MIXED METALS

2/10/96

MIXED METALS -- Until about 1970, Gold was the material of choice for electronic connectors. Gold is a fairly good conductor, does not corrode, and is soft enough that it would tend to flow to fill in gaps between connectors. However when the price of Gold soared in the early 1970s, a number of alternatives were developed

As of 1997, a variety of metals are used for socketed component contacts and sockets. These include Gold platings of various thicknesses, Lead-Zinc, Copper, Silver, Beryllium-Copper (sometimes mistaken for Gold), etc. When dissimilar metals are used in the socket and component in the presence of electrical voltages and moisture in air (especially salt air), the less "noble" of the metals may potentially corrode rapidly resulting in increased resistance, higher voltage drops, and eventually to intermittent/unreliable contact operation.

Bare copper will corrode after exposure to sulfates in many common papers. It is rarely used. Tin corrodes, but tin sockets are often designed to form gas tight contacts that will not corrode at the contact because Oxygen is prevented from reaching the metal contacts. Repeated insertion of soft, thin gold platings into tin sockets may damage the gold surface as the tin side tries to dig out a gas tight contact. Few sockets of any kind are designed for repeated component insertion (Something to bear in mind when testing components). Repeated insertion of components into any non-Zero Insertion Force socket will eventually damage both the socket and component.

Corrosion (as well as thin platings) can be removed by reseating and/or mechanical cleaning with a pencil eraser. It is sometimes claimed that mixed metal corrosion is a serious problem, but many users with broad experience report no problems even with micro ohm sensitive analog circuits. It has been suggested that many "corrosion" problems may be due to organic tars from tobacco smoke or human skin oils rather than corrosion from metal mixing. It has also been suggested that small dust particles may be a greater problem in achieving reliable connections than is corrosion.

The best advertised mixed metal problem appears to be a problem with Gold-Tin contacts called "fretting corrosion". Fretting is a mechanical problem where small vibrations transfer tin to the gold surface after which both tin surfaces can oxidize. Fretting purportedly can be minimized or eliminated by using an appropriate contact lubricant. Some people claim this works. Others assert that it does not work and leaves the additional problem of removing lubricant from connectors.

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Last Update: content 5Nov99 -- spelling 12oct02

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