General notes about the use of dissimilar metals

Good engineering practice for connectors requires contacts made with corrosion resistant materials and avoids pairing those with grossly different galvanic potentials. Reactivity (measured as electronegativity or half-cell potentials) predicts the likelihood of "galvanic corrosion". This is a continuing problem when humidity is present, creating the effect of a short-circuited battery within a contact pair.

An extreme example of this would be a pairing of gold and aluminum with inadequate contact pressure. Here, with oxygen and moisture present in the air, the aluminum even more certain to oxidize than with both parts made of aluminum.

Corrosion products like oxides, sulfides, carbonates, etc. are usually insulators that interrupt a connection. Even poorly insulating types can exhibit semiconductor effects at the metal / oxide junction, creating unwanted rectifying effects. One symptom of this is demodulation of any stray RF signals present on the lines. Digital signals may experience poor rise time, which can show up as a computer crash or chaotic behaviour of any peripheral using an affected connector.

Stabilant 22 treatment is especially helpful when one must deal with poor connector designs. It reduces corrosion by keeping humidity and oxygen from creating the unwanted electrochemical action and reduces mechanical wear in all cases.

Service experiences in the early days of personal computers

• Many of the first enhancement boards made to fit the Apple Maclntosh™ computer used a connector clip which snapped down over the 68000 microprocessor chip on the main board. The initial design had gold plated contacts which exerted only a low pressure over a fairly large area of the corresponding solder-alloy plated IC contact.

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The design of that clip, together with the shielding effect of the board to which it was attached, created a higher temperature environment in the vicinity of the contacts. Additionally, the The clip’s molded polycarbonate housing would absorb small amounts of moisture when cool, releasing it when heated up. It was not unusual to find that the enhancement board became erratic after two to three weeks of operation.

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Examination of the hard black film that formed at the junction of the two metals revealed that it was substantially composed of oxides of tin and lead. A redesign of the clips did significantly reduce the failure rate; nevertheless, a number of companies servicing this equipment still found it prudent to apply Stabilant 22 to the contacts, and they reported that this took care of the marginal cases.

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• Connector problems with older graphic accelerator boards were addressed with the application of Stabilant 22. This alone was found to increase the Mean Time Between Failures from a two week period to about seven weeks. Even this magnitude of improvement in MTBF was not sufficient in the application, and additional contact cleaning was necessary before applying Stabilant.

On the basis of solving similar problems with other types of connectors, our recommendation was that the gold plate be changed to solder alloy plate, and that the contact pressure be increased either by the use of a thicker contact stock or the use of a small dome protrusion on the contact. We found that under these circumstances, the use of Stabilant 22 on a solder to solder contact pairing, resulted in a MTBF of years rather than weeks.