Microstructure Observation, Accumulated Equivalent Stress, and Creep Strain Simulation of SPD Disconnector Solder Joint Based on Isothermal Aging

Surge protective device (SPD) is a widely used type of overvoltage protection device, and the good resistance characteristics of metal oxide varistor (MOV) inside determine its ability to suppress transient overvoltage. However, the deterioration of MOV after long-term operation will make the temperature rise gradually. The disconnector made from disconnection mechanism and low-temperature solder joint can prevent the burning of MOV caused by increasing temperature, and its solder joint inside is vital to the successful disconnection. However, the cracks and voids will gradually appear inside the low-temperature solder joint during the operation, which seriously affects the reliability of SPD. Microstructures of SPD disconnector solder joint were assessed at 20 °C (natural operation) and 135 °C (aging operation) in this article. Besides, the energy-dispersive spectrometer (EDS) is used to compare the compositions and element migration at 20 °C and 135 °C. Furthermore, the accumulated equivalent stress and creep strain distribution on cracks and voids in solder joint are analyzed by the finite-element analysis (FEA). It is shown that the solder joint at 135 °C not only changes its microstructures and compositions compared 20 °C but also appears larger voids and cracks. Their appearance is due to the large accumulated equivalent stress and creep strain by analyzing the 2-D simulation model of the solder joint. Moreover, the increase of the radius of electrode corners can effectively reduce the accumulated equivalent stress and creep strain, and it will slow down the generation of cracks and voids and improve the reliability of SPD operation.