechanical stress is taken up by the
solder providing a certain amount of
compliancy. Now the enemy becomes
thermal cycling. If the assembly is
continually turned on and off, the
components become hot and then cool.
Thermal Management Techniques
for Assemblies - 1.4
TOC
REF
GLOSSARY QUIT
These conditions cause the solder to change
states many times and eventually can cause a
crack in the solder due to the solder joint
fatiguing under the strain of the continual
change from plastic to brittle. The greater the
difference between the high and low ends of
the temperature excursion and the greater the
number of cycles, the sooner the problem can
become noticeable. The first indication is an
intermittent signal; the final is an open joint.
Extra
1.5a
Thermal Management Techniques
for Assemblies - 1.4
TOC
REF
GLOSSARY QUIT
For all of the obvious reasons it is
important to keep the assembly as
cool as possible. Components can be
individually provided with a heat sink
to transfer or dissipate the excess
temperature to the air or to another
solid member that may be cooler.
Thermal Management Techniques
for Assemblies - 1.4
TOC
2221
7.2
7.2.1
8.1.10
Fig
8-8
REF
GLOSSARY QUIT
When the heat is transferred to the printed
board, the design must be able to
accommodate the distribution adequately.
Organic resins used to manufacture
laminate for printed board manufacture or
the reinforcements used in such laminate
are not good conductors of heat, therefore
the function must be transferred to the
metal (usually copper) planes.
Thermal Management Techniques
for Assemblies - 1.4
TOC
REF
GLOSSARY QUIT
There are many techniques for heat sinking
components. Included in these are the
simple method of contact with a heat sinking
plane to the more exotic of mounting the part
in a specially designed finned metal bracket
used to allow the ambient air to cool the part.
Additionally, with some components,
thermally conductive adhesive or paste can
help transfer the heat effectively.
Extra
1.6b
Thermal Management Techniques
for Assemblies - 1.4
TOC
Thermal Management Techniques
for Assemblies - 1.4
REF
GLOSSARY QUIT
TOC
REF
GLOSSARY QUIT
Section 1.5
Testing Techniques and Procedures
Design Considerations
TOC
REF
GLOSSARY QUIT
Developing a test strategy should be done at
the beginning of any design. It is important
to involve everyone from the manufacturing
cycle in the decision process that determines
what will be tested and when it will be tested.
At a minimum, the testability review team
should include:
• the board manufacturer
• the assembly process engineer
• testing experts from both disciplines
• the circuit designer
Extra
1.7
Testing Techniques and
Procedures - 1.5
TOC
2221
3.5
REF
GLOSSARY QUIT
The circuit engineer knows the robustness
and maturity of their circuit; s/he determines
whether in-circuit testing is necessary or
whether functional testing will suffice. Incircuit testing consists of applying test signal
directly to a device’s input terminals and
sensing the results directly from the device’s
output terminals. In this manner it can be
determined if the part is functioning as
intended, or in its simplest form, electrical
continuity.
Testing Techniques and
Procedures - 1.5
TOC
REF
GLOSSARY QUIT
Functional testing analyzes the unit under
test. This is usually done from the
connector(s) of the board assembly and
circuitry is considered a complete functional
testing entity. The circuit is stimulated
through the connector by applying specified
inputs to exercise the circuit. By sensing
the circuit outputs the operator can
determine if the circuit is operating correctly.
Testing Techniques and
Procedures - 1.5
TOC
REF
GLOSSARY QUIT
Both in-circuit and functional testing are
performed on the entire assembly,
therefore, the decision as to what
approach to take many times is
predicated on the complexity of the
assembly. In theory, functional testing
as a go/no go condition would be all that
is necessary to ensure that a circuit
works properly.
Testing Techniques and
Procedures - 1.5
TOC
3.5.3.1
3.5.4
3.5.4.1
REF
GLOSSARY QUIT
Some customers require that this test is
performed while the assembly is at an
elevated temperature. Burn in is the
process of electrically stressing a
device at an elevated temperature for
the purpose of sorting out infant
mortality.
