I am not sure of how/why this occurs, but I have an idea of perhaps how it
works.

When the solder is applied, a Copper/Tin intermetallic layer forms at the
interface, this layer has no lead, thus making the Tin essentially 100%, if
you ignore the Copper content.  As the solder builds on top of this
intermetallic, at say the "proper ratio", of 63/37, the Total Tin in the
deposit remains above the theoretical level because of the intermetallic
layer.  As the thickness of the deposit increases, the contribution from the
intermetallic layer decreases, and the apparent Tin level in the total solder
decreases.

This is the only explanation that seems to make any sense, but it is only a
guess.

Rudy Sedlak
RD Chemical Company

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