Terminal plating: tin, nickel and silver

Three jobs, one coating

A terminal plating has to do three things. It protects the base metal from oxidation and corrosion, it keeps contact resistance stable over the life of the joint, and it survives the forming and crimping process without cracking or flaking. A finish that does one job well can be weak on another.

The base metal is usually copper or brass. Copper oxidises in air, and the oxide changes resistance and solderability. The coating slows that process, but it also introduces a new interface between the coating and the base metal. That interface is where adhesion, intermetallic growth and galvanic effects live.

The choice is not a hierarchy. Silver is not simply better than tin, and nickel is not simply more durable. Each one trades contact resistance, corrosion resistance, hardness, temperature capability and cost in a different way.

Tin: the general purpose default

Tin is soft, reasonably conductive and easy to plate. It forms a thin oxide that breaks down under normal contact pressure, which is why a tin plated joint can work well without a high contact force. It is the default for copper and brass lugs and terminals in indoor and general industrial service.

Tin is also sacrificial to copper at pores and scratches. That is useful because the tin corrodes first and the copper underneath stays intact, but the corrosion product has to go somewhere. In a confined joint, tin oxide can build up and raise resistance, especially in a wet or chloride environment.

At elevated temperature, tin and copper form intermetallic compounds at the interface. A thin layer is usually acceptable in a bolted joint, but it changes the surface and can affect a soldered connection. For a part stored hot for a long time, ask about the storage and temperature history.

Silver: low resistance, with a caveat

Silver has the lowest contact resistance of the three common finishes, and it holds up at higher temperatures than tin. That makes it the usual choice for high current joints, busbar interfaces and terminals that run warm.

The caveat is tarnish. Silver reacts with sulphur compounds in the air and forms a dark film. The film is not as insulating as some oxides, and it can often be displaced by contact pressure, but it changes the appearance and can raise resistance in a low pressure connection. In a sulphur rich environment, the finish needs protection or a different choice.

Silver is also softer than nickel and more expensive than tin. It is worth the cost when the joint is current critical or temperature critical, not as a general upgrade on every terminal.

Nickel: hardness and corrosion

Nickel is harder and more corrosion resistant than tin or silver. It resists wear, which helps on a terminal that is mated and unmated repeatedly, and it stands up to higher temperatures without the intermetallic behaviour of tin on copper.

The trade-off is contact resistance. Nickel has higher resistance than tin or silver, so it is a poor choice for a high current joint unless the contact area is large enough to compensate. It is more often used on a steel or brass component where hardness and corrosion resistance matter more than the last milliohm.

Nickel plating also has an internal stress that depends on the process. A thick or poorly controlled deposit can crack at a bend. If the terminal is formed after plating, check the adhesion and the bend behaviour.

Thickness and significant surfaces

A minimum thickness is not an average. The drawing should say which surfaces are significant and where the minimum applies. The inside of a barrel, the edge of a palm and the thread of a screw all plate differently, and a single number across the part is not realistic.

A thin cosmetic flash covers the metal for appearance but does not provide a reliable barrier. A working coating is usually several micrometres, with the exact value set by the product standard or the application. Over-specifying thickness adds cost without improving the joint; under-specifying it turns the finish into a short lived cosmetic layer.

Plating also changes dimensions. A coating on a threaded part affects the fit, and a thick layer inside a barrel reduces the clearance for the conductor. For a precision terminal, the plating allowance belongs on the drawing.

Base metal and mating metal

Plating does not remove the galvanic couple between dissimilar metals. Tin plated copper against aluminium still forms a couple, and the tin layer does not change the potential difference by much. An aluminium conductor needs a bimetallic transition or a suitable joint compound, not just a tin plated palm.

The same applies to a silver plated terminal on a tin plated busbar. The two finishes can be joined, but the joint needs a contact surface that is clean and a torque that is specified. Mixing finishes without a note is a common source of a joint that looks correct and behaves differently after a few thermal cycles.

At a plated joint, the coating on each side is part of the contact interface. If one side is heavily tarnished or has a thick oxide, the contact resistance is set by the film, not by the coating name on the drawing.

Verification after crimping

The common thickness methods are X-ray fluorescence, coulometric testing and a cross-section. XRF is fast and non-destructive but needs a calibration for the coating and the substrate. A cross-section is slower but shows the layer and the adhesion at the same time.

Adhesion is checked by a bend, a burnish or a tape test, depending on the standard. For a crimped terminal, the most useful check is after crimping: the plating should not crack or flake at the barrel bend. A coating that passes a flat coupon test can still fail on a formed part.

Record the thickness, the method and the measurement points. If a certificate is supplied, check that it names the standard and the measured values, not just the finish.

• Plating standard and minimum thickness on the drawing
• Significant surfaces and measurement points defined
• Adhesion method and acceptance stated
• Post-crimp visual check for cracks and flakes
• Contact resistance or millivolt drop where the joint is critical
• Certificate with measured values, not only a finish name

A selection sequence

Start with the current and the allowable temperature rise. If the joint is high current or runs warm, silver moves to the front. If the environment is corrosive but the current is modest, nickel may be the better trade. If the joint is a general indoor connection, tin is usually enough.

Then check the mating surface and the environment. A silver terminal on a tarnished busbar is not a silver joint. A tin terminal in a chloride atmosphere needs a thickness and a test that match the exposure.

Finally, check the process. A coating that cannot survive the crimp or the bend is not a candidate, whatever its data sheet says. Prove it on a formed sample.