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The tracks used as an edge connector are gold coated to improve contact with the female connector: bare Cu oxidizes, and also wears out if they are inserted and removed repeatedly. Au cannot be applied directly to Cu, so electroless Ni + immersion Au (ENIG) is used.

Well, the question is: since Ni has good resistance to abrasion and oxidation, why are never edge tracks plated with Ni only? I don't think it's because of the lower resistivity of Au: with or without Au there are always the microns of Ni in the middle, practically no resistance. And female connector contacts are also Au coated.

Anybody knows? Thank you!

I am NOT answering my question. I am writing here just because I needed a place that is common to all contributions.

My real need was not exactly about edge connectors, but making printed contacts for a sliding switch, this will be to choose tuned circuits for a 'ham' receiver. I aim to make the construction easier for the DIY builder, that's why I looked forward to avoid the gold process, not because of cost but added complexity.

I am delighted to have been quickly and correctly informed in my very first tour into StackExchange. Thanks to everyone!

Dave Tweed
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LW1ECP
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    Just tinning (or is it called wetting?) is also an alternative. – Harry Svensson Aug 30 '19 at 17:38
  • What is the resistance of gold, silver, copper & nickel? – Solar Mike Aug 30 '19 at 17:39
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    @SolarMike Audio connectors are nickel plated and seem to do just fine. So why so rare on PCBs specifically? – DKNguyen Aug 30 '19 at 22:12
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    @DKNguyen All audio connectors? Most of my audio cables are gold plated... unless it is cheap v expensive ie a story of quality. The speaker connections on the back of amplifiers seem to be mostly gold plated... At least on mine - cost or quality? The co-ax cables are gold plated as well... also gold is very tarnish & corrosion resistant... – Solar Mike Aug 31 '19 at 06:06
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    @SolarMike I see far more nickel plated than gold plated connectors. How many gold plated connectors do you see on stage equipment which get much heavier use than home audio? Zero from what I've seen and they seem to work just fine. All my XLR cables, studio monitors, and instruments come with nickel plate. I don't think I've actually seen an XLR cable or studio monitors with gold plate. I only ever seem to see gold plate on things marketed to the home audiophile market and not the professional audio market. So then we're back to the question, why does nickel seem to work but so rare on PCBs? – DKNguyen Aug 31 '19 at 15:03
  • Thank you for all your answers. Yes, I didn't pay attention that most audio connectors I know are Ni plated (or at least have a silverish look). But I am used too to having to spray contact cleaner/lube on them when hearing noise when moving them. All 2.54mm pin headers on PC motherboards are Au plated and most mating sockets too. – LW1ECP Sep 01 '19 at 00:20

4 Answers4

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And female connector contacts are also Au coated.

Not your question, but you want to mate like metals to like metals -- gold to gold, tin to tin, etc. If you don't, then in environments with any moisture you get (possibly slow) galvanic corrosion, and in high vibration environments you get something called "fretting corrosion".

TimWescott
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I've wondered the same thing myself on occasion.

I'm pretty sure the answer has to do with contact resistance (as opposed to the bulk resistance of the materials). A gold-to-gold contact has lower resistance and better long-term reliability than a nickel-to-nickel contact. Even a microns-thick layer of gold is significantly softer than the underlying nickel.

Dave Tweed
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  • I think you've just hit the nail! Contact resistance. Yes, I can imagine it is better if both touching surfaces are soft. Thank you! – LW1ECP Aug 31 '19 at 23:53
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Yes, it's possible to use nickel as an edge connector contact, we've done it for high voltage/current connector contacts (24~240V). I don't think the nickel is very good for low level contacts. Gold is good for high level and low level contacts.

You can perfectly well apply gold to copper, however the ultra-thin layer of gold will diffuse into the copper and effectively disappear. That's why the nickel layer is sometimes referred to as a "barrier".

Spehro Pefhany
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Contact resistance. Edge connectors aren't typically required to sustain high insertion/withdrawal cycles.

Many pro audio grade XLR connectors have heavy silver plating on both M&F. It doesn't last forever in stage and studio applications and the entire cables are frequently thrown out.

Peter
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