Ted, et al,

I have no data to offer regarding immersion tin coatings and tin whisker propensity, but I felt obliged to remark on Ted Stern's comments I've excerpted below: It appears from Ted's remarks that he may be confusing the phenomenon of "DENDRITIC GROWTH" with "WHISKER GROWTH" (I've seen the two phenomena confused many a time). 

DENDRITES (which take the appearance of beautiful "fern-like" leaf patterns tracking ALONG a surface) may form under the conditions cited by Ted:
Moisture + Soluble Ionic Contamination + Voltage Differential = DENDRITES  (Steve Zeva's www site has a beautiful example)

WHISKERS  on the other hand (which often appear as filaments or "hair-like" growths OUTWARD from a surface) require NONE of the above factors in order to form.  Whiskers result from MECHANICAL stress relief within the tin layer.  Potential sources for the mechanical stress are MANY and include factors such as residual stress in tin post plating, intermetallic formation INTO tin grain boundaries, environmental stresses (CTE mismatches), substrate stress/corrosion, applied mechanical stress (bending, lead forming, surface scratches, etc.) ....

For examples of TIN WHISKERS, you may wish to visit the NASA Goddard Space Flight Center Tin Whisker Homepage (which may be down temporarily for maintenance):
http://nepp.nasa.gov/whisker

Peace

Jay Brusse
Sr. Components Engineer
QSS Group, Inc at NASA Goddard
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Date: Wed, 3 Apr 2002 09:19:54 -0600
From: Ted Stern <[log in to unmask]>
Subject: Re: Tin whiskers and immersion tin coatings

Dear Ms. Christian:
......
......
It is my opinion, albeit limited to the immersion tin we manufacture, that organic and/or inorganic codeposition agents incorporated into the tin coating serve to reduce the rate of intermetallic growth but not whisker growth. Although codeposition agents may lessen the propensity, given the proper
conditions (moisture, sufficient soluble ionic contamination, and voltage differential), whisker growth may still occur.
Regards,
Ted Stern