Contact Resistance of Oxidized Metals: Dependence on the Mating Material

The contact resistances of several oxidized metals were determined with smooth spherical probes that were made of various noble and base metals. It was found that contact resistance decreased sharply as the hardness of the probe was increased. Oxidized hard metal probes sometimes gave contact resistances which were less than those from probes made of soft noble metals. The reason is postulated to be that the plastic deformation, hence contact area, of a hard probe under mechanical load is less than that of one which is soft. Contact pressure at a fixed force is, therefore, greater, and the fracture or displacement of oxide on the surface is thereby enhanced. It may not be possible to apply sufficiently high pressures to displace films on hard base metals like nickel when the probe is plated with an easily deformed metal, such as tin-lead solder, and even tin-lead mated to itself may not give low values of contact resistance when the surface films are more than a few hundred angstroms thick at the forces ordinarily used in probing. Finally, a probe made of solid pure gold is a more sensitive indicator of contamination on a hard contact surface than is a hard probe, like electrodeposited gold on nickel underplate.