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
---------
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