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The proposed revision of IPC-4101 - Specification for Base Materials for Rigid and Multilayer Printed Boards contains new
slash sheets describing FR-4 base materials compatible with lead free assembly. It has been reported in countless papers and
presentations that the new silver/tin/copper and other lead free solders mandate processing temperatures 20-30°C higher than
the time proven tin/lead solder. The six new lead free specifications were developed and incorporated into the IPC-4101B
document in order specify FR-4 base materials with more robust processing windows for lead free assembly. While it is true
that some “more mature” FR-4 products can be used successfully in lead free assembly,most fabricators and assemblers have
found that no single FR-4 product works in all situations.
This paper discusses the new tests incorporated into IPC-4101B,the development of the requirements for lead free FR-4 and
the solutions to various requirements / issues that have surfaced due to the lead free assembly movement.

The regulation requiring lead-free solders for printed circuit boards (PCBs) has presented a number of challenges to our industry at virtually every stage of the process. For the brominated epoxy resins used to make a large fraction of the laminates that serve as the starting material for PCBs,the requirement for higher thermal stability has led to increased use of phenolic curing agents in place of dicy. The good news is that brominated formulations are available that can be used to make laminates that meet the highest IPC specifications for glass transition temperature (Tg > 170 °C) and decomposition temperature (Td > 340 °C). However,phenolic curing agents lead to some compromises in other properties,especially toughness and copper adhesion. A general comparison of the properties of phenolic cure vs dicy cure will be made,along with some guidelines for application. The thermal stability of non-brominated resins is inherently greater than that of brominated resins,and meeting the highest Td specifications is not a challenge,even with dicy cure. Therefore non-brominated resins are well suited for lead free applications. Furthermore,unfilled non-brominated resins have improved dielectric properties and lower densities. However,non-halogenated resins are more expensive and exhibit greater water absorption. Achieving high Tg’s (> 170 °C) can be a challenge,but this is now possible with commercially available materials. A comparison between brominated and non-brominated resins will be made.

Silicones are often used to protect electronic applications designed for cold environments. The low temperature flexibility
of silicones is well recognized,but not as well understood. While the actual Tg of silicones is about -120°C,they do
transition from a soft rubber to a harder rubber around -45°C. At the transition their hardness,strength and modulus
increase slightly while elongation decreases slightly. Very soft silicone gels show the greatest change,becoming more
rubbery and in some cases showing tears. These tears can self-heal within a few weeks at warmer temperatures.
The specific temperature where these changes take place is shown to be dependant on the rate of cooling. Slow cooling
will show this transition around -45°C for many silicone elastomers. Analytical tests may indicate performance limits that
are too conservative. On the other hand,rapid cooling and short dwell times often used in thermal shock and cycling tests
may not detect stress changes that could occur in slower cooling conditions unless a sufficiently long low temperature soak
is included in the testing regime. Lower temperature versions of silicones are available that do not show property
transitions until -80°C or even until -120°C.

While glass fibers are commonly used to reinforce circuit board substrates,they have a high dielectric constant and loss.
Cyclic olefin copolymer fibers have a lower dielectric constant and loss. By combining these fibers with glass fibers in
unique hybrid cloths,we have made circuit board substrate materials with a dielectric constant of 3.08 and loss tangent of
0.013 using standard epoxy resins that are common in FR-4 glass reinforced substrates. The comparative glass materials had
a dielectric constant of 4.49 and loss of 0.019. In another embodiment,the cyclic olefin copolymer fibers were melted to
form the resin component,yielding a substrate with dielectric constant of 3.25 and loss tangent of 0.0013. In the last
example,a special low dielectric resin was used,giving a substrate with dielectric constant of 2.8 and loss tangent of 0.0009.
Substrates made from this fiber have passed Peel Strength,Solder Float,Water Uptake,and have a low coefficient of thermal
expansion.

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CTE mismatches during temperature cycling can cause significant movement of components and materials relative to one
another,which in turn stresses solder joints and wirebonds. Silicones are often used to protect electronic devices,and the
large CTE of silicones can be of particular concern,though the very low modulus of these materials tends to greatly mitigate
stress. While a few examples of potential damage caused by thermal expansion of silicones do exist,a level of understanding
of the magnitude of expansion pressures would be helpful in design,validation,and thermal reliability testing.
A study was begun to measure the pressures developed during thermal expansion of typical silicone encapsulants used in
electronic applications. A range of products was evaluated to encompass very soft gels to medium hard elastomers. Harder
elastomers were found to generate 0.2 psi/degree C rise in temperature,while soft gels generated <0.01 psi/C and extremely
soft gels <0.001 psi/C. Application variables such as part design geometry are discussed and shown to potentially multiply or
concentrate the generated pressures. Validation of experimental results was made with a case study.

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As electronic packages keep shrinking in size,high-speed high-accuracy package assembly equipment are becoming more
sensitive to mechanical vibrations. Alignment,positional and pick & place accuracy can be compromised during high speed
operation due to sensitivity to mechanical vibrations which are generated within the assembly equipment,as well as from the
floor and neighboring machines. Vibrations can also affect process yield as a result of diminished accuracy. Semiconductor
package manufacturers are under constant pressure to increase output,at the same time are challenged by tighter alignment
accuracy requirements.
This paper describes the methodology used to characterize the vibration response of manufacturing equipment under
vibrating floor environment,and that of the manufacturing floor under vibration induced by operational equipment. Case
study of pick & place equipment shows significant differences in vibration levels were found in different building facilities
and factory floor conditions. Also described in the paper is reason why a standard/specification is needed for purchased
assembly equipment. Industry standard needs to be developed that can help specify how much vibration can be transmitted
from the machine to the floor,and how much internal and external vibration the machine should be able to withstand without
affecting performance or quality and reliability. Examples of possible solutions that manufacturers and equipment suppliers
can adopt to meet future needs for precision high-speed placement equipment are also discussed.