Kirk,

The silver may dissolve in the molten solder unless it is protected by a
barrier layer of copper or nickel.  Typically the nickel is a better barrier
than copper in equal thicknesses, but for normal soldering both will work
well.  The final tin or solder coating is to protect the copper or nickel
from oxidation (passivation) and retain solderability.  The intermetallic
formation and copper or nickel dissolution during soldering is not an issue
in this case where the dwell time is short.

Phil Hinton

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