Thank you Jay... this is exactly what I was looking for.

bill...

At 08:01 AM 01/30/2003 -0500, you wrote:
>I work in support of NASA Goddard Space Flight Center where since 1998 we
>have been involved in tin whisker related research.
>         http://nepp.nasa.gov/whisker
>In June 2002 I published a technical paper that may address one of Bill
>Raymond's questions about pure tin-plated capacitors and resistors -- "can
>they form tin whiskers"?.   The answer is - "yes" they can (at least I can
>say so based upon direct experience with respect to ceramic chip capacitors).
>
>J. Brusse, "Tin Whisker Observations on Pure Tin-Plated Ceramic Chip
>Capacitors", Proceedings of the AESF SUR/FIN Conference, June 24-28, 2002,
>pp. 45-61
>
>The paper and associated slide presentation can be downloaded from the
>NASA Goddard Tin (and OTHER Metal) Whisker www site:
>         http://nepp.nasa.gov/whisker/reference/reference.html (3rd and
> 4th items under the NASA publications reference list)
>The paper details several examples where very high population densities
>(hundreds per square mm) of tin whiskers have been observed to form
>on  the terminations of some pure tin-plated ceramic chip capacitors after
>exposure to multiple temperature cycles.  In these observations whisker
>lengths have ranged from a nominal few 10's of microns (0.010 mm) to as
>much as 0.250 mm.
>
>In addition, we have an area of the www site that provides many
>photos/summary of one extreme case (covered in the paper, but the www site
>has many additional photos not in the paper).
>                 http://nepp.nasa.gov/whisker/experiment/exp5/
>In this example, tin whiskers were observed on pure tin plated ceramic
>chip capacitors after extensive T-cycle followed by ambient storage for
>several months.  The www site is a bit out of date, so today we can say we
>have noted some whiskers are approaching 0.250 mm in length (max) with
>population densities in the 100's per sq. mm.  Interestingly, the max.
>whisker length at the end of T-cycle was ~0.100 mm.  After an additional
>year of ambient (office) storage, some of the whiskers more than doubled
>in length.  NOTE:  in this example capacitors were mounted by CONDUCTIVE
>ADHESIVE (not soldered)  INSIDE of a hybrid microcircuit with minimum
>trace spacing (exposed) inside this hybrid of ~0.125 mm thus posing a
>legitimate shorting risk from whisker debris or even bridging to
>components adjacent to the capacitors.  For those who wonder what might
>have happened if the capacitors parts had been SOLDERED?  The tech paper
>has one example (different application) where capacitors were soldered via
>vapor phase with Sn63Pb37 at 217C.  Areas of the termination that were NOT
>covered in the solder fillet (i.e, top side of chip) formed numerous small
>(~0.03 mm) whiskers after T-shock or T-cycle. No follow-up data was made
>available about subsequent storage/growth potential.
>
>The above discussion relates directly to ONE TYPE of capacitor/resistor
>construction--ceramic chip caps.  It should be noted that there are many
>and SUBSTANTIALLY different  constructions  for DIFFERENT TYPES of
>caps/resistors (e.g., some tantalum chip caps have tin-plated copper
>leadframes, some resistor chips use tin-plated over Ni plated steel end
>caps, and on and on...).
>
>Assessing whisker risk is a very difficult task indeed.
>There are no consensus models to describe the fundamental growth mechanism.
>There are no industry-accepted "accelerated" test conditions.  As such no
>one is yet ready correlate experiments such as those above to application
>conditions that may be substantially DIFFERENT  (e.g., 100 cycles
>-40C/+90C = ???? hours at 25C).
>
>I hold on to hope that researchers today will someday develop and validate
>models to describe the fundamental growth mechanism(s).  In the meantime,
>within the NASA community (where hi-rel/long duration is frequently the
>name of the game) our group advises against use of parts
>(electrical/mechanical/PWB finishes, etc.) bearing pure tin finishes.  We
>are realists too and understand that strict prohibition of tin is not
>always practical, therefore, we are also evaluating ways in which we can
>assure ourselves that we have taken appropriate mitigating steps when tin
>is used.
>
>Sincerely,
>
>Jay Brusse
>
>Jay Brusse
>QSS Group at NASA Goddard
>Sr. Components Engineer
>(301) 286-2019
>(301) 286-9778 (fax)
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