Mike,

Part of the answer to your question lies in the following:

This ?metal? layer can be a very high tin concentration on the solder side
and a very high copper concentration on the other side - meaning the base
metal side. In the case of PCB's, this mostly means copper.

Whether copper extends, or can do so, above the IMC layer, I cannot answer
but I believe it should not. How much is too much has always been a question
needing an answer. I think solder joint experts as Werner, and other
soldering and solderability experts, should have this answer to your
important question.

Earl

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