Tin has two potential problems.

A shelf life problem, unaffected by the atmosphere it is stored in, where
spontaneous alloying occurs as the Copper and Tin form an "intermetallic".
It is my understanding that when all of the Tin becomes part of the
intermetallic layer, the surface is no longer solderable.

The other issue is oxidation.  Tin Oxide is particularly difficult to solder
over, and if it is thick enough, not even the most aggressive flux will work.
 This issue is strongly affected by the atmosphere the board is stored in.

Both problems are accelerated by heat.

I hope this helps.

Rudy Sedlak
RD Chemical Company

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