Janus-faced Cu-core periphery formation and Bi phase redistribution under current stressing in Cu-cored Sn58Bi solder joints

The morphology evolution of a Cu-cored Sn58Bi solder joint under current stressing was investigated, where additional driving force caused by current density gradient together with electromigration was considered. One interesting phenomenon observed was that the Cu core periphery showed a Janus-faced microstructure with half white Bi phase and half gray Sn layer, which is supposed to be the polarity effect induced by electromigration. Another noticeable microstructure change, i.e., the Bi phase redistribution along the current direction in the current crowding region of the solder joint, was attributed to the current density gradient at the SnBi phase boundaries. Due to the resistance reduction of the redistributed SnBi matrix, a preferential path for current flow was established and the electrical performance of the Cu-cored Sn58Bi solder was promoted.