Mark:

I swear I am going to copy what I am writing here, so I can simply paste it
into a message, as I retype this about once every 6-12 months when this kind
of question comes up.

Plating metal involves the transfer of an electron to the metal in the
plating bath to change if from a soluble salt to the metal form.

Where that electron comes from defines the type of plating.

In electroplating the electron is pumped over wires from the rectifier.

In electroless plating, the electron is supplied by another chemical in the
plating bath. (called a "reducing agent")

In immersion plating, the electron is supplied by the substrate metal under
the gold plating.  What happens is that the Nickel donates electrons to the
Gold, and in the process the Nickel goes into solution, and the Gold
deposits.  It is an exchange.

The implications of this are that in immersion plating, once the Nickel is
completely covered up, plating ceases, as the Nickel can no longer go into
solution, so immersion plating is always thin.

With electroless plating, the electrons are coming from the solution, so
plating can go on as long as you leave the part in the plating bath, so, the
plating can be as thick as you want.

From a user point of view, electroless baths tend to be "meta stable",
(defined for me as a pyramid resting on its point) and tends to plate where
ever it wants, on dust particles, tank walls, and other things that you do
not want.  Immersion baths are much more stable.

Rudy Sedlak
RD Chemical Company