Base Metal Contacts: An Exploratory Study of Separable Connection to Tin-Lead

A study was made of solder plated butt contacts to determine the feasibility of designing separable connectors for high reliability low energy circuits without gold or other precious metals. The chemical reactivity of tin-lead and the penetrability of films that formed on it were determined. It is little degraded by sulfur vapor, 0
2
, SO2, H
2
S, CO, and NH3. However, films from NO2 and Cl
2
can grow to large thickness, limited only by time and concentration of the reactant. Corrosion of tin-lead was promoted by a synthetic dust and by salt spray. The penetrability of tin-lead depends on contact pressure, and, owing to creep, on temperature and time. Except at light loads with certain geometries, 0.025 mm (0.001 inch) tin-lead is penetrated nearly to the copper substrate. Reconnection to such spots may be less reliable than to fresh areas, particularly if the contacts are aged between separation and remating. The ability of films on tin-lead to be broken is dependent on contact geometry, load, and wipe. The most effective geometries are cones and short (2.5 ram) wedges having an acute included angle (60°), followed by cones and wedges with an obtuse angle (120°), followed by hemispheres of increasing radius (0.38 to 1.5 mm). The cones and wedges had rounded tips of 0.05 mm radius. The order of merit may be different with thickly plated contacts (much in excess of 0.025 mm); but thick coatings are undesirable on contacts because of their deformability. When good contact is obtained, however, as in the mating of clean surfaces, the ranking of probe geometries in maintaining good contact in aggressive atmospheres is different, with the 1.5 mm radius hemisphere being best. It may be possible to design reliable butt connectors with tin-lead coated contacts for critical applications. However, additional studies need to be undertaken. These include: development of field experience in varied environments, study of long term behavior of mated contacts such as cold welding at elevated temperature, and rematability of contacts that have been aged in corrosive environments between initial use and subsequent matings.