Immersion plating is a chemical replacement reaction between two metals.  The
reaction is driven by the difference in electromotive potential between the
metals (sort of like a reverse battery).  The more noble metal is deposited,
and the other metal dissolved into solution.  As gold is the most noble
element, it will immersion plate on most other metals, (given that a proper
electrolyte and complexing chemical is present for the dissolving metal).
The key is finding  electrolytes that can "hold" both the metal to be plated
and the species to be dissolved - over time and at high capacity.

In the most common application on circuit boards, previously plated
electroless nickel is dissolved and gold deposited to give "electroless
nickel/immersion gold" surface finishes.  Other immersion reactions common
are Immersion silver and immersion tin (copper being replaced in both
reactions).

Denny Fritz
MacDermid, Inc
Waterbury, CT

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