Tin whiskers are often related to plating type in which the high stress
bright tin plate will often initiate whisker growth; however, reflowing will
decrease but not eliminate tin whiskers from growing in on metal substrates.
I have seen several cases where whiskers tin whiskers grew on brass
substrates that had been plated and then fused.  A compressive stress
apparently develops as the part cools from fusing.  I have produced whiskers
in a vacuum chamber during temperature cycling; so a humid condition is
conducive, but not the total cause.  Bending to put the tin in compression is
also an inducement, but I have not been able to produce it on demand.
Reaching certain temperatures (about 120C) and cooling slowly seems to
decrease occurrences, probably due to a decrease in dislocation density in
the coating.  The single crystal that shoots out of the coating appears to be
completely disrupted by practically any contaminant such as lead or even
copper as is seen in Sn99.3 Cu 0.7. I have not been able get whiskers to
start in any of the high-tin alloy solders even when coated on brass plates
and bent to produce compression and stored at 60 Deg. C and 65%RH.

Conformal coating is also an improvement, but have seen a case where they
penetrated a sprayed urethane coating.

International Tin Research Institute (UK) has a good paper on tin whiskers
that also gives some good references.  I understands that have done work on
trying to induce whisker growth in many of the lead-free solder.

I was not aware of the NASA site that J. Kadesch mentioned, but I will go
look at it with great interest.

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