Electrical contact rivets: materials, head design and staking

What a contact rivet is

A contact rivet is a two-material part. The head carries the face that meets the opposing contact. The shank passes through a hole in a carrier, usually a copper or brass blade, and is deformed to hold the assembly together. The head is made from a contact material chosen for arc and resistance behaviour. The shank is made from a ductile, conductive material chosen for forming and current transfer.

The two are joined at the factory by cold welding, brazing or a press-sintered bond. That interface is not decorative. It sits in the electrical path between the contact face and the blade, so a poor bond shows up as heat and as a drifting resistance reading.

Rivets are common in relays, contactors, thermostats, small circuit breakers and appliance switches. Their advantage is assembly speed: one staking stroke sets the contact face and the mechanical joint at the same time. Their weakness is that the joint is hidden after staking, so the process has to be controlled rather than inspected afterwards.

The three interfaces that decide performance

It helps to think of a rivet as three interfaces rather than one part. The first is the contact face, where the arc lands and where constriction resistance is generated. The second is the bond between the head and the shank, which carries current and must survive the heat of the arc without cracking. The third is the clinch between the shank and the carrier, which carries the mechanical load and the current into the blade.

A design review that only looks at the head material misses two of the three. A fine silver face on a cracked bond, or a perfect bond on a loose clinch, will fail the same life test. Each interface needs its own callout and its own check.

Contact materials for the head

Fine silver has the lowest contact resistance of the common face materials and the poorest resistance to erosion and welding. It suits low-current signal and measurement contacts where resistance matters more than life. Silver nickel, usually 90/10 or 80/20, keeps most of the conductivity and adds hardness and better arc resistance. It is a workhorse for domestic and industrial relays at moderate currents, but it can weld under high inrush.

Silver tin oxide is the cadmium-free choice for switching loads. Its oxide dispersion resists erosion well, but the tin oxide film on the surface raises contact resistance, especially at low force. Modern grades add indium oxide, bismuth oxide or tungsten oxide to refine the microstructure and stabilise resistance. Silver cadmium oxide remains one of the most forgiving materials for welding and resistance, but cadmium is restricted in many markets and its use needs a regulatory check rather than an assumption.

Silver tungsten, typically 50/50 or 60/40 by weight, is for severe duty: high current, frequent arcing and a need to resist erosion. It is hard, has higher resistance than silver nickel, and needs more contact force to establish a stable spot. It is a specialist choice, not a general upgrade.

• Choose by load profile first: current, voltage, inrush, inductance and operations per life.
• Treat silver content as one variable, not the specification. A 90% silver grade can behave very differently from another 90% grade because of the oxide system and the manufacturing route.
• Ask for the material designation and the supplier's data sheet, not just a trade name.
• If the part must meet a cadmium restriction, confirm the exemption position for the exact application and market.

Shank materials and the current path

The shank is usually copper, a copper alloy or brass. Copper gives the best conductivity and stakes well in the annealed condition, but it work-hardens quickly and can crack if the clinch asks for too much deformation in one stroke. Brass is easier to form and machine and is cheaper, at the cost of conductivity. Copper nickel is used where a harder shank or a diffusion barrier is needed.

Plating matters more than many drawings admit. A silver-plated shank lowers the resistance at the blade interface and resists tarnish. A nickel barrier under the silver limits diffusion between the contact material and the shank at high temperature. Bare copper in a humid enclosure will oxidise, and the oxide sits directly in the current path.

For a bimetal rivet, the shank may be a separate wire cold-welded to a contact head, or a clad strip formed into a rivet. The bond line should be visible in a cross-section and free of voids and oxide inclusions. If the supplier cannot show a bond-line image, the bond is being taken on trust.

Head geometry and the contact face

Head diameter sets the available face area and the erosion allowance. Head height sets how much material can be lost before the face sinks to the shank. A head that is generous in diameter but thin in height will erode through early, even though it looks substantial from above.

The face form changes the first point of contact. A domed or radiused face starts with a small area, which raises the local pressure and helps break through an oxide film. A flat face gives a larger apparent area and a lower resistance once seated, but it is less tolerant of tilt and can trap wear debris. A conical or chamfered face is used where the contact must locate against a mating shape.

The edge of the head should carry a radius. A sharp edge concentrates the electric field during opening and erodes faster than the surrounding face. The same radius avoids a plating build-up that can flake at the rim. If the drawing shows a sharp corner, expect the supplier to add a deburr or a small radius and to ask which one you meant.

Staking: hole, protrusion and clinch form

Staking deforms the shank to fill the carrier hole and form a foot on the far side. The hole clearance, the length of shank above the blade and the punch shape do most of the work. A hole that is too tight grips the shank and forces it to buckle instead of filling the hole. A hole that is too loose lets the rivet sit off centre and leaves a gap that the clinch cannot close.

As a starting point for a cold-formed rivet, allow a hole roughly 0.05 to 0.10 mm larger than the shank diameter, and a protrusion above the blade of about one to one and a half shank diameters. Both figures are illustrative and depend on the shank material, the blade thickness and the clinch type. They are a place to start a conversation, not a universal rule.

The punch form decides where the metal goes. A roll clinch turns the shank outward into a ring. A star clinch splits and spreads the end into petals. A radial or spread clinch pushes the shank outward into a controlled bulge. A flat stake simply upsets the end. Each has a different bearing area and a different sensitivity to hole clearance.

What to put on the rivet drawing

A useful rivet drawing states the two materials separately and then the joint that connects them. It also states the carrier it will be staked into, because the rivet cannot be validated without the blade thickness and hole size it will meet.

• Head material designation, grade and manufacturing route
• Shank material, temper and plating, including any barrier layer
• Head diameter, height, face form and edge radius
• Shank diameter and tolerance, and the bond-line requirement
• Carrier material, blade thickness range and hole diameter tolerance
• Clinch type, staked height and foot diameter
• Pull-out or push-out force, and whether it is a lot test or a type test
• Contact resistance limit at a stated test current
• Any cadmium, lead or RoHS declaration required for the market

Inspection that proves the joint

The first check is dimensional: staked height, foot diameter and head diameter after forming. The second is destructive. A pull-out or push-out test on a sample from each lot shows whether the clinch is carrying load. A torque test is useful where the contact is a threaded terminal or where the blade is likely to rotate.

The third check is a cross-section. Cut through the rivet axis and look at three things: whether the shank fills the hole, whether the clinch has a sound fold without cracks, and whether the head-to-shank bond is continuous. A polished and etched section also shows grain flow, which is the best evidence that the shank was formed rather than torn.

Resistance should be measured on the finished assembly, not on a loose rivet. A four-wire reading from the contact face to a point on the blade includes the bond and the clinch, which is what the application actually sees. Record the test current, the force and the probe positions so the number can be repeated.