Influence of pre-existing void in the solder joint on electromigration behavior of Cu/Sn58Bi/Cu joints

The influence of the pre-existing void in the Cu/Sn-58Bi/Cu solder joint on electromigration behavior was investigated by in-situ SEM observation and focused ion beam (FIB) analysis. The results show that the void with smooth internal surface has no influence on the Bi atom migration, while the one with a sharp notch may easily result in Bi atom congregation and further lead to the formation of cracks there, indicating that the void with a sharp notch can deteriorate the reliability of the solder joint undergoing current stressing. In the meantime, the pre-existing void near the cathode can enhance the electromigration diffusion flux of Bi atoms and accelerate the ripening of the microstructure of the solder matrix. The comparative analysis reveals that the inhibition of Bi atom migration by the notch tip and the imbalanced shear stress along both sides of the notch tip induced by the Bi congregation are the main reason leading to the formation of cracks, while the reduction of the effective contact area and the high Joule heating are the root cause of the higher electromigration diffusion flux in the solder joint with a pre-existing void near the cathode side.