@noiob I will note also that the amount of gold used in ENIG is incredibly tiny, the plating thickness is usually specified as 1 μ" which is a horrible unit, but which equates to somewhere around 100 gold atoms of thickness
so "gold is expensive" isn't really a major concern here, when you need so little of it to prevent the nickel from oxidizing
-F
@Felthry no, I get that, gold plating is cheap, see shitty HDMI cables
@noiob we've seen gold-plated *TOSLINK* cables
-F
@noiob you know, optical fiber. well known for being enhanced by gold plating
-F
@Felthry well, the plating is more for optics anyways
@noiob Because gold prevents oxidation of the surface
if you're asking why not use a HASL finish (where the board is coated in solder), the answer is surface flatness--HASL can be bumpy enough to make some many-pinned packages fail to get all their pins attached. the problem is particularly pronounced with BGA packages that have more than a few dozen balls, which is why you should always go for ENIG over HASL for designs with large BGA parts in them
-F