Testing Techniques and
Procedures - 1.5
TOC
2221
3.5.3.5
Extra
1.7b
REF
GLOSSARY QUIT
Functional testing saves cost, however
if the circuit fails the test, either at
functional test or burn in, the fault must
be located. This usually requires
special test points on the critical nets at
specified node points so that the fault
can be isolated. These points are the
same points that may be used to
perform in-circuit testing using a bed-ofnails tester.
Testing Techniques and
Procedures - 1.5
TOC
REF
GLOSSARY QUIT
Thus from a layout point of view, the
critical circuit elements must still be
fanned-out to locations away from the
part to probe the portion of the circuit
being evaluated. Probe points are
spring loaded pins that are positioned in
a bed-of-nails tester. They are usually
on some standard grid to simplify the
probe point fixture design.
Testing Techniques and
Procedures - 1.5
TOC
3.5.6.2
REF
GLOSSARY QUIT
The recommended land size on the
board to accommodate probe points
is 1.0mm [.040”] for a round land, and
0.9mm [.036”] for a square land.
3.5.6.5
Fig
3-1
The land may be a via or just a plain
circuit feature, or even a wide
exposed conductor.
Fig
3-3
Testing Techniques and
Procedures - 1.5
TOC
REF
GLOSSARY QUIT
Assembly testing can require as many
as one point for every node or net in the
circuit (full nodal access), or only for
those nodes or nets that are critical
components that are hard to establish
as being at fault in circuit performance
(partial nodal access).
Testing Techniques and
Procedures - 1.5
TOC
REF
GLOSSARY QUIT
A strategy is developed at the start of the
layout to provide the direction as to
where to leave room for the test points.
Manufacturing must be involved to make
sure that the tooling hole concept
matches the fixture of the tester, and if
the board is tested while still in a panel
format, how the inputs are applied.
Boards are usually separated from the
panel for functional testing.
Testing Techniques and
Procedures - 1.5
TOC
2221
3.5.6.1
REF
GLOSSARY QUIT
Bare board testing is done to prove
that the printed board has full continuity
and that there are no short circuits. In
order to facilitate communication of
problem areas, and accurately locate
probe sites, it is essential that grid
locations and net names be specified.
Testing Techniques and
Procedures - 1.5
TOC
3.5.1
REF
GLOSSARY QUIT
The probe points are long solid pins
that make contact with the conductors
on the outer surfaces of the board.
Probes must make contact with the end
of every net. A voltage is applied to
each conductor set to ensure electrical
continuity.
Apdx.
A
Testing Techniques and
Procedures - 1.5
TOC
REF
GLOSSARY QUIT
Section 1.6
Reliability Terms and Issues
Design Considerations
TOC
REF
GLOSSARY QUIT
Designing for reliability means
considering all the factors necessary to
establish the probability that a device or
assembly will function properly for a
defined period of time under the influence
of specific environmental and operational
conditions. This is the expected product
life or the printed board failure rate.
Reliability Terms and
Design Issues - 1.6
TOC
REF
GLOSSARY QUIT
This is usually identified in terms of
Mean Time Between Failures (MTBF);
the higher the number of hours, the
more reliable a part is. However, some
products only function once in their life
cycle, therefore a different measure to
determine reliability is established.
Reliability Terms and
Design Issues - 1.6
TOC
2221
7.0
REF
GLOSSARY QUIT
Stress conditioning is applied to the
product in terms of temperature cycling,
vibration, and/or shock to determine that
the product will survive these Highly
Accelerated Stress Test (HAST)
conditions.
Extra
1.7c
Reliability Terms and
Design Issues - 1.6
TOC
REF
GLOSSARY QUIT
The main failure mechanism in a printed
board assembly, besides a component
failure, is a cracked plated through-hole or
a cracked solder joint. Both conditions can
occur due to the thermal exposure of the
assembly to temperature variation. The
larger the range between the upper and
lower temperature working conditions, the
greater the stress and potential for a failure.
