Key takeaways
- A bimetal contact uses a switching face on a conductive backing; a trimetal adds an intermediate layer for the joint or the heat path.
- The layer ratio matters as much as the materials, because it sets the heat spread and the amount of face material available for erosion.
- The joint to the carrier is part of the contact assembly and needs its own specification and test.
- Specify the face material, the layer sequence, the thicknesses, the joint and the inspection on one drawing.
Why a contact is not one material
A contact has two jobs that pull in different directions. The face has to resist arc erosion, welding and contact resistance. The body has to conduct heat and current to the terminal and join reliably to the carrier. A single material rarely does both at the best cost, so the part is built as a stack.
A bimetal contact bonds a switching face to a backing. A trimetal contact adds a third layer between them, usually to improve the bond, act as a diffusion barrier or spread heat. The name describes the number of layers, not a fixed material combination.
The layer choice is a design decision with commercial consequences. Silver alloy at the face is expensive, so the design wants the thinnest face that will survive the life. Copper or brass in the backing is cheap and conducts well, but it is soft and may not weld to the carrier without an intermediate layer. The stack is the compromise between those pressures.
When a bimetal is enough
A bimetal is the simpler stack: a contact face bonded directly to a backing strip. It suits a contact where the switching duty is moderate, the backing is compatible with the carrier and the heat path is short. A silver copper bimetal on a brass carrier is a common example, and it is often the lowest cost option that meets the electrical requirement.
The direct bond is the weak point to check. Silver and copper form a good metallurgical bond, but the interface can diffuse at temperature and change over life. If the contact runs hot, a barrier layer may be needed even when the electrical duty is modest.
A bimetal also limits the face thickness. The face has to be thick enough to survive the erosion allowance and thin enough to keep the cost sensible.
When a third layer earns its place
A trimetal adds an intermediate layer for a reason. It may be a diffusion barrier that keeps the face and the backing from interacting, a compliant layer that absorbs thermal expansion, or a layer chosen so the underside can be brazed or welded to the carrier. Each reason leads to a different material and a different thickness.
The extra layer costs money and adds a process step, so it has to solve a real problem. If the contact runs hot, a copper intermediate can spread heat and lower the face temperature. If the carrier is aluminium, an intermediate layer may be needed to make a sound joint. If the face is a silver oxide composite, the intermediate layer may be part of the manufacturing route rather than a separate design choice.
Ask the supplier to state what the intermediate layer does. A drawing that specifies a trimetal without saying why leaves the supplier free to choose a stack that meets the drawing and not the function. When the layer is there for the joint, the joint test is the proof. When it is there for heat, a temperature rise test is the proof.
Layer thickness and ratio
The face thickness sets the erosion allowance. Too thin and the contact wears through before the life target. Too thick and the cost rises without a benefit, and the heat path through the face is longer. The backing thickness sets the heat spread and the mechanical support for the joint.
For illustration, a contact might use a face of 0.3 to 0.8 mm on a backing of 1.0 to 2.0 mm, with an intermediate layer of 0.1 to 0.3 mm. Those ranges are illustrative, not a specification. The right values depend on the current, the arc energy, the contact force and the life target, and they should be confirmed by a thermal and life test.
The ratio between the face and the total thickness also affects the manufacturing route. A very thin face on a thick backing is harder to bond without damaging the face, and a very thick face on a thin backing can distort during the joint.
| Construction | Layer sequence | Typical use | What to control |
|---|---|---|---|
| Bimetal | Face on conductive backing | Moderate switching duty, short heat path | Bond quality and face thickness |
| Trimetal, barrier | Face, barrier, backing | Hot contacts or a reactive face | Barrier continuity and diffusion |
| Trimetal, joint | Face, joint layer, backing | Carrier material that needs a helper layer | Brazability or weldability |
| Face on carrier | Face joined directly to the carrier | Simple pads and buttons | Joint area and carrier flatness |
The stack is illustrative. Material grades and thicknesses must be set against the electrical duty and the life test.
Joining the layers and the carrier
The layers can be bonded by roll cladding, explosive welding, brazing or diffusion bonding. Each route gives a different interface. Roll cladding is common for strip and gives a long, continuous bond. Brazing gives a local joint with a filler metal that must not soften the face. Diffusion bonding gives a strong interface but needs heat and pressure that can affect the backing temper.
The joint to the carrier is a second process and often the one that fails. A contact tip brazed to a brass blade has a braze fillet, a heat affected zone and a residual stress. A welded button has a nugget and a small heat affected zone. Both need a defined area, a defined energy and a test that measures the result, whether that is a shear test, a pull test or a bend test.
Specify the joint on the assembly drawing, not only on the contact drawing. Give the joint type, the filler if any, the coverage, the maximum void and the test. Then state the process sequence, because joining the contact after the carrier is formed is different from joining it before.
The carrier and the heat path
The carrier is part of the contact assembly even when it is a separate stamping. It carries the current from the contact to the terminal, and it spreads the heat. A thin carrier with a narrow neck will run hotter than a wide one, and the contact face will erode faster at the higher temperature.
Give the carrier material, temper and thickness, and say whether it is the same part as the terminal or a separate piece. If it is stamped, add the burr direction and the flatness. A carrier that is twisted after forming will tilt the contact face, and the tilt changes the force and the wipe in the assembly.
The finish on the carrier matters too. A silver plated carrier with a bare contact face is a common combination, but the plating has to stop where the contact is joined, or the joint is made through a coating that may not be compatible. Masking, selective plating or a post-join plate are all valid routes; the drawing has to say which.
Specifying and inspecting the assembly
A useful drawing states the face material and its thickness, the intermediate layer if any, the backing material and thickness, the overall thickness, the joint method, the joint coverage and the finish. It also states the test: a shear or pull value, a hardness range, a bond line check or a thermal result.
Inspection follows the same list. Measure the overall thickness and the face thickness on a cross section, check the bond line for voids, test the joint on a sample and check the face hardness. For a silver oxide face, add the oxide content and the distribution. A visual check of the face tells you very little about the layers behind it.
Keep the cross section with the first article. A photograph of the bond line at a stated magnification is worth more than a certificate that says the part conforms.
Frequently asked questions
What is the difference between a bimetal and a trimetal contact?
A bimetal has a switching face bonded to a backing. A trimetal adds an intermediate layer, usually to act as a diffusion barrier, to spread heat or to make the joint to the carrier more reliable.
How thick should the contact face be?
Thick enough to survive the erosion allowance for the required life, and no thicker. The value depends on the current, the arc energy and the contact force, so it should be set by a thermal and life test rather than by a rule of thumb.
Does a trimetal always last longer than a bimetal?
No. The extra layer helps only when it solves a real problem such as diffusion, heat or joint compatibility. A trimetal with the wrong intermediate layer can add cost and still fail.
How is the contact to carrier joint inspected?
With a shear or pull test on a sample, a cross section of the joint and a check of the joint area and voids. The test and the acceptance value belong on the assembly drawing.
Sources and further reading
- ASTM B539, Standard test methods for measuring resistance of electrical connections
- ISO 17672, Brazing, filler metals
- Qumek contact assembly drawing and first article notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Specifying a layered contact?
Send the stack, the duty and the joint. The engineering desk will reply with a note on the layer thicknesses and the test that proves the bond.
Send a contact drawing