Dominant effects of Sn orientation on serrated cathode dissolution and resulting failure in actual solder joints under electromigration

• Excessive serrated cathode dissolution strongly depends on the Sn grain orientation.
• Sn grain orientation is a dominant factor that controls the direction of the serrated teeth.
• Producing joints with fine Sn grains is one of the approaches to improve the reliability against current-induced failures in solder joints.

Excessive metal dissolution is one of the major electromigration-induced degradation mechanisms in interconnects, and it often produces a distinctive serrated cathode interface with most of the serrated teeth inclined toward a specific direction. In this study, actual flip-chip solder joints were systematically analyzed to understand this highly interesting morphology. It was unequivocally established that the Sn grain orientation is a dominant factor that controls the direction of the serrated teeth. When the c-axis of a Sn grain was nearly parallel to the electron flow direction, serrated dissolution occurred, with the serrated teeth inclined toward the c-axis. These observations were rationalized based on the diffusion anisotropy of Cu in Sn.