Immersion gold (ENIG)
A chemical displacement reaction deposits a thin gold layer over electroless nickel. Its purpose is protection: the gold stops the nickel oxidising before assembly, and the nickel acts as a diffusion barrier over copper. Thickness is deliberately small.
Hard gold
Electroplated gold with a hardener, typically cobalt or nickel, applied to a defined area such as a card edge or a test point. It is thicker, harder and far more resistant to abrasion. Because it is electroplated, the area needs a plating bus or a conductive connection.
Practical rules
- Soldered pads → immersion gold.
- Repeated insertion / wiping contacts → hard gold.
- Gold wire bonding → a different specification again; discuss the wire and loop before choosing.
- Mixed boards → it is common to have ENIG overall and hard gold only on the connector fingers, applied in a selective step.
State the contact application in the fabrication note. "Gold" alone does not tell the fabricator which process to run. That note is read at pcb manufacturing alongside the Gerber set, so the finish call should be explicit there rather than assumed from a drawings legend.
