I think g_axelsson knows what he is talking about. I was a gold plating specialist for 10 years and his post and the reference he gave (read it - very informative!) made a lot of sense to me. Assuming he is right, it also explains to me why people are having so much trouble with the gold under solder mask - it's almost non-existent. At $920 gold, 1 square inch of 3 micro" gold would be worth $.0276 and would weigh .00094 grams, if I calculated right. It would take 66 boards, each with a gold plated area under the mask of 4" x 4", to make 1 gram. Are these boards really worth messing with?
Immersion plating is exactly the same thing as cementation, except that the solution is formulated so that the gold bonds tightly to the nickel and eventually will form a barrier to prevent further penetration of the solution. As gold is deposited, nickel dissolves. At some point, when all the surface nickel is covered with gold, the gold stops plating. In the Reference, the gold thickness is said to be self-limiting. Since the rate of immersion deposition continually decreases before completely stopping, the thickness could be controlled by the time spent in the solution.
Electroless plating occurs from a bath that contains a reducing agent. The nickel is chemically reduced onto the copper (or onto the nickel already deposited) and forms a bond. Unlike immersion plating, no copper dissolves and the the nickel thickness is unlimited. True electroless gold baths work the same way. Here again, time in the tank determines thickness.
Flash plating is simply electroplating. It's thickness is easily controlled and is proportional to the current density (amps/sq.ft., e.g.) and to the time spent in the tank.
The adhesion of all 3 types of plating is dependent on how well the substrate was prepared (cleaning, removal of oxides, etc.) before plating. All 3 types of solutions are composed of multiple ingredients, which control such things as reduction, color, brightness, adhesion, and solution conductivity.
As far as adhesion is concerned, electroplating is usually the best, then electroless plating, then immersion plating.
Although being in this business for 40 years, I couldn't figure out WHY the large area under a solder mask would be covered with gold. There didn't seem to be any logical reason for it, unless you wanted to waste gold - and, believe me, PCB manufacturers have NEVER wasted gold.
The reason that silver cementation on copper is not limited is that the silver continually sloughes off, thus providing fresh nickel sites for the silver to deposit. The copper is never completely covered with silver.