Hi Paul,
IPC-SM-785 recommends an accelerated aging process, not a burn-in, prior to
accelerated reliability testing, to bring the solder joint microstructure to
about the level of a 1-year-old solder joint. This prevents unrealistically
optimistic results--many test results reported in the literature however
ignore this recommendation and thus give artificially long fatigue lives.
I am with you in finding shear tests not particularly helpful. The absolute
values are meaningless in terms of reliability--the fracture surfaces tell me
however if there was good wetting or not. Fine-grained solder joints are
stronger and will give you higher shear stress values; but properly wetted
solder joints have ample strength for anything happening to them (with the
exception of single-sided PCAs), and there is no correlation between shear
(or pull) strength test results and fatigue reliability.
Studies with finishes have not shown a significant first-or second-order
impact on SJ reliability; data on lead-free solders are too incomplete and/or
improperly obtained and thus a specific reliability model is as yet not
obtainable. The available data do however indicate that the choice of solder
does likely have no more than a second-order effect--the recommendations in
IPC-D-279 will still serve you well.
Werner Engelmaier

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