Key takeaways
- Silver content is a starting point, not a specification. The oxide system and the manufacturing route change behaviour at the same silver level.
- Weldability has two meanings: the ability to join the tip to the carrier, and resistance to sticking between contacts. Specify both.
- The backing layer and the joining process decide bond strength and heat rise more than the contact grade alone.
- Inspect the joint with a shear or peel test and a cross-section, not by looking at the top face.
What a contact tip or pad is
A contact tip is a piece of contact material joined to a carrier, usually a copper or brass blade, a terminal or a busbar. A pad is the same idea in a different shape: a flat segment rather than a shaped tip. Neither carries its own mechanical fixing. The joint, whether it is a weld, a braze or an adhesive, has to carry both the current and the separating load.
That is the main difference from a contact rivet. A rivet stakes through the carrier and holds itself. A tip is only as good as the bond beneath it. The top face gets the attention because it is visible and because it meets the arc, but the bond line is where most field failures start.
Tips and pads are used where the contact shape has to follow the blade, where the contact material is difficult to form into a rivet, or where a local wear surface is needed on a large copper conductor. They are common in contactors, circuit breakers, disconnectors, battery terminals and high-current switchgear.
Forms and how they are joined
The common forms are a solid tip with a shaped face, a flat pad, a segment cut from strip, and a profiled contact with a radius or a groove. The form is usually driven by the carrier geometry and by the space available for the arc to run.
Resistance welding is the fastest joining method for small and medium tips. Current passes through the tip and the carrier, and the interface heats under force. It suits fine silver, silver nickel and silver tin oxide with a suitable backing, and it is easy to automate. The risk is shunt current and electrode wear, which show up as inconsistent nugget size.
Brazing gives a larger bonded area and is tolerant of material combinations that do not weld well, including silver tungsten and silver graphite. It needs a filler alloy, a flux or a controlled atmosphere, and a heat cycle that does not soften the carrier more than the design allows. Laser welding is used for local, low-heat joins and for seam sealing, but it is a line bond rather than an area bond.
| Method | Heat input | Best fit | Main risk |
|---|---|---|---|
| Resistance weld | Local, fast | Small to medium tips, high volume | Shunting, electrode wear, weak nugget |
| Brazing | Whole assembly | Silver tungsten, graphite, large pads | Filler voids, carrier annealing |
| Laser weld | Very local | Seam sealing, thin carriers | Limited bonded area, porosity |
| Adhesive | None | Low-current or cosmetic parts | Thermal ageing, conductivity |
The method must be chosen with the contact material and the carrier, not after both are fixed.
What silver content buys
Silver is the conductive phase. More silver generally means lower contact resistance and better current sharing across the face. It also means a softer material, more tendency to weld under high inrush, and usually a higher price. The relationship is not linear, because the non-silver phase controls the arc behaviour.
A 90% silver nickel grade and a 90% silver tin oxide grade have the same silver content and very different performance. The nickel particles are ductile and conductive; the tin oxide particles are hard and resistive. The first gives a low resistance contact with moderate erosion resistance. The second gives better erosion resistance with a higher and more variable resistance. Quoting only the silver percentage hides that difference.
Silver content also affects the joining process. A high-silver, soft tip deforms easily under welding force and can flow out of the joint. A hard, oxide-dispersed tip needs more force and more energy to form a nugget. That is why the joining method and the material grade have to be developed together.
Two kinds of weldability
Weldability can mean how easily the tip is joined to the blade, or how strongly the two contacts weld to each other when they close. A grade can be easy to join and prone to sticking. State which property the application needs.
The common silver grades
Fine silver is used for signal and low-current contacts where resistance is the priority. It joins easily and has the lowest resistance of the group, but it erodes and welds readily under load.
Silver nickel at 90/10 or 80/20 is a general-purpose switching material. It has good conductivity, reasonable erosion resistance and a lower cost than the oxide grades. It is widely used in relays and small breakers. Under high inrush it can stick, so the contact force and the opening speed need attention.
Silver tin oxide is the main cadmium-free option for load switching. It resists erosion and has good anti-welding behaviour when the oxide is well dispersed. Its weakness is a resistive surface film, which is why low-force applications need a grade with additives or a different material.
