Electro-thermo-mechanical Reliability of Low Melting-point Solder Joints in Metal Oxide Varistor Packages under High Surge Current Densities

Metal oxide varistor (MOV) is a ceramic electronic component, which is extremely sensitive to overvoltage and is widely used in overvoltage protection devices. The resistance of MOV increases as the voltage across its terminals rises, thus it serves as a voltage divider for the electronics. In order to connect the MOV to the circuit, there is a common method at present to interconnect the electrode with the MOV by soldering process. Under extreme conditions, the solder joint becomes the key point of the entire device, so it is necessary to evaluate the performance of the solder. For the scenario of varistor applications, there are two main aspects to evaluate the performance of the solder: surge current test and short circuit current test. Currently, there is a main issue, that is, the joint in MOV device soldered by low melting-point eutectic Sn-58Bi solder usually fails when subjected to surge current. To investigate the root cause of solder joint failure and the fundamental mechanism under surge current, this study first examines the temperature rise and stress response characteristics of Sn-xBi (x=58, 40, 30, 20, 10) solder joints under the current surge. Subsequently, the influence of Bi content on temperature and stress distributions of solder joints during the surge test is analyzed to further elucidate the failure mechanism. The study identifies three factors significantly influencing temperature and stress distributions in the whole surge test, and their effects and underlying mechanisms are thoroughly studied and analyzed.