John,

"Tin Pest" is the allotropic transformation of tin from the body-centred
tetragonal "beta" form (the soft shiny metal with which we are familiar) to the
diamond cubic "alpha" form (a gray brittle material) which is theoretically the
more stable phase at temperatures below about 13C.      I attach some photos
that illustrate that change.

As you can see it is quite a different phenomenon from whisker growth, which
is certainly not stopped by 50-100ppm lead.

I understand that it got the name "tin pest" ("pest" meaning something like
"plague") because the increase in volume that occurs with the transformation
results in what look like pustules or blisters forming on the surface of the
tin.   The grey colour adds to the effect so that the tin looks as though it is
suffering from a disease.

Nihon Superior were prompted to undertake this study because of the concern
expressed by some people that tin pest could be a problem with lead-free
solders exposed to low temperatures in serviced.  Obviously solder that had
undergone this transformation would no longer be much use as a joining material.

From the information you provided on available ATSM tin grades it seems that
there should be no problem selecting one for the manufacture of a lead-free
solder that, accoriding to the Nihon Superior study, would be free of the risk
of "tin pest".

Keith Sweatman
Nihon Superior Co., Ltd.

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