Reliability Terms and
Design Issues - 1.6
TOC
REF
GLOSSARY QUIT
The number of thermal cycles that a part
will experience during its service life is also
a significant factor in determining reliability.
Accelerated test conditions will many times
indicate early failures or infant mortality
conditions, however to prove true reliability
often requires long exposure to conditions
intended to simulate field exposure.
Reliability Terms and
Design Issues - 1.6
TOC
2221
7.3.3
Table
7-4
REF
GLOSSARY QUIT
As an example:
automotive under-the-hood products
must survive both extreme cold and hot
temperatures.
Reliability Terms and
Design Issues - 1.6
TOC
REF
GLOSSARY QUIT
In developing a new board structure,
many companies find that it is beneficial
to incorporate a test coupon into the
board production panel. The coupon can
be specially designed to provide
information on the reliability of the final
product. The test coupon allows
destructive testing of the board structure,
without sacrificing a good board.
Reliability Terms and
Design Issues - 1.6
TOC
REF
GLOSSARY QUIT
Plated through-hole structures can be
destructively conditioned and then
examined in order to determine the
robustness of the design and the
manufacturing processes. A high quality
manufacturing process will not improve on
the reliability of the design; however, a
poor quality manufacturing process can
have a detrimental effect on the reliability.
Reliability Terms and
Design Issues - 1.6
TOC
REF
GLOSSARY QUIT
Balanced construction of the printed
board about the center of the board and
on individual layers can improve the
performance of the plated through-hole
survival. Hole diameter, aspect ratio,
plating thickness, as well as copper
plating ductility are some of the factors
related to hole failures.
Reliability Terms and
Design Issues - 1.6
TOC
REF
GLOSSARY QUIT
For the assembly the common failure is
the solder joint.
Material properties of the substrate
compared to the component’s
coefficient of thermal expansion
suggest keeping this mismatch as low
as possible.
Reliability Terms and
Design Issues - 1.6
TOC
REF
GLOSSARY QUIT
Leaded parts, or parts that have good
solder volume between the part termination
and the board land pattern, are more likely
to be able to withstand thermal swings,
thereby extending the fatigue life of the
solder joint. The lead or solder joint
compliancy helps to offset the stresses that
occur when the board expands at a greater
rate than the component body.
Reliability Terms and
Design Issues - 1.6
TOC
REF
GLOSSARY QUIT
The choice of material reinforcement
in many instances can help limit the
expansion of the board in the X and Y
axis, thus improving the conditions of
CTE mismatch.
Reliability Terms and
Design Issues - 1.6
TOC
2221
7.3.3
Table
7-4
REF
GLOSSARY QUIT
Balancing all the elements of the design to
arrive at the optimum solution is the
challenge of the printed board designer.
For the product that sees dramatic thermal
stress during its life cycle, the evaluation is
crucial; for a product that must function in a
benign environment the issues are not as
critical.
Reliability Terms and
Design Issues - 1.6
TOC
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Section 1.7
Purpose of Tooling Holes
Design Considerations
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A tooling hole is a physical feature in
the form of a hole, or slot, on a printed
board fabrication panel or assembly
panel. Tooling features are used
exclusively to position a printed board
or assembly during fabrication
processes, assembly, and test
processes.
Purpose of Tooling Holes - 1.7
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This includes:
• registration of phototooling
• positioning core layers during lamination
• panels during drilling
• boards at bare board testing
• panels or boards during automated
assembly
• functional test
Purpose of Tooling Holes - 1.7
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Layer-to-layer registration is the
manner in which lands and holes are
related to one another within a
multilayer board. Tooling holes are
used to assure the accuracy of the
relationship.
Purpose of Tooling Holes - 1.7
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The designer is responsible for indicating
the tooling holes that stay with the board
or panel.
