Retarding Electromigration in Lead-Free Solder Joints by Alloying and Composite Approaches

The voids induced by electromigration (EM) can trigger serious failure across the entire cathode interface of solder joints. In this study, alloying and composite approaches showed great potential for inhibiting EM in lead-free solder joints. Microsized Ni, Co, and Sb particles were added to the solder matrix. Cu and Sn particles were added to the melting solder to form in situ Cu6Sn5, which formed a barrier layer in the underbump metallization of flip-chip solder joints. The polarity effect induced by EM was observed to be significantly inhibited in the alloyed and composite solder joints. This indicates that the Sn-Ni, Sn-Co, Sn-Sb, and Cu6Sn5 intermetallic compounds may act as barriers to obstruct the movement of the dominant diffusion species along phase boundaries, which in turn improves the resistance to EM. However, Sb particles could induce crack formation and propagation that might lead to joint fracture.