Good question Jack,
The property that makes it uncorrodable (a very thick-non porous oxide film) also makes it very difficult to solder. In the old days we could use an active acid flux but that is no longer true. The current non-clean type of flux would not even penetrate the oxide on a tin-nickel pad and it would end up non-wetted (not even de-wetted). Gold, tin, tin-lead, silver, Pd, etc all solder very nicely, so they need to be put over the tin-nickel to prevent the oxides from forming.
The other factor is that the plating bath is difficult to control and you have to watch it to make sure it does not plate stressed. I once saw the traces 'jump' off the boards after etching, the tin-nickel was so stressed. But like I said, the sail boat metal parts are remarkable!
Ok, I'm probably showing my ignorance here,
but with a stable non-corrodable finish like
Sn-Ni, why do you need the gold?
Jack (a PCB layout guy)
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