Hi Dave,

At the IPC/JEDEC Lead-free Conference in San Jose in April I presented a
paper reporting an investigation by Nihon Superior into the tin pest phenomenon.
 I presume that there are ways of getting that paper via the IPC.

As you know "tin pest" is the transformation of tin from the form with which
we are all familiar, the body centred tetragon "beta",  to the brittle gray
diamond cubic "alpha'" form.  The expansion in volume that occurs with the
transformation combined with the brittleness of the alpha tin results in the
effect, apparent on the blistering on the surface of those now famous Plumridge test
specimens.    The alpha tin is theoretically the more stable form at
temperatures below about 13C but it does not nucleate easily so you need to get to a
temperature around -40C before there is enough driving force to nucleate
trigger nucleation.

It is already known that the transformation is suppressed by trace level
impurities and additions and in the Nihon Superior study the effect of various
additions to high purity tin at the 0.01% level was studied.     As expected,
lead turned out to be the addition that was most effective in suppressing the
transformation.   Of course that is well below the RoHS limit for lead and the
good news is that you do not really need to do anything to ensure your lead-free
solder has the required amount lead except make sure it is made from
"standard grade" "99.9%" tin which has a naturally occurring level of lead at the
required level.   That is in any case the sensible choice from the cost point of
view and there are no other properties important in a solder that are
compromised by lead at that level.   I do not know but I suspect that the famous
Plumridge test specimens were made (with the best of intentions) with high purity
("99.99%) tin.

Best wishes
Keith Sweatman
Nihon Superior Co., Ltd.

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