Reduction of AgSnO2 contact resistance by changing the brazing method and corresponding improvement of an 18.5kW contactor

The replacement phase of AgCdO by Cd free material, especially AgSnO₂ in contactors manufactured in Europe, can now be considered complete. However, AgSnO₂ still has not replaced AgCdO in contactors for avionics due to their specific switching requirements of low currents because higher and more unstable contact resistances are observed during service life of these devices. This can cause significant concerns in low current applications with the risks of transmission failures of electrical information of control and measures. The strong influence of contact force on the contact resistance has already been presented (Holm 2013). To further improve the performance, this study focuses on the influence of surface finishing methods needed depending of the assembly processes used. To produce the assembled contacts of an 18.5kW contactor, induction brazing was replaced in this study by resistance brazing and with total suppression of finishing treatments. First, the two production processes and the visual appearance of parts produced by these two processes are presented. The study then focuses on the influence of the suppression of surface finishing treatments of the parts produced by resistance brazing. To validate this suppression, a natural aging of 2 years was performed and contactor poles were tested to measure the possible gain on contact resistance in low-level switching depending of the contact force. The temperature rise at new and during AC1 and AC3 service life of contactors are compared, as well as the low-level behavior at 10VDC, 20mA. In conclusion, the advantages and constraints of the process change are presented, as well as the perspectives offered, especially for avionics and space devices.