A relay is rated for a hundred thousand operations, and the contact pair erodes differently on every one of them. By the time the resistance has drifted far enough to matter, the datasheet number has been irrelevant for years.
This note is for buyers and design engineers specifying electrical contacts and contact materials. It covers how alloy choice, arc erosion and joint design interact, and which measurements actually predict whether a switch will last.
Silver content is only the headline number, while arc erosion and resistance decide whether a switch lasts
Silver carries the current and the oxides that form on its surface are conductive, which is why it dominates low and medium power contacts.
Adding nickel, tungsten or metal oxides trades some conductivity for resistance to welding and to arc erosion. The percentage tells you how much silver is present, not which of those trades was made.
Two materials with the same silver content can behave very differently in a circuit that breaks inductive loads, because the arc energy is what does the damage.
Arc erosion is a wear mechanism with a rate you can measure
Every break draws an arc, and the arc removes a small amount of material from one contact and deposits it on the other. The transfer is not symmetrical.
The rate depends on current, voltage, inductance and the speed of separation. A contact that lasts in a resistive circuit can fail quickly in a motor circuit with the same nominal rating.
Measuring mass loss across a controlled number of operations gives a rate you can use. Without that rate, the rated life is a marketing figure rather than a design input.
Contact resistance is the number that drifts before anything fails visibly
Fresh contacts measure in the milliohm range. As the surface roughens and oxides build in the eroded pits, that number climbs, and the heat it generates accelerates the process.
A four-wire measurement at a defined current is the only reliable way to see this. Two-wire readings include the lead and probe resistance and hide the change you are looking for.
Trending that number over a life test tells you how much margin a design really has, and it usually shows the end arriving earlier than the rating suggested.
Riveted, welded and staked joints fail in different ways
A riveted contact loosens under thermal cycling and the joint resistance rises before the contact itself wears out.
A welded contact holds better but concentrates heat at the weld nugget, and a poor weld is invisible until it separates under load.
Staked contacts depend on the deformation being consistent, which is a tooling question rather than a materials one. Each joint style needs its own acceptance check.
Silver migration and the insulation that quietly degrades
In humid conditions and with a DC bias, silver can migrate across an insulating surface and grow a conductive path that no contact erosion would predict.
The failure appears as leakage or as a short between adjacent poles, and it is often blamed on contamination rather than on the material choice.
Conformal coating, wider spacing and lower bias all help. The point is to recognise migration as a design consideration rather than a manufacturing defect.
Bounce and weld-through are the two ends of the same problem
Contacts that bounce on make produce a series of short arcs, each one eroding the surface and each one generating a burst of interference.
Contacts that weld on make stick closed, and the circuit stays energised after the command is removed. Both behaviours are decided by the material pair and the closing force.
Testing both means running make-break cycles under the real load, not a resistive load chosen for convenience.
Ambient conditions change both erosion and resistance
High ambient temperature softens the material, accelerates oxidation and shortens the life of every contact in the enclosure.
Humidity adds the migration path and changes the friction between the surfaces. A contact tested at laboratory conditions may never see the environment it was specified for.
Run the life test at the enclosure temperature the product will actually see. The derating that follows is usually larger than the design team expects.
Joining the contact to the terminal is part of the same problem
A contact that is brazed or soldered introduces a second interface, and that interface has its own resistance and its own failure mode.
Where the assembly uses solder, the paste choice and the reflow profile decide whether the joint is sound. The solder paste market offers options, and the difference shows up in voiding rather than in the datasheet.
Treat the contact plus its joint as one component. Testing them separately produces results that do not survive assembly.
Cost per operation beats cost per piece every time
A contact that costs less and lasts half as long doubles the cost of every operation, and adds the labour of the replacement to the calculation.
For equipment that is serviced in the field, the labour is usually the larger number, and the material choice becomes a service cost decision rather than a purchasing one.
Model the cost per operation across the expected life. It is the only comparison that includes both the part and the consequences of it failing.
What to write into a contact specification
Name the alloy family and the joint style, then state the load type the contact will break. Resistive, inductive and capacitive loads are not interchangeable.
Add the acceptance criteria for resistance at end of life, and the test method used to measure it. The contacts and contact materials picture is wide enough that a vague specification will always be filled in by the cheapest interpretation.
Finally, require the life test report with the first delivery. A supplier who has run the test will send it without argument.
Arc suppression changes the erosion rate more than the material does
The same contact pair fitted with a suppression network erodes far more slowly, because the network absorbs the energy that would otherwise cross the gap.
That does not make suppression optional equipment. It means a contact specified without one is being asked to do work the circuit should have handled.
Deciding where the energy is absorbed, in the contact or in a capacitor and resistor, is a system decision that is often left to whoever designs the enclosure.
Certification testing is a sample of one
A type test proves that the samples submitted passed the standard. It says nothing about the lot you receive two years later.
Where contact material is source-controlled, a supplier change can invalidate the assumption the certification rested on, and nobody notices until the failures arrive.
Adding a material change notification clause costs nothing at the negotiation stage and protects the whole downstream approval.
FAQ: questions buyers ask
Does higher silver content always mean a better contact?
No. Silver conducts well and its oxides are conductive, but it also welds and erodes under heavy arcs. Adding nickel, tungsten or oxides trades conductivity for resistance to those failure modes, and which trade is right depends on the load.
Why does a contact rated for a high current fail in a small motor circuit?
Because the rating is usually established on a resistive load. An inductive load stores energy that has to go somewhere when the circuit opens, and that energy erodes the contact far faster.
How should contact resistance be measured?
With a four-wire method at a defined test current, and as a trend across a life test rather than a single reading. Two-wire measurements include probe and lead resistance and hide the drift that matters.
What causes a relay to stick closed?
Welding on make, usually after repeated inrush current or after the surface has already been roughened by erosion. Closing force, material pair and the load profile all contribute.
Is silver migration a real risk in dry environments?
It is much slower without moisture, but condensation during temperature cycling can provide enough. Coating and spacing are cheaper than the field failure that migration causes.
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