Contact matters

If silver is good, gold must be better. For contact materials, it isn’t necessarily so. M-Series and S-Series toggle switches from NKK Switches are rated for 6 A at 125 Vac and 50 A at 125 Vac, respectively. These 6 A fixed contacts were damaged by 38 A of inrush current. In general, silver contacts are better able to withstand the arcing from high inrush current. The main disadvantage of silver contacts is tarnishing. The dome contact on the left shows considerable tarnishing, as opposed to the clean contact on the right. The electrical resistivity of a material tells how well the material conducts electrical current. Conductivity is the reciprocal of resistivity. Good conductors of electricity have low resistance or high conductivity. Switch contacts are made from a variety of metals, but gold and silver are the most common. Each has advantages and disadvantages. The big plus for gold is that it doesn't corrode or tarnish. However, its higher cost is a disadvantage, and it's less conductive than silver. Another problem with gold contacts is the potential for oxidation from organic compounds, simple versions of which are present in the atmosphere. In vapor form, they can be absorbed by the gold contact surface. Sliding or fretting between contacts causes a chemical reaction between the gold, which acts as a catalyst, and organic compounds. This reaction produces long-chain compounds or polymers. One way to get around these drawbacks is to place a thin gold layer over a copper contact. Doing so demands barrier plating. Due to the cost of gold, the plating is usually thin, on the order of 1 to 2 microns. Without a barrier between the two materials, copper can migrate to the gold surface, where the exposed copper begins to oxidize forming a copper-oxide layer that acts as an