A New Mechanism for Lateral Erosion

Although miniature dry reed sealed contacts are in increasing demand in telecommunications, their use is often limited to low current switching. Attempts to use dry reed contacts to switch higher currents lead to resistance failures or failures to release after a relatively small number of operations. These premature failures are always due to the formation of pips and craters resulting from localized arc erosion. It has long been recognized that the delocalization of the erosion would lead to better utilization of the contact material and consequently to a longer contact life. A new mechanism promoting lateral erosion is demonstrated and shown to operate under various electrical loads. The spreading of the erosion is linked to the microstructure of the precious metal layer with the most favorable structures having a high density of voids. The generation of the voids depends on the method of deposition of the contact metal layers. They can be readily produced in the case of electroplating by codepositing foreign molecules which can be decomposed by a subsequent heat treatment. A comparative study is presented of the wearability under electrical load of reed sealed contacts using either conventional dense gold or heat-treated void-containing gold as the contact material. Under diversified electrical stresses, the void-containing gold contacts show an improvement in mean life to failure ranging from a factor of seven to ten.