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I'd wish to insert a small 12x33mm PCB into a USB slot multiple times a day. Because of that high number of expected mating cycles, the surface finish of the edge connector should be chosen wisely.

The PCB will draw 2 microamps of current in power-down mode, and up to 8 milliamps at peak levels. And the components are likely to last for over 20 years, so it the PCB.

The contact wear will be caused by spring contact and the wear is sliding.

"Common" surface finish options for increased mating cycles are:

  • ENIG - Electroless Nickel, Immersion Gold
  • ENEPIG - Electroless Nickel, Electroless Palladium, Immersion Gold
  • electrolytic nickel gold - (sulfamate) nickel with hard (or soft) electrolytic (acid) gold layer on top (often 3:1 layer height ratio)
  • galvanic gold a.k.a. hard (plated) gold a.k.a. electrolytic gold a.k.a. electroplated gold. This is gold with hardening additives (like cobalt) without the nickel base layer.

The "Blackpad effect" seems possible with hard plated gold and ENIG. So these PCB finishes can only be applied selective not over the complete board, or when applied full board, loose the rework capability.

I do need an edge contact connector (a.k.a. "finger") finish mainly for high mating cycling low wear. I do not necessarily need to meet any spec its contact resistance upper limit value. Still having a connection that works, and doesn't do harm to the spring contacts of the USB female connector is sufficient. There is no soldering to be done on these contacts.

Which (sufficient, not the best) surface finish to choose for these contact fingers, and why?

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2 Answers2

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The goal of plating is to protect copper or other metals from oxidation (which increases contact resistance). Since gold doesn't oxidize contacting surfaces need a sufficient amount. I can tell you from personal experience that a gold flash <1um doesn't do much.

Here is some info from a manufacturer:

KAT Electroless Nickel/ Immersion Gold (ENIG) KAT ENIG is the industry standard for producing uniform mid-phos EN deposits with a thin topcoat of immersion gold, over copper substrates. This PCB finish is highly resistant to corrosion, and is both solderable and aluminum wire bondable. It is an ideal contacting surface.

Electroless Nickel, Electroless Palladium, Immersion Gold (ENEPIG) ENEPIG is formed by the deposition of electroless nickel, followed by electroless palladium, with an immersion gold flash. ENEPIG has exceptionally wide application: it is suitable for soldering, gold wire bonding, aluminum wire bonding, and contact resistance. KAT UF (ENEPIG) Universal Finish This unique Ni/Pd/Au surface finish is solderable, aluminum and gold wire bondable, and a superb contact surface. The recommended thickness is 3 – 5 microns (120 – 200 µins) of electroless nickel, 0.16 micron (4 – 6 µins) of electroless palladium, and 0.02 to 0.04 micron (1 – 2 µins) of immersion gold.

The thicker the gold the better, how long will it last? There isn't much information on that. From Viasystems surface finishes

enter image description here

If your really worried about it and this is essential for your product, I think I'd do some mechanical testing while measuring the contact resistance (Here as an example) Otherwise, stay away from the really thin flashes and the thicker the better. Remember that thicknesses and some definitions vary from manufacturer to manufacturer (such as the definition of hard and soft gold) so make sure you know what your getting.

From this article on contact resistance:

Golden Rule No.3: Gold Coatings Can Be Used For High Durability

Coating a contact with pure (i.e. soft) gold generally results in a connector with low durability and high insertion forces (i.e., high coefficient of friction), especially when the thickness is greater than 0.13 microns (5 microinches). In practice, gold coatings are usually hardened by adding small amounts cobalt or nickel to the gold. Such coatings are defined as ‘hard gold’ and produce coatings with a low coefficient of friction and excellent durability characteristics. Hard gold coated contacts can generally withstand hundreds to thousands of durability cycles without failing. The durability of hard gold coatings can be enhanced by using an underlayer having a hardness value that is greater than that of gold and which will provide mechanical support. Nickel is generally recommended as an underlayer for this purpose. Lubricants are also effective at increasing the durability of gold coatings. Generally, lubrication can increase the durability of a gold contact by an order of magnitude.

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  • Do you know what kind of lubricant is used for this? Would mineral oil be effective? – Dmitry Grigoryev Mar 16 '17 at 08:38
  • Even **ENIG** has a durable "_Contact Resistance (1 million + cycles)_" according to [page 15 of Viasystems surface finishes](http://www.smta.org/chapters/files/UMW_Viasystems_Surface_Finishes.pdf#page15) – Pro Backup Mar 16 '17 at 09:39
  • [Circuitree.com going beneath the surface of surface finishes](https://www.scribd.com/document/2877521/Going-Beneath-the-Surface-of-Surface-Finishes) writes: _ENIG is used for touchpads, **spring connectors**, wiping contacts, and aluminium wirebonding. Only the most difficult plug-in connectors will require electrolytic nickel gold._ – Pro Backup Mar 16 '17 at 10:03
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The thicker the gold the better is not always the case. If you are soldering to the pad, gold embrittlement can occur if too much gold is present. An Au-Sn intermetallic phase occurs that is weak mechanically (relative to the solder) and very brittle. The general rule is it occurs if the gold concentration exceeds 3% by weight. The typical gold thickness of 2 to 5 microinches for ENIG plating is good for PCBs considering typical pad sizes for contacts.

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