By popular demand, Dave tears down the dodgy tactile switch from his aircon, and compared with a couple of others.
Some might describe me as a plating expert. Those dark patches on either side of the snap dome looked like they lined up with the two contacts on either side of the post in the center. I believe those contacts and their respective legs leading out of the body are a single piece and, speculating further, I believe those legs are tin-plated which would mean the two contacts on the inside are also. The black stuff is probably tin oxide as a result of fretting corrosion (its the result of repeated, low amplitude motion between the snap dome and the contacts). I don't think I would say it's bad plating, its just a very cheap way of doing it, both because tin is relatively cheap and because it enables the leg/contact piece to be stamped and plated as a single piece.
Of course I'd have to throw it under the XRF to make sure but the shipping costs....
use more power microscope, looks like arcing spots
Of course these pushbutton switches cannot take a pcb wash, you have to use no clean solder. But that mistake I have seen noobs make.
I'm suspicious the switch didn't get a drop of water inside, as the disc is oxidized black above the contacts only, so electrolysis occurring at those two gaps.
Omron I find the best, I have zero hassles with them but they are expensive. Gold is optional.
E-switch I did not like, just too cheap, they melt easy and plastic cover.