Hi Joe
There is a very good paper on the subject of tin wiskers in the proceedings of current IPC Expo "Tin Whisker Growth in Electronic Hardware" by D. F. Ciambrone of Raytheon Corporation in El Segundo, CA
It is very well reseached with 100 references. The bottom line is contained in the first two paragraphs of the conclusions copied below:
"The electronics industry is again rediscovering the tin
whisker problem. Tin whiskers still present serious
problems for high reliability electronics industries
such as medical apparatus, military hardware and
space equipment.
The decision to use tin as a primary coating on
electronic component contacts and printed circuit
boards should be done with great care at this time. If
tin coatings are used, a requirement for a nickel under
coating should be considered. The use of bright tin
electroplated coatings should be avoided."
Peace,
Joe