James:

Metals in solution are too low in electrons, and thus have a positive charge.
 Solid metals, in their shiney form, have the proper number of electrons.
When a more "active" metal (meaning a metal that does not bind its electrons
tightly) is immersed into a solution of a metal that does bind its electrons
tightly, like Gold, the electrons shift from the substrate metal, (the
substrate under Gold is almost always Nickel) to the metal in solution,
causing the metal in solution to plate out spontaneously on to the substrate
metal....in other words, a Nickel coated PCB, when immersed into a solution
of Gold, will cause the Gold to plate out spontaneously on to the Nickel, and
in the process some of the Nickel is dissolved and goes into solution.

This continues until the substrate Nickel is covered, and no more Nickel is
available for this chemical reaction, and you wind up with around 7-8
microinches (0.2 micron) of Gold from this type of plating.

Hope this helped...

Rudy Sedlak
RD Chemical Company

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