Hi Bill,
What I said in 1994, and many times before and this that date, still holds 
even—or perhaps more so—for Pb-free solders.
Ahh, the vicissitudes of the Pb-free idiocy.
The problem is less so with reliability predictions for product in the 
one-cycle-a-day arena, in both the cases of SnPb and the SAC-flavor of the month the 
creep process is complete at the operating temperatures and essentially 
complete in the off-state (room temperature). My tentative recommendation for 
product reliability is to use a model developed for SnPb [that expressly excludes 
straight Coffin-Manson and Norris-Landzberg] and use a safety factor of about 
3. That can be fine-tuned as actual field reliability data become available 
[beware of the billions of Pb-free solder joints claimed; this is not data, is 
anecdotal at best (or at its worst) and for throwaway products].
Recent data at various conferences show that SAC solder joints perform equal 
to or somewhat worse than control-SnPb solder joints in accelerated testing. 
Side-by-side accelerated testing will always show a higher stress range for SAC 
solder joints [higher modulus of elasticity] and a smaller strain range [much 
slower creep rates] as compared to control-SnPb solder joints. So depending 
on test environment and test arrangement, the hysteresis loops being the 
measure of the cyclic visco-plastic strain energy and thus creep-fatigue damage, 
will be more or less the same.
Because we do not have acceleration models for SAC solders, prediction of 
accelerated test results is not possible at this time. Nor for that matter is 
extrapolation to product reliability from the results of accelerated testing 
[that is the reason that IPC-9701A is not a performance standard for Pb-free 
solders].
QUESTION: "Would someone please say again what the dwell time at extremes 
should be for lead-free solder systems in order to experience the full CTE 
hysteresis loop?
ANSWER: We do not know enough do give an answer to this question—but see the 
3rd paragraph above for a practical answer.

Esteemed or otherwise,
Werner



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