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PLATING
The plating is the contact surface.
The alloy underneath carries the current, but what touches the other half is the plating. It is chosen for resistance, wear and atmosphere in that order.
- Gold, thin
- Stable resistance · adequate for a part mated once
- Gold, thick
- Repeated mating · the same surface with a longer life
- Tin
- Permanent joints · cheap and solderable, and the oxide is resistive
- Tin over nickel
- A barrier under the tin · stops the tin diffusing into the alloy
- Nickel, shell only
- Wear surface on backshells · not used as a contact surface
- Palladium nickel
- A hard alternative to gold · wear resistant, and it needs a gold flash
Illustrative plating reference
| Plating | Typical use | How it fails |
|---|---|---|
| Gold 0.4 µm | Mated once, dry | Worn through at low cycle counts |
| Gold 0.8 µm | Repeated mating | Cost, where the cycles are not needed |
| Tin | Permanent joints | Oxide film raises resistance over time |
| Nickel shell | Backshell exteriors | Wears through to the zinc at the joint |
Why gold is specified by thickness.
Because it wears. A thin layer is a short life, and the failure is a rise in resistance rather than a visible defect, which makes it hard to attribute to the connector.
The one worth avoiding.
Gold on one half and tin on the other. The pair is a corrosion cell, and the tin side is what suffers.