Emerging issues in the physics and control of electromigration in SnPb and Pb-free solder bumps

Electromigration is the movement of metal atoms in the direction of current flow. It is caused by the transfer of momentum from the passing electrons to atoms in the lattice. First discovered in thin-film aluminum conductors some thirty years ago, electromigration is a significant concern for solder bumps on chip scale packages (CSP). As the understanding of solder electromigration expands, new opportunities for risk mitigation have emerged. This paper addresses the physics of electromigration in solders with particular emphasis on the time and temperature dependant factors that affect accelerated testing. Pb-based solders are contrasted with Pb-free solders by realizing that the microstructures of these solders are substantially different and the risks from electromigration are different as well.