Contact plating: silver, gold and nickel compared

What a contact finish has to do

A contact finish has four jobs. It keeps the resistance low and stable. It resists wear over the life of the part. It limits corrosion and tarnish in the service environment. And it does all of that at a cost the programme can carry. No single metal wins on all four, which is why the choice depends on the duty.

The base material matters as much as the finish. A silver plate on a rough copper surface copies the roughness, and a thin plate over a pore is a pore with a thin roof. A gold plate over a nickel barrier behaves differently from gold directly on copper, because the nickel stops the copper from diffusing through the gold.

The application sets the priority. A high current switch wants low resistance and resistance to arc erosion. A signal contact wants a stable resistance at low force and a film that does not build up. A connector that is plugged and unplugged many times wants wear resistance. The same three metals are used for all three, in different thicknesses and combinations.

Silver

Silver has the highest electrical conductivity of the common contact metals, and it is the default for current carrying contacts and switchgear. It also has good thermal conductivity, which helps to move heat away from the contact spot. For a joint that carries tens or hundreds of amps, silver is hard to beat on resistance.

The weakness is tarnish. Silver reacts with sulphur compounds in the air and forms a film that raises contact resistance, especially at low force and low voltage. The film is soft and is usually broken by the wipe of a sliding contact, which is why silver works well in switches and less well in a static low force connector. Silver is also soft, so it wears and transfers under repeated operation.

Silver plating thickness for a contact is typically in the range of 2 to 10 µm, with heavier deposits on high current parts. Those figures are typical and illustrative, not a specification. The drawing should state the minimum thickness on the functional surface and the test method, because a thickness measured on a test coupon is not the same as a thickness in a blind hole.

Gold

Gold is chosen for one property above all: it does not form a resistive oxide. That makes it the finish for low voltage, low current signal contacts, where the contact force may be too low to break through a film. A gold to gold contact can hold a stable resistance for a long time in a clean environment.

Gold is soft and expensive, so it is used thin. A thin gold layer over a rough or porous underplate has pores, and the pores expose the layer below. In a humid or sulphurous environment, the base metal can corrode through the pores and the resistance rises. The underplate and the surface finish are therefore part of the gold specification, not an afterthought.

Gold thickness for a contact is often in the range of 0.5 to 3 µm, over a nickel barrier of 1 to 3 µm. These are typical ranges for illustration. A dry circuit contact and a connector that sees many mating cycles need different values, and a thick gold on a poorly finished surface can still fail.

Nickel

Nickel is hard, corrosion resistant and a good diffusion barrier. Those properties make it the usual underplate between a copper alloy and a gold or silver top layer. It stops the copper from migrating into the gold, which would otherwise raise the resistance over time.

Nickel is rarely the final contact finish. Its own contact resistance is higher than silver or gold, and it forms a thin oxide that can be a problem at low force. It is used as a final finish on some terminals and on parts that need wear resistance more than low resistance, but that is a different duty from a switching contact.

The nickel layer also affects the appearance and the adhesion of the top layer. A nickel layer that is too thin may not cover the copper, and one that is too thick can crack under bending. The thickness and the plating bath control the stress, and stress is what decides whether the finish survives a formed bend.

Thickness, coverage and the drawing

State the minimum thickness on the surfaces that carry current, not an average over the whole part. A contact that is plated all over may be thin at the contact face and thick on a corner, depending on the rack and the current density. Selective plating or masking is often the better route for a high cost finish.

Say how the thickness is measured. X-ray fluorescence is common and fast, but it measures a small spot and can be affected by the underplate. A cross section gives a direct reading but destroys the part. A coulometric method works for some coatings. The method belongs on the drawing because two methods can disagree on the same part.

Add the adhesion requirement. A bend test, a tape test or a thermal shock test can show whether the layer is bonded. A finish that passes a thickness check and fails adhesion will flake in service. For a formed contact, test the finish after forming, because the bend is where the coating is most highly stressed.

Wear, porosity and fretting

Wear removes the finish at the contact spot. A soft finish wears faster, and a thin finish reaches the underplate sooner. The result is a change in resistance and, in a humid environment, a corrosion cell at the exposed area. The wipe of the contact helps, because it moves the contact spot to a fresh area, but it also wears the finish.

Porosity is the other route to failure. A plated layer is not perfectly dense, and the pores connect the surface to the base metal. Corrosion products can grow through the pores and lift the finish. A thicker layer reduces porosity, and a good underplate limits the damage when a pore exists.

Fretting is the small amplitude movement that occurs in a connector under vibration or thermal cycling. It wears the finish and produces oxide debris at the contact spot. Gold is resistant to oxide growth but still wears, and a nickel underplate can crack under fretting. A contact design that limits movement at the contact spot is as important as the finish.

Choosing the finish

The table below compares the three finishes on the points that decide a contact programme. Use it to narrow the choice, then confirm the thickness and the test with the supplier and a life test.