In low voltage connector designs, precious metal coatings are used at the contact interface to ensure electrical continuity. Failure can occur when base metals diffuse to the contact surface, oxidize and form an insulating film that reduces conductivity. The increase in contact resistance from the conductance loss results in a voltage drop. If the magnitude of the voltage drop is of the same order as the circuit voltage, an erroneous signal results and a "failure" occurs. Susceptibility to this type of failure is a function of temperature and time, and was first demonstrated in studies conducted by Bell Laboratories in the late 1960's. These studies showed that the useful life of a low voltage connector with 40 (-inch thick gold contact material was thousands of years at 65(C, but only days at 200(C.
To counteract problems caused by base metal diffusion, both electroplated connectors and connectors made from clad materials are manufactured with a layer of nickel between the base metal and the precious metal contact surface. Research has shown that the effectiveness of this nickel barrier in preventing diffusion depends not only upon the temperature range in which the connectors operate and the time of exposure, but also on the manufacturing technology used to apply the coating. Connectors made from clad inlay materials exhibit excellent electrical stability after exposure to elevated temperatures for protracted periods of time. Electroplated connectors are more prone to failure after identical exposures.
The prevention of insulating film formation is of particular importance to automotive system designers. Designing for electrical stability under the hood presents a significant challenge. Under-hood interconnects for computerized ignition systems, mass air flow sensors, automatic transmissions, cruise control devices, anti-lock braking systems, and new generation suspension control systems may be required to operate at temperatures ranging from 100(C to 200(C. Film formation on the contact surface caused by diffusion could lead to system failure and result in high warranty costs or expensive liability claims. Because diffusivity is both a material and structure property, the design process involves selecting a spring material, coating it with a contact material, and evaluating system resistance to film formation. The most important decision for prevention of film formation is selection of coating technology.
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