In response to comments by Guenter Grossman and James Canner about reflow
temperature eliminating the risk of whiskers, just review some of the
reports on the NASA tin whisker web site. Annealing and melting no doubt
help, but they can't be counted on to eliminate the risk. We just don't know
enough about the problem to be able to say what it will take, as Canner
acknowledges by his references to current research. Remember that there is
no known way to accelerate the growth of whiskers, so in order to be able to
say confidently that a proposed strategy will totally prevent the growth of
whiskers long enough to cause a short circuit for ten years, one must wait
ten years for the data. Remember that whiskers can grow by the millions; the
number needed to cause a failure is one, if it is long enough and happens to
make contact with the wrong surface. Pure tin plating is much more likely to
support the growth of such a whisker than is tin-lead, and that is why mil
specs for many components have for years prohibited its use.
For those who are interested in pursuing this question further, see
"Mitigation Strategies for Tin Whiskers", by M. Osterman CALCE-EPSC, (August
28, 2002)
http://www.calce.umd.edu/lead-free/tin-whiskers/TINWHISKERMITIGATION.pdf.
Here is an excerpt relating to reflow.
The melting point of tin is approximately 232 C. It has been
reported that reflowing or "fusing" pure tin plating is an effective means
to reduce whisker formation. Again, the limitations of this approach have
not been identified; the effect could be time dependent, affected by the
substrate, the environment, or by any number of other potential variables.
It has been observed that scratches on pure tin finishes can become sites of
whisker growth. In addition, bending a tin finished surface in such as way
as to cause a compress load in the finish has been observed to increase
whiskers formation. Therefore, handling the parts after reflow may
compromise the effectiveness of this mitigation strategy.
Pros: May be able to push responsibility to parts suppliers. Limited
evidence that the method reduces the risk of tin whisker formation.
Cons: Added process step. Tin whiskers have been observed to occur
on heat-treated tin-plated surfaces. May damage temperature sensitive
devices. Solderability may be compromised.
Effectiveness: Some documentation appears to show that growth to is
reduced by this process [Dunn 1987 by M. E. McDowell, "Tin Whiskers: A Case
Study (USAF)," Aerospace Applications Conference, pp. 207-215, 1993.]. Some
documentation shows that tin whiskers will grow in fused tin [K. Cunningham
and M. Donahue, "Tin Whiskers: Mechanisms of Growth and Prevention," 4th
International SAMPE Electronics Conference, June 1990, pp. 569-575.]
Erik de Kluizenaar correctly points out that billions of tin-plated
components have already been fielded, so that should give some indication
that the risk is limited. That is small comfort to the people responsible
for the satellites in orbit that are useless due to tin whiskers. What
should be clear is the more tin plating and the smaller the gap between
terminations, the more failures due to tin whiskers can be expected. The
reason people are contemplating adopting a termination finish known to
present the risk is because alternate finishes are either more expensive or
will soon become illegal.
Gordon Davy
Baltimore, MD
410-993-7399
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