The board manufacturer is responsible
for determining the tooling holes needed
for board fabrication.
Purpose of Tooling Holes - 1.7
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Since it is preferred to assemble boards in
panel format, many designers specify the
assembly panel as the required purchase
item with its inherent tooling holes or
features needed by the assembly
processes. The holes are often used to
position the board mechanically on the
conveyer and define the relationship
between the tooling hole location and the
location or position of the components.
Purpose of Tooling Holes - 1.7
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If the tooling holes are not required
after depanelization (for programming,
test, etc.), it is preferred to place them
in the borders of the panel.
Purpose of Tooling Holes - 1.7
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Tooling holes are unsupported holes
(i.e. a hole in a printed board that does
not contain plating or other type of
reinforcement), and are toleranced
tightly in order to avoid movement
between the tooling pin and the board.
This is especially important if the holes
are being used for registration.
Purpose of Tooling Holes - 1.7
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9.2.1.1
9.2.1.2
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Pins are usually very precise with
tolerances in the range of 0.025mm
[.001”] or less. The holes also have very
precise tolerances which are generally in
the range of 0.05mm [.002”]. Maximum
Material Condition (MMC) and Least
Material Condition (LMC) are terms used
to describe the relationship between the
hole and the pin.
Purpose of Tooling Holes - 1.7
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Line to line conditions are considered as an
interference fit, thus the MMC of the hole
(when the hole is smallest) is usually
considered with as small a clearance as
possible with the MMC of the pin (when the
pin is as large as possible). A 0.025mm
[.001”] clearance is usually sufficient provided
that the hole does not get too large or the pin
too small.
Extra
1.7d
Purpose of Tooling Holes - 1.7
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The tolerances for tooling holes are tighter
than those used for leads of components.
To assure accuracy of the tooling system,
they are handled with special care by the
manufacturer.
Purpose of Tooling Holes - 1.7
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Since tooling systems generally have several
holes involved in the system, various
methods are used to avoid mechanical stress
as the pins and holes are aligned.
A popular tooling system used by board
manufacturers employs round pins positioned
in slots. This reduces the surface contact of
the pin with the board and avoids a stress
build-up.
Purpose of Tooling Holes - 1.7
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Often the slots are positioned at the
centers (near the edge) or at the four
corners of the panel.
Another technique for reducing the stress
is to use pins that are diamond shaped in
a round hole.
Purpose of Tooling Holes - 1.7
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Section 1.8
Purpose of Stiffeners
Design Considerations
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Stiffeners are mechanical parts that are
added to a printed board in order to reduce
the possibility of bow & twist, they also
provide rigidity to the assembly. Stiffeners
can come in a variety of configurations with
the simplest form being a piece of sheet
metal, or sheet metal bent into an “L”
shaped bracket, to that of extruded metal
shapes specifically designed to be attached
to a printed board and provide the stiffness
needed to resist bending.
Purpose of Stiffeners - 1.8
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8.2.7
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Stiffeners may be made from a variety of
different materials. They may be made of
a dielectric (do not conduct electricity) or
they may be made of metal (do conduct
electricity). Steel and aluminum are the
most popular metals with aluminum being
the choice if the stiffener is an extruded
shape. While metal stiffeners are popular,
they must have an adequate finish to
protect the metal from corrosion.
Purpose of Stiffeners - 1.8
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The proper spacing must be maintained
between the stiffener and components
mounted on the board and conductors.
The component mounting sequence must
be considered so that the stiffener is not in
the way of the insertion or pick-and-place
heads. Therefore, it is important for the
designer to have a clear understanding of
the assembly operation and the physical
clearances required. Electrical clearance
between the stiffener and the conductors
can be achieved in a variety of ways.
Purpose of Stiffeners - 1.8
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If the electrical clearance conditions are
tight, fiber or plastic insulators can be
located between the stiffener and the
conductors. It should be noted that moisture
traps can occur between the board and the
stiffener depending on how it is mounted or
secured. Another way to provide isolation of
the metal is to make the protective coating
serve the dual function of protecting and
insulating. A few examples of these are
epoxy coated metal and porcelainized steel.
