Connector contact plating and fretting corrosion

What fretting corrosion actually is

Two contacts that are clamped together still move. Vibration, thermal expansion and the small movement of a housing under load push the surfaces against each other by a few micrometres. The asperities at the interface weld under pressure, then shear apart. Fresh metal is exposed, oxidises within minutes, and the next movement grinds the oxide into the contact patch. Contact resistance climbs, the joint runs warmer, and the warmer joint expands further. That loop is fretting corrosion.

The movement needed is small. A displacement of 10 to 100 micrometres is often enough, which is well below what a visual inspection would call loose. The damage is also local. A connector can look clean on the outside while the contact interface is full of dark oxide debris.

This matters because fretting is often mistaken for a plating failure. The plating may be intact and within thickness. The problem is that the design asked a thin film to survive movement it was never given the force or the wipe to resist.

Why the plating decides the result

Copper alloys oxidise readily. A bare copper contact forms a resistive film quickly, and that film is worse in humid or sulphur-bearing air. The plating is a barrier, but every barrier has a failure mode. Tin oxidises slowly in air, gold is noble but porous when thin, nickel is hard and corrosion resistant but has higher resistance, and silver conducts well but tarnishes.

The practical question is not which metal is best. It is which metal keeps the interface conductive for the number of cycles and the environment the connector will see. That depends on normal force, wipe distance, plating thickness, underplate and the motion profile. A plating callout without those numbers is only half a specification.

Tin, gold, nickel and silver compared

The four finishes below cover most connector contacts in industrial and electrical equipment. The table is a starting point for a design review, not a substitute for the connector maker's test data.

Tin: the default with conditions

Tin is the usual choice for power and terminal connectors because it is cheap, easy to plate and forgiving of ordinary handling. It also has a useful property: the oxide that forms is soft enough to be displaced by wiping action at a reasonable normal force. A contact that slides a short distance on mating can break through the film and establish a low-resistance patch.

The condition is that the design provides that force and that wipe. A tin contact with light force, no wipe and continuous micromotion is a fretting risk, even in a dry room. The oxide debris has nowhere to go and the resistance rises. Thin tin also wears through after repeated mating, exposing the underplate or the base metal.

Tin plating on a high-current contact is usually applied at 1 to 3 micrometres, with the exact figure set by the connector standard or the customer drawing. Thicker deposits help wear life but can flake at a tight bend if the deposit is stressed. Tin whiskers are a separate concern for fine-pitch signal contacts and for long service in temperature-cycled equipment.

Gold, the underplate and porosity

Gold does not form a stable oxide at room temperature, which is why it holds a low contact resistance at forces that would fail a tin contact. That property is the reason gold appears in signal circuits, low-current sensing and connectors that are mated many times.

The gold is rarely used alone. A nickel underplate, commonly 1 to 3 micrometres, sits between the copper alloy and the gold. The nickel blocks copper diffusion and gives the gold a hard surface to sit on. If the gold is too thin, pores and wear tracks expose the nickel. The contact still works, but the interface is now nickel or copper, and the fretting behaviour changes.

Hard gold, alloyed with a small amount of cobalt or nickel, resists wear better than pure soft gold. Soft gold is more ductile and covers better at the same thickness. For a connector that is mated once and then left alone, a thin gold flash can be adequate. For one that is cycled, the wear path and the number of cycles decide the thickness.

Harsh environments change the ranking

Humidity, sulphur, chlorine and salt change which finish wins. Silver tarnishes quickly in air containing sulphur, and the film is resistive unless the contact wipes through it. Tin oxidises faster at high temperature and high humidity. Gold remains stable, but a porous deposit can corrode the nickel or copper underneath and push resistance up.

Mixed flowing gas testing, described in IEC 60068-2-60, is one way to compare finishes under a controlled corrosive atmosphere. Salt mist testing to ISO 9227 is useful for marine and outdoor equipment, but it is a corrosion test, not a fretting test. A connector can pass salt mist and still fail in vibration. Ask for the test that matches the failure you are trying to prevent.

Temperature adds a second effect. A connector that runs warm expands and contracts with each load cycle. That movement is exactly the micromotion that drives fretting. In equipment with daily load cycling, the contact interface sees thousands of small cycles even if the connector is never unmated.

What to put on the drawing and in the RFQ

A plating callout should name the finish, the thickness range, the underplate and the areas that must be plated. Add the contact resistance limit, the normal force, the expected mating cycles and the environmental test. If the contact is crimped or welded to a conductor, say so, because the heat from a weld can damage a thin plating.

• Finish and thickness, for example tin at 1 to 3 micrometres minimum, measured on the contact area
• Underplate type and thickness where gold or silver is used
• Contact normal force and wipe distance at the mating interface
• Maximum contact resistance at a stated test current and method
• Mating cycles and the motion profile expected in service
• Corrosion and vibration tests, with the standard and the acceptance limit
• Any area that must stay bare for welding, crimping or earthing

Verifying the interface, not just the deposit

Plating thickness can be checked by X-ray fluorescence on the finished part, and adhesion by a bend or tape test on a coupon. Those checks confirm the deposit. They do not confirm that the contact will survive vibration.

For that, use a contact resistance measurement before and after a vibration or thermal cycling sequence. A four-wire measurement at a low test current is more sensitive than a simple continuity check. Record the resistance at the start, after the conditioning and after a defined number of mating cycles. A rise of a few milliohms may be normal; a rise that continues with each cycle is a warning.

If the part is safety related, ask for the test fixture and the mounting method as well as the result. A contact tested loosely on a bench is not the same as a contact tested in its housing with the cable harness attached.