Electromigration and Annealing Kinetics of Cu pillar

Cu pillar bump was annealed or current stressed at various current densities. Based on the results of electromigration life time test, the resistance to electromigration of Cu pillar bump was much better than conventional solder bump. Also, intermetallic compounds (IMCs) growth behaviors were investigated by in-situ scanning electron microscopy (SEM) observation. Only Cu6Sn5 was observed in the Cu pillar/Sn interface after reflow. However, Cu3Sn formed and grew at Cu pillar/Cu6Sn5 interface with increasing annealing and stressing time. The growth kinetics of total (Cu6Sn5 + Cu3Sn) IMC changed when all Sn phases in Cu pillar bump were exhausted. The complete consumption time of Sn phase in electromigration condition was faster than that in annealing condition. Kirkendall voids, which would be detrimental to the reliability of Cu pillar bump, were observed at both Cu3Sn/Cu pillar and Cu3Sn/Cu under bump metallization interfaces.