Tuan,

One of the best references to tin whiskers is S.C Britton, Transactions of
the Institute of Metal Finishing. 52, 95 (1974) which was also published by
International Tin Research Insitute, London England as "Spontaneous Growth of
Tin Whiskers on Tin Coatings, 20 Years of Observations" Publication No. 487.

In general the thin tin coatings that are fine grained and bright as-plated
are subject to tin whisker growth when subjected to compressive stresses.
 The fused coatings  will also produce tin whisker growth if not thermally
treated after fusion to induce larger stress-free crystal size.  The
stress-free may be negated by later applied stresses such as thermal CTE
differences between the substrate and the coating.  

Lead in the greater than 2% by volume range will inhibit whisker growth.
 Also a nickel plate under the tin is better than a copper under the tin
coating.  A brass substrate is the worst.  Thin pure bright tin platings
either fuse or unfused are the prime prducers of tin whiskers. The type of
tin pating bath that you used is important in suppressing whisker growth.
 The bright tin platings are the worst and matte methane sulfonic being one
of the better ones.  Lucent Technolgies has one out that they claim has a
low-whisker probability.  I would prefer to heat treat even the low
probability coatings to increase crystal size and internal stress (this is
also called annealing)

Your description of tin whiskers growing out of a tin-lead deposit on
instressed pads does not seem like tin whisker to me as it does not fit in
any of the tests that others have run to produce tin whiskers over the last
50 years.  Dendritic growths which are   
produced by electromigration seems to better fit with your description.

Phil Hinton
Hinton "PWB" Engineering  

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