Dave,
There has been incidents of solder joint fatigue failures in avionics, space,
automotive, medical, computer, telecomm applications, etc..., involving
LCCCs, PLCCs, QFPs, TSOPs and others. Of course, these incidents are not much
talked about. Sources of problems went from a design being unfit for a a
given application (high ramp rates resulting in overstress, lack of
compliance or too large of a CTE mismatch) to not enough solder, gold
embrittlement etc... most of these problems could have been avoided by
upfront design-for-reliability work (as opposed to "one of those
after-the-fact dilemmas"). I also wonder about all those NTFs
(no-trouble-found) where the board is just thrown away without failure mode
analysis??
On the subject of life predictions, there are several engineering approaches
to choose from, each with their own scope of application, strengths and
weaknesses. Coffin-Manson is a subset of Morrow's generalized fatigue law
where the fatigue life is correlated to cyclic inelastic strain energy.
Strain energy is used in: IPC model for leaded assemblies, Darveaux's model
(chap 13 in Lau's Ball Grid Array Technologies), "Solder Reliability
Solutions" (Clech, Proceedings, SMI'96, p. 136) and others. For SMT
assemblies, the strain energy approach has been found to work better because
it accounts for the entire stress/strain cycle (hysteresis loops). The
Coffin-Manson approach is strain driven only (but it works in some cases).
National labs are also working on 3D finite-element models with
microstructure evolution, which will provide detailed fundamental insights in
solder fatigue. Last I heard, computations were being switched from a Cray
(where it takes one week to get the first couple of hystereris loops) to a
massively parallel machine.
Jean-Paul
________________________________________________________________
Jean-Paul Clech
EPSI Inc., P. O. Box 1522, Montclair, NJ 07042 - USA
tel: +1 (201)746-3796 fax: + 1 (201)655-0815
Home page: <A HREF="http://members.aol.com/Epsiinc1/index.html">http://members
.aol.com/Epsiinc1/index.html</A>
- click on the above or copy: http://members.aol.com/Epsiinc1/index.html
__________________________________________________________________
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