The following is what I know about gold plating processes on PCBs.  It is
the result of
conversations with two Engineering Managers at a PCB FAB shop, and with
other, less
knowledgeable people.  These conversations were at times contradictory and
were certainly
confusing.  I have bullets by the items I believe are absolutely true.

Do you have other sources? Can we bounce this between the three of us to
'fill out' the
holes?  We need a quick reference guide to understand these differences.

I believe we (as a manufacturing group) require less then 20 mincroiniches
of gold to maintain
reasonable solderability.

Flash (Immersion) Plating
*               Most common in the industry.
*               Limited to 3-5 microinches in thickness.
*               Soft in nature ("Dead-Soft") less then 80 on a hardness
scale.
*               The plating is a substitution process where nickel is
replaced by gold.
*               When the nickel is gone, the process stops.
*               Most baths are in the thinner side of spec.
        Possible to cheat and apply higher levels (8 or even 10 is
possible).
        Gold cell structure is incoherent.
        Plating is laminar in nature

Electro-less Plating
*               Rarely done.
*               Unstable process, gold suspended in plating liquid can
"fall" easily.
        Same as flash, (seems likely)?
        Less predictable result.
        Possible to reach 10 microinches in thickness.
        Gold structure is incoherent.

Electro-Plating
*               Still fairly common.
*               Required to reach higher gold thickness (10- unlimited?)
*               Slightly less controllable then flash.  Generally has a
variation of 10 microinches.
*               Plating is based on an electrical current, and so requires a
single electrode, thus
*               cannot be applied after etching.
*               Etching requirements change and require an ammonium etchant.
*               Cobalt is used in this process.
*               Plating is columnar in nature.
*               Gold hardness is either a soft gold (82 on hardness scale)
or a hard gold, though
*               hard gold is the default.

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