Immersion gold is the result of a displacement reaction where the nickel
substrate acts as the reducing agent. As soon as the nickel is coated by the
gold deposit the reaction stops and accordingly is self limiting.  I do not
beive that you can sucessfully exceed a thickness of 5 microinches.  
An electroless process involves an external chemical reducing agent that can
sustain the deposition to any desired thickness.
Electroless nickel immersion gold has excellent solderability characteristics
and the thin gold does not have any adverse effects on the solder joint.

George Milad
Atotech USA Inc

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