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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