Keith,

Thanks for the information.  I think we better be careful not to store our
pure tin SRMs below 13 C.  A phase transformation would ruin them.

John


At 09:51 PM 10/10/2005, you wrote:
>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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