Purpose of Stiffeners - 1.8
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Stiffeners may be screwed, glued, or
riveted to the board. Riveting appears
to be the preferred method because it
makes a positive connection and will
not come apart easily. With riveting,
the stiffener becomes an integral part of
the board.
Purpose of Stiffeners - 1.8
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The use of screws requires an extra
assembly operation which takes additional
time. And the use of an adhesive or glue
requires that the adhesive is compatible
with the surface conditions of the board
and the stiffener. Depending on the
method of attachment, the techniques for
documentation of the stiffener
requirements also vary.
Purpose of Stiffeners - 1.8
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If the stiffener is glued to the board and is
made of laminate, it could become part of
the master drawing and be fabricated at
the same time the board is laminated; if
screwed on, it could be part of the
assembly drawing; and if riveted, it could
be the first assembly operation.
It should be noted that some companies
have a separate drawing for riveting
stiffeners.
Purpose of Stiffeners - 1.8
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No matter what the technique,
stiffeners prevent flexing of the printed
board thus reducing the incidence of
solder or copper foil cracking during
shock and vibration exposure.
Extra
1.7e
Purpose of Stiffeners - 1.8
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• Who assigns the class to which a printed
board belongs?
–
–
–
–
the printed board user
the bare board fabricator
the assembly manufacturer
the government procuring agency
Answer: the printed board user
Quiz1
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• The major function of a design team, if
assembled prior to starting the layout, is to
ensure which condition?
– that the time-to-market schedules can be met
– that customer requirements are properly
addressed
– that all factors for design and production are
considered
– that bill of material and electronic diagrams are
ready to release
Answer: that all factors for design and production are considered
Quiz1
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• If a tooling pin is 2.85mm to 2.87mm [.112” to
.113”], what is the size of the optimal tooling
hole?
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2.85-2.90mm [.112-.114”]
2.90-2.95mm [.114-.116”]
2.90-3.00mm [.114-.118”]
2.95-3.10mm [.116-.122”]
Answer: 2.90-2.95mm [.114-.116”]
Quiz1
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• How is the location of tooling holes
determined by the assembler?
– the holes are visually inspected and are not plated
through
– the holes are documented and indicated on the
master drawing
– the holes are documented and indicated on the
assembly drawing
– the holes are visually inspected and have no
electrical significance
Answer: the holes are documented and indicated on the
assembly drawing
Quiz1
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• Proper tooling hole systems consist of a
certain number of holes which are positioned
in certain locations on the board. What are
their characteristics?
–
–
–
–
two tooling holes in opposite corners
three tooling holes near each of three corners
two tooling holes located on the break-away tabs
a specific number and location to facilitate the
assembly operation
Answer: a specific number and location to facilitate the
assembly operation
Quiz1
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• What are the 3 most common dielectric
thicknesses of copper-clad laminate used to
produce double-sided printed boards?
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–
–
–
–
0.50mm [.020”]
0.75mm [.030”]
1.00mm [.040”]
1.50mm [.060”]
2.00mm [.080”]
2.40mm [.090”]
Answer: 0.75mm [.030”], 1.5mm [.060”], 2.4mm [.090”]
Quiz1
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• Copper thickness is defined in ounces based
on the weight of a square foot of foil material.
What is the thickness of one ounce copper?
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–
–
–
–
–
–
0.009mm [0.0004”]
0.017mm [0.0007”]
0.025mm [0.0010”]
0.035mm [0.0014”]
0.050mm [0.0020”]
0.070mm [0.0028”]
0.075mm [0.0030”]
Answer: 0.035mm 0.0014”
Quiz1
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• What is the most commonly used resin
system for producing rigid printed boards?
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–
–
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PTFE
epoxy
polyimide
cyanate ester
Answer: epoxy
Quiz1