terminal plating, surface finish, tin-plated contacts, gold-plated contacts, silver plating
Underneath the plating, most automotive terminals are copper alloy: brass, bronze, or a high-performance copper alloy where temperature or current demands it. The plating on top is the surface that actually touches the mating contact, and it is chosen for the circuit rather than for price alone.
Tin is the default. It is inexpensive, it crimps well, and it performs on power circuits and higher-current signal circuits. Its weakness is fretting corrosion. Tin forms an oxide, and under the micro-motion produced by vibration that oxide is broken and reformed repeatedly until an insulating layer builds up and contact resistance climbs. Tin systems tolerate this because contact normal force is high enough to keep breaking through, which is also why tin contacts carry a low mating-cycle rating.
Gold is used where the circuit cannot tolerate rising resistance: low-current, low-voltage signal circuits, sensor lines, safety-critical circuits, diagnostic connectors that are mated repeatedly, and anything living in high vibration or high temperature. Gold does not oxidise, so contact resistance stays stable, and durability is far higher. It costs more, and it is normally a thin layer over a nickel underplate.
The rule that catches buyers is the mixing rule. Tin and gold should not be mated to each other in a permanent connection: the dissimilar pair accelerates fretting and wear on the tin side, and the resulting joint is worse than either metal on its own. This matters commercially because plating is often the only difference between two part numbers in one family, and under schedule pressure a suffix change reads as a small concession.
Harness Code states plating explicitly on every terminal it quotes, and never offers a plating change as a like-for-like substitution without naming it as a change.
Related terms
Terminal / contact
A terminal, or contact, is the metal part crimped to a wire and inserted into a connector housing to make the electrical connection. Terminals are specified by family, gender (pin or socket), wire cross-section range, and plating, typically tin or gold.
Mating cycles
Mating cycles are the number of times a connector can be mated and unmated while still meeting its electrical specification. The rating depends mainly on contact plating and contact normal force, and it is usually far lower than buyers assume.
Crimp and crimp height
A crimp is the cold-formed mechanical and electrical joint between a terminal and a wire. Crimp height is the measured height of the finished conductor crimp, and it is the main control on whether that joint survives vibration and temperature cycling in service.
Cross-reference and drop-in equivalent
A cross-reference maps one manufacturer part number to a comparable part from another family or another manufacturer. A drop-in equivalent is a cross-reference matching on form, fit and function, usable without any change to the harness, the tooling or the mating half.