Gold flashed silver contacts - is conventional thinking correct?
Norman Carnt, Finder’s international technical manager throws some light on the practise of applying gold flashing to relay contacts. Apparently gold does not always glitter…

Conventional wisdom has it that the practice of gold flashing silver or silver based contacts in relays and switches enhances the integrity of the electrical connection achieved by the contact faces. It is a ‘fact’ held by users and endorsed by some suppliers that the contact resistance will be lower and more consistent if the contact faces feature a thin ‘flash’ of Au - typically in the region of 0.2-1.0µm in thickness. On the face of it, this would appear to be not unreasonable.
There is a general and correct understanding that the noble metal – gold – is impervious to the oxidisation and sulphurisation that affects silver under most everyday environments. Additionally people tend to believe their eyes and are familiar with the dark silver sulphide tarnish that forms on silver, and are much happier when they do not see it.
However, the reality is not all that it seems - a fact that many people within the industry have known for a long time. The difficulty is that conventional wisdom seems to be entirely laudable and well rooted, whilst the scientific evidence may seem a little esoteric.
Generally silver is affected by two tarnish mechanisms. Firstly, that which forms as a result of oxidisation due to the ever-present oxygen in the atmosphere. This starts to develop on a clean silver surface almost immediately and grows with time. Whilst this can and does cause contact resistance problems, it is generally only with low level switching applications. This is largely due to the fact that the oxide film (Ag2 O) is very soft and breaks down easily under the effect of even modest contact pressure. Also, it is often destroyed by the heat produced by the passage of current.
Secondly, sulphurisation occurs due to the ever-increasing sulphur levels polluting our atmosphere. Ag2S films tend to be far more tenacious than Ag2O and if contacting problems occur it is more likely to be due to sulphurisation.
Surely then, Au flashing silver contact material must be a good Idea. But in fact, at best gold flashing offers no guaranteed advantage and at worst, it is something to avoid. The problem lies with the fact that the gold is applied as a ‘flash’, i.e. it is very thin and as such it is porous, leaving pore holes through to the silver substrate – typically in the order of a few µm diameter. If it were thick – typically 4µm or more - then the intrinsic attributes of gold would apply, but this is not the case for the cosmetic sub micron flash commonly employed.
Nevertheless, wouldn’t an Au flash covering 99% of the contact face be better than no gold at all? But the answer is no. The short reason for this is that the silver/silver sulphide migrates through the pores and then spreads across the face of the gold flash. But more interestingly, in doing so it creates a dense, amorphous, pore free, Ag2S film across the contact face that is several times the diameter of the pore itself. Such Ag2S is far more effective at increasing contact resistance than the variant that would develop on an unplated silver surface; this being quite different – light, spongy and crystalline in nature. In conclusion, gold is good only if it’s thick. So for applications where consistent ultra low contact resistance is needed, it is necessary to check that the relay of choice’s contacts are gold plated (4µm or more), not merely gold flashed.