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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@HilmarSt
Having a closer look at my watch I can see that the gold plating has very small holes, like punched with a needle.
I suppose that this is caused by Spring Contact Probes (circuit board test needles) used in the factory to program or to test the watch.
Humidity will go through these holes and cause corrosion in the copper layer, ending up in pushing off the gold plating, only remaining the corroded copper. This will also explain the still gold plated pad(s), the needles did not penetrate their gold layer.
The factory should use less sharp pointed contact probes to avoid this, for already punched contacts one may try to use Silver Conductive Paint to cover the holes (not yet tested by me).