> I don't believe the difference is in the mechanical bond. I believe tin w=
as
> originally used but lead had to be added to prevent "whiskering" which
> caused problems even in the old circuits.

Yes, whiskers are a real problem with pure tin, they grow in seconds - real=
ly - I have seen real time videos of them growing and the speed at which th=
ey grow is astonishing.

SMD solders use silver and other metals as part of the alloy to stop whiske=
rs (as well as stop silver leaching from terminations). Apparently it doesn=
't need a lot of other metal in the alloy to stop whiskers.



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