Mark,

Plating Gold out of solution entails the reduction of the gold ion to the
gold metal through the supply of electrons:

In electrolytic plating the electrons are supplied through electric current
and rectification in a plating cell. This process will continue to plate as
long as the plating conditions are sustained.

In electroless plating the electrons are supplied from a reducing agent
(chemical constituent of the bath). Here also plating will continue
indefinitely as long as the plating conditions are maintained.

In immersion plating the electrons are supplied from the substrate that is
being plated, example during the plating of immersion gold over nickel, the
nickel metal gives up electrons and dissolves into the gold bath, the gold in
turn picks up those electrons and is plated on the nickel surface.  Immersion
then becomes self-limiting because as soon as the nickel surface is coated
with gold the electron supply is stopped.

George Milad
HDI Consulting
IPC Plating Committee Chair
[log in to unmask]
(516)901-3874