An observation and explanation of interior cracking at the interface of solder by electromigration

The electromigration (EM) of a solder joint under high-current stressing causes damage and reduces its service life. Previous studies have suggested that EM-induced cracks typically initiate at the cathode corner where the electron current enters, and propagate across the intermetallic compound and solder interface. Herein, however, another type of propagation is experimentally demonstrated whereby cracks initiate from voids located in the middle of the interface and propagate in two directions. This phenomenon was investigated further with finite element simulations. These simulations showed that, when the solder corners become rounded via the wetting phenomenon that occurs during solder reflow, the current density around the voids is greater than that at the solder corner. Under these conditions, cracking will initiate from the side of an existing void, instead of the corner from where electron current flows into the solder ball, when tensile stress is present at the void and compressive stress at the electron current entrance.