Silver cadmium oxide vs silver tin oxide: performance and regulation

Why this comparison keeps coming up

Silver cadmium oxide was for decades the default contact material for medium-power switching. It combines low contact resistance with strong resistance to welding and steady arc erosion. It is also a cadmium-bearing material, and cadmium is restricted in many markets because of its toxicity. That combination puts design teams in a familiar position: a material that works well and a regulatory direction that points away from it.

Silver tin oxide is the main cadmium-free alternative. It has improved steadily, and modern grades with oxide additives close much of the gap. It is not a drop-in replacement. The resistance behaviour, the welding behaviour and the process window all differ, and a switch that is not validated can trade a regulatory problem for a reliability problem.

This guide compares the two on the properties that decide contact life. It does not give a regulatory verdict, because the rules depend on the application, the market and the date. Confirm the current position with a compliance specialist before committing a design.

What the two materials are

Silver cadmium oxide is a silver matrix containing fine cadmium oxide particles, typically in the range of 8 to 15 percent by weight. It is made by internal oxidation of a silver cadmium alloy or by powder metallurgy. The cadmium oxide particles are fine and well distributed, which gives the material its characteristic balance of resistance and anti-welding behaviour.

Silver tin oxide is a silver matrix containing tin oxide particles, often with a smaller addition of indium oxide, bismuth oxide, copper oxide or tungsten oxide. Tin oxide does not dissolve in silver in the same way that cadmium oxide does, so the manufacturing route is different. Most grades are made by powder metallurgy, and the additive system is a large part of what distinguishes one supplier's grade from another.

The microstructure explains the behaviour. Cadmium oxide decomposes at arc temperature and the decomposition absorbs energy, which helps to cool the arc and limit welding. Tin oxide is more stable, so the mechanism is different: the oxide resists erosion and increases the melt viscosity, but it also forms a resistive surface film.

Arc erosion and welding

Both materials resist erosion well at moderate current. Silver cadmium oxide tends to hold a stable face over a long life, with gradual rather than dramatic loss. Silver tin oxide can show better resistance to deep pitting at high current because the oxide network is thermally stable, but the face can become rough and oxide-rich.

Welding is where the two diverge most. Silver cadmium oxide has a strong reputation for resisting sticking under inrush, because the cadmium oxide decomposition cools the molten bridge. Silver tin oxide is more prone to weld in the basic grades, and the additive system is used to improve it. Grades with indium oxide or bismuth oxide are usually better than a plain tin oxide grade, but the improvement depends on the load and the force.

Do not assume that a cadmium-free grade is automatically worse. The best formulated silver tin oxide grades can match silver cadmium oxide in many applications. The point is that the grade matters, and a generic silver tin oxide callout does not capture it.

Contact resistance and film behaviour

Silver cadmium oxide usually has the lower and more stable contact resistance. The cadmium oxide particles are fine and the surface does not build a thick insulating film as readily. That is why it has been popular in low-force relays and in applications where the voltage drop across the contact is critical.

Silver tin oxide tends to form a tin oxide film on the surface. The film is thin but resistive, and it becomes more important as the contact force falls. At low force, the film may not break down reliably, and the resistance can drift upward over life. At higher force, the film breaks and the resistance stabilises.

This is the single most common reason a cadmium-free switch disappoints. The new material is fitted to a mechanism designed for the old one, the force is unchanged, and the resistance rises. The fix is usually more force, a different face form or a grade with a better additive, not a return to cadmium.

The regulatory question

Cadmium is restricted under several frameworks, including RoHS in the European Union and equivalent rules in other markets. Electrical contact materials have historically relied on exemptions, and those exemptions have been reviewed and narrowed over time. The position is not the same in every market, and it can change during a programme.

The practical approach is to treat the regulatory status as a design input with an owner. Ask three questions: which markets will the product be sold in, which exemption or threshold applies in each, and what is the expected review date. If the answer depends on an exemption that could close, plan the alternative now rather than at the end of the programme.

A material declaration and a full material composition statement should be part of the part record. If a supplier cannot state the cadmium content and the relevant declaration, that is a gap in the evidence, not a detail to resolve later.

Making the switch without losing reliability

A material switch touches four things: the contact force, the face form, the thermal path and the life test. Start with the force. If the new material needs more force to break the film, check whether the spring can supply it without increasing bounce or overstressing the hinge.

Then look at the face. A domed face concentrates the force and helps break the film. A flat face distributes it and gives a lower resistance once seated. The right choice depends on the force and the load. A small change in face radius can make more difference than a change in silver content.

The thermal path matters because a higher-resistance material generates more heat at the same current. Check the temperature rise at the rated current, not just the contact resistance at a low test current. If the design is already close to its thermal limit, the switch may need a larger face or a shorter path to the carrier.

• Recheck the contact force and the spring stress at the new material's requirement.
• Review the face form and the edge radius, not only the material.
• Run a temperature rise test at rated current, not only a micro-ohm reading.
• Repeat the welding test with the real inrush, including lamp and capacitive loads.
• Confirm the material declaration for every market in scope.

A validation plan that settles the question

Build the plan around the failure modes the application actually has. If the risk is welding, run a make-and-break test with the real inrush and measure the force needed to separate the contacts after each operation. If the risk is resistance drift, measure at low force and at the end of life, not only on a new part.

Use enough samples to see variation. Contact materials are made in batches, and the microstructure can vary within a specification. Test parts from at least two batches, and keep the batch records with the results.

Finally, compare the two materials on the same mechanism and the same test circuit. A comparison across different springs or different carriers tells you about the assemblies, not about the materials. The useful result is the one where only the contact material changed.