To just round out the very good answers given by other technetters:
Consider
Tin is roughly ten times the price of lead. Tin and lead are priced by the tonne
and silver by the ounce.
A tolerance of typically +- 0.5% is aloud by most specs on notional alloying
constituents.


So in respect of the above
Ask yourself which side of the tolerance is going to be taken by the
manufacturers and compare what is likely to be supplied as  the notional 63/37
or 63/36/2  with the theoretical eutectics? Conclusion: The actual alloy
supplied is going to be closer to the theoretical eutectic than as supposed in
previous replies.

So far as why have 60/40 and so on..:
Soldering reportedly goes back a few thousand years. Far enough anyway that the
chemical symbols are dervied from Latin rather than English. In other words
Soldering is old enough that a working formula for a mixture of tin and lead
would have been given as "take ye 3  parts of tin and add to ye 2 parts of
lead". Later this would have been restated as 60Stannum/40Plumbum and later
still when science crept in somebody did some measurements and found 63/37 and
so on into modern numbers with decimal places.


Mike Fenner
BSP
______________________________________


It would be interesting to know the history of this discrepancy between the
real and "generally accepted" figures. Perhaps it was the result of earlier,
less accurate measurements. In practice it may have been beneficial as
intermetallics and oxidation deplete this "excess" tin.

David Whalley


>Dave Hillman is absolutely right-- the appropriate phase diagrams show that
>the real eutectic Sn/Pb solder is 61.9%Sn/38.1%Pb (mass percent) (MP=183C),
>just as  62.5%Sn/36.1%Pb/1.4%Ag (MP=179C) is one of the true eutectic
>mixtures of this ternary alloy system. However, 63Sn/37Pb (Solidus=183,
>Liquidus~185 to 186C) and 62Sn/36Pb/2Ag (S=177C, L=189C) have become the
>'nominal' industry eutectics. In both cases the difference between solidus
>and liquidus may be small enough, that for practical purposes the existence
>of a pasty temperature range may be negligible. However, 62Sn/36Pb/2Ag a 177C
>solidus--189C liquidus difference is not exactly small and could make the
>difference between adequately wetted and inadequately wetted solder joints.
>
>Werner Engelmaier

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