Contact brazing and welding: how to choose the joining process

The decision is a material pair

A contact joint has to do three things: carry the current with a low resistance, hold the contact in place against force and vibration, and survive the thermal cycles of switching. The joining process is the route to those three, and the route depends on the two materials being joined and the area available.

Resistance welding passes a current through the parts and uses the resistance at the interface to generate heat. Brazing melts a filler metal between the parts without melting them. Laser welding melts a small volume with a focused beam. Each has a window where it works well, and each has a failure mode that the drawing and the inspection plan have to cover.

The common mistake is to choose the process before the material pair. A silver tungsten face on a copper backing, a silver tin oxide face on a brass carrier and a silver nickel face on a copper alloy all behave differently. A supplier that offers one process for every combination is a supplier that has not shown you the exceptions.

Resistance welding

Resistance welding is the default for contact buttons and small tips on a strip or a carrier. The parts are clamped between electrodes, a current is passed for a short time and the interface heats and bonds. The cycle is fast, the heat is local and the process is easy to put on a production line.

The variables are current, time, force and electrode condition. A change in any one moves the nugget size. The interface has to be clean and flat, because a gap or an oxide film concentrates the current and produces an inconsistent joint. Contact materials with a high silver content conduct very well, so the current has to be high and the time short, which puts pressure on the power supply and the electrode cooling.

Electrode wear is the hidden variable. A worn electrode spreads the current over a larger area and the nugget shrinks. A supplier that dresses electrodes on a schedule and records the count is easier to control than one that dresses them when a joint looks wrong. Ask for the schedule and the nugget check.

Brazing

Brazing uses a filler metal with a melting point below the parts. The filler flows into the joint gap by capillary action and forms a metallurgical bond. It tolerates a wider range of material pairs than resistance welding, and it can cover a larger joint area in one pass, which suits a contact pad on a carrier or a tip on a blade.

The process needs a controlled gap, a clean surface and a flux or a protective atmosphere. The filler must wet both materials and must not alloy with the contact face in a way that changes its switching behaviour. A silver based filler is common for electrical joints, but the exact grade and the joint gap decide the strength and the resistance.

The heat is the trade-off. Brazing raises the whole part, or a large region of it, to the filler temperature. That can soften a work hardened carrier, distort a thin blade or change the temper of a spring. If the carrier is heat treated, the braze sequence has to come before the final heat treatment, or the heat treatment has to be repeated after the joint. A drawing that ignores the sequence can produce a joint that is strong and a carrier that has lost its spring.

Laser welding and other options

Laser welding melts a small volume with a focused beam. It is fast, it can be automated and it produces a narrow heat affected zone, which suits a delicate contact on a thin carrier. The joint is a fusion weld rather than a filler joint, so the material pair has to be weldable without a helper layer.

The catch is fit-up. A laser needs the parts to close a small gap, and it needs a consistent surface. A reflective silver surface can be difficult for some wavelengths, and a contact that is not held flat will produce a weld that looks complete and carries little current. Fixture design matters as much as the laser settings.

Other routes exist: ultrasonic bonding for some material pairs, friction welding for round parts and diffusion bonding for specialised assemblies. They are not general purpose, but they can be the right answer for a specific geometry. The point is not to pick a favourite process. The point is to match the process to the joint area, the materials and the volume.

Heat, temper and the material next to the joint

Every joining process puts heat into the parts, and the heat does not stop at the joint. The carrier may be work hardened, the contact may be a composite with a temperature sensitive oxide, and the plating may be a barrier that has to survive the cycle. The heat affected zone is part of the design, not an accident.

For a spring carrier, the loss of temper in the heat affected zone lowers the contact force. The force was set by the design, and the joint quietly removes it. A post-join stress relief or a design that keeps the joint away from the spring zone can solve the problem. A drawing note that says the carrier must not be annealed is not enough; the process has to be able to meet it.

For a composite contact, the heat can change the oxide distribution near the bond line. That may be harmless, or it may change the erosion behaviour at the edge of the face. A cross section and a life test on the joined part are the only way to know. Testing the contact before the joint misses the effect.

Inspection that proves the joint

A visual check finds a missing fillet and little else. The joint has to be tested mechanically and examined metallurgically. A shear test pushes the contact sideways and measures the load at failure. A pull test pulls it away from the carrier. A bend test works for a flexible carrier. The right test depends on the geometry and the load direction in service.

The cross section shows the bond line, the voids, the nugget or the filler coverage, and the heat affected zone. It is the only check that shows whether the process did what the drawing asked. Keep a cross section from the first article and from each significant process change.

For electrical performance, a contact resistance or millivolt drop measurement across the joint adds the functional check. A joint can pass a shear test and still have a high resistance if the bonded area is small. The two tests answer different questions and belong together.

Choosing the process

The table below compares the three main routes on the points that decide a programme. It is a starting point for a discussion with the supplier, not a substitute for a trial joint on the actual materials.