Roger -

There is more to the choice between electroless nickel/immersion gold and
electrolytic nickel/electrolytic gold than thickness control.

To plate electrolytically, all the pads to be plated must be electrically
connected (bussed) together.  While this can be done as one of the first
steps in fabrication - while the copper foil covers the entire surface, the
nickel gold first process is not widely used.  The etch step to form copper
traces and isolate pads undercuts nickel/gold badly - frequently causing
slivers of conductive gold to short out adjacent traces.  And, the sidewalls
of the pad, exposed in the etch step, are bare copper.

Electroless nickel/immersion gold is usually about the final step in board
fabrication.  Any copper surface exposed to the plating chemistry should
plate, whether it is electrically connected to any other circuitry, and
certainly NOT connected to an external souce of current - DC rectifier.

While electroless thicknesses should be more uniform than electroplate,
proper design of plating areas and plating thieves used near isolated pads
should make electroplating a very controllable process.

Denny Fritz
MacDermid

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