Hi Bob

I can't give you any advice concerning the scale factors for PCB's with
woven Kevlar. But we made a lot of investigation about the reliability of
SMT solder joints on this type of material.
Be careful !! A PCB with Kevlar can have an anisotropic CTE. The Kevlar is
not always symmetrically woven ( depends on the distributor ). This means
there are not the same amount of Kevlar strings in both weaving directions.
As a result, we observed a stunning crack growth rate in the solder joints
of ceramic components that are in the direction with just a few strings. on
the other hand, components with plastic packages show fast growing cracks
in the solder joints if set in the direction with many strings. I got
really angry reactions from DuPond when I published this results. The folks
these explained me that no woven Kevlar is used anymore in PCB's ( due to
the problems explained in my report ) but a fleece with an irregular
distribution of the Keflar strings leading to a less pronounced anisotropy
in the CTE ( hopefully ). However, DuPond never gave us examples of this
material for testing when I offered them the possibility of performing a
series of reliability tests. So I can't tell you anything about this
material. Anyway, it seems as if woven material is still used.

Guenter

Federal Institute of Technology
Reliability Laboratory
Zurich
Switzerland


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