Silver cadmium oxide still offers an excellent balance of low resistance and anti-welding performance, but cadmium restrictions mean it needs a market-by-market check. Silver tungsten and silver graphite cover the extreme ends: tungsten for erosion resistance at high current, graphite for welding resistance in high-inrush and capacitor-switching duty.
- Match the grade to the load: inrush, inductance, current and operations per life.
- Ask for the oxide system and the additive, not only the silver percentage.
- Check the joining route before choosing a grade that cannot be welded or brazed to the carrier.
- Confirm the regulatory position for cadmium and lead in each destination market.
Backing layers and brazeability
Many tips are supplied with a backing layer on the joint face. The backing is usually copper, nickel or a silver-rich alloy, and its job is to improve wetting, reduce oxide at the interface and give the weld or braze a consistent substrate. A backing also stops a reactive contact material from forming a brittle compound with the carrier.
The backing is not a free addition. It adds a thermal resistance between the arc and the blade, and it changes the height of the assembly. If the tip is staked or clamped, the backing thickness belongs in the stack. If the tip is brazed, the backing has to be compatible with the filler and the furnace atmosphere.
For silver tungsten and silver graphite, a brazeable backing is often essential. These materials do not resistance weld reliably to copper, and a direct braze can leave voids because the filler does not wet the contact phase. A nickel or copper backing gives the filler something to bond to and keeps the joint continuous.
Specifying the tip and the joint
A tip drawing should describe the finished assembly, not just the contact piece. State the carrier material and thickness, the contact material and grade, the backing, the joining method and the bond acceptance criteria. If the supplier is free to choose the joining method, ask for the method in the quote so it can be reviewed.
- Contact material designation, silver content and oxide or additive system
- Tip or pad dimensions, face form and edge radius
- Backing material and thickness, if used
- Carrier material, temper and thickness
- Joining method and any filler alloy or atmosphere
- Bond coverage, void limit and shear or peel strength
- Contact resistance limit at a stated current and force
- Plating, if any, and the areas that must stay unplated
Inspection and life evidence
The first check is visual: tip position, tilt, squeeze-out and discolouration. The second is dimensional: height after joining, tip flatness and carrier distortion. The third is destructive. A shear test pushes the tip sideways off the carrier and gives a number that can be tracked lot to lot. A peel test is useful for pads with a large bonded area.
A cross-section shows what a shear test cannot: void distribution, bond-line thickness, grain growth in the carrier and whether the filler reached the edges. For brazed joints, look for a continuous fillet and an absence of dark, unbonded zones. For welded joints, look for a nugget that spans the interface rather than sitting entirely in the tip.
Life evidence should come from a test that resembles the application. A make-and-break test at the rated load, with the actual closing force and opening speed, is more useful than a generic endurance figure. Record the material batch, the joining parameters and the test circuit, so a change in any of them can be traced to a change in results.
Frequently asked questions
Does a higher silver content always give better performance?
No. Higher silver usually lowers resistance but can reduce erosion resistance and increase the tendency to weld. The oxide system, the particle size and the manufacturing route matter as much as the percentage. Choose by load profile, not by silver content alone.
Can silver tin oxide tips be resistance welded?
They can, with the right backing and welding schedule. The oxide dispersion makes the material harder and more resistive than silver nickel, so the process window is narrower. Prove the nugget with a shear test and a cross-section before release.
Why do some tips have a copper or nickel backing?
The backing improves wetting during brazing or welding, limits oxide at the joint and gives a consistent substrate. It also prevents a brittle reaction layer between the contact material and the carrier. The backing thickness belongs in the assembly stack.
How do I check that a tip is bonded properly?
Use a shear or peel test on a sample from each lot, plus a cross-section to check void coverage and bond-line continuity. A visual check of the top face tells you nothing about the joint underneath.
Sources and further reading
- IEC 60947-4-1, Low-voltage switchgear and controlgear, contactors and motor starters
- ISO 17672, Brazing filler metals
- Qumek contact tip drawing review and joining process 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.
Need a second opinion on a tip joint?
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