We are making a USB device but with the USB tracks on the PCB itself, something like this -

Like this

My concern is regarding the life of such a product. How can I make it last as long as it would if I were to add a 'normal' USB connector to the PCB.

The copper thickness we have used is 60 um and Gold thickness on the USB tracks is about 0.5 um. Will this suffice?

  • 2
    \$\begingroup\$ I doubt anyone could tell you if this would suffice, because you haven't told us what your requirements are. 10 insertions? 1000? 1 million? \$\endgroup\$ – brhans May 22 '18 at 11:33
  • \$\begingroup\$ Sorry for not mentioning that. About 10,000 insertions. \$\endgroup\$ – Amorphous May 23 '18 at 7:47

I think you would see a degradation of performance when you started to wear through the gold finish and into the copper/nickel underneath. The best way to get around this would be to put on as much gold as you can, but this gets expensive.

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.

Source: Technical Report Golden Rules: Guidelines For The Use Of Gold On Connector Contacts

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Source: Watchplating.com

In my experience a hard gold finish usually works best with anything gold, but is also more expensive. Evaluate the tradeoff of cost vs quality.

How long will it last? You'll have to test it yourself. However it's worth knowing that standard connectors (that are not on the PCB end) have 30um flash and still function through 10k cycles:

Two key factors: Contact design: Major effect on the durability of the spring force, will eventually reduce to a level that will not give good contact. Gold plating: Thicker the gold, longer it takes to wear out.

Number of mating cycles:
USB2.0 Full Size: 1,500 mating cycles, gold flash plating.
USB2.0 Mini: 5,000 mating cycles, 15µ” gold plating.
USB2.0 Micro: 10,000 mating cycles, 30µ” gold plating.
USB3.0 Full Size: 5,000 mating cycles, 30µ” gold plating.
USB 3.0 Micro: 10,000 mating cycles, 30µ” gold plating.
Type C: 10,000 mating cycles, 30µ" gold plating.

Source: https://gct.co/usb-connector


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