Arcing Failure of ROHS Compliant Electromagnetic Relays
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Electronic relay contacts have traditionally been made of materials, primarily Ag/CdO, which are resistant to welding under short, high power pulses. However, since 2006, RoHS prohibits the use of cadmium in electronics, driving the elimination of Ag/CdO and its replacement with Ag/SnO2. The reliability of relays made with Ag/SnO2 contacts has been shown to be vendor specific. This thesis focuses on developing an understanding of the metallurgical and design factors that vary by manufacturer and their effect on welding susceptibility of Ag/SnO2 electromagnetic relay contacts and other related relay failure mechanisms. In addition, it aims to predict a safe operating area of power and energy over which specific relay contacts will not weld under high power DC conditions.
Relays from various manufacturers were subjected to capacitor discharge pulses of 250 V at 10-80 uF to characterize relay reliability. Failure analysis was then conducted on the welded contacts using scanning electron microscopy (SEM) and wavelength-dispersive spectroscopy (WDS) in order to address material properties and design variations that affect the welding susceptibility of relays. The incidence and extent of degradation is correlated to material characteristics including contact composition, oxide content, hardness, contact geometry, and surface roughness using a physics of failure approach. The relays with a higher percent content of indium oxide exhibited a greater reliability than those without. Both power and energy were then varied to further investigate the one cycle to failure boundary region and a failure map is presented.