Thermomigration and electromigration in Sn8Zn3Bi solder joints

Individual effect of thermomigration (TM) and combined effect of TM and electromigration (EM) on the microstructural variation in Sn8Zn3Bi was investigated by stressing line-type Au/Ni-P/Cu-Sn8Zn3Bi-Au/Ni-P/Ni solder joints with a 5 × 103A/cm2alternating current (AC) or direct current (DC) at 110°C. Due to the different thermoelectric characteristics of Cu and Ni wires, a thermal gradient of 196°C/cm could be established across the solder joints according to the finite element simulation. In AC current stressing, there is no EM effect and only TM dominates the migration. Microstructural study shows that Zn atoms migrate towards the lower temperature side during TM. In DC current stressing, it is found that both EM and TM play important roles depending various experimental conditions. And the energy change during the EM and the TM is estimated to be δωem3.2 × 10-28Joule and δωem2.2 × 10-28Joule, respectively. Upon different current directions in DC current stressing, there is a counteractive or accelerated effect between TM and EM on Zn migration, resulting different microstructures at the cathode side in the solder joints.