Electromigration in Pb-Free Solder Joints in Electronic Packaging

Pb-free solders have been adopted to replace Pb-containing SnPb solders in the electronic packaging industry due to environmental concerns. Electromigration is one of the critical reliability issues for fine-pitch Pb-free solder bumps. This chapter reviews the unique features of electromigration in flip-chip solder bumps, emphasizing the effects of current crowding and Joule heating. It also describes the changes in the physical properties of the solder bumps during electromigration, such as bump-resistance changes, the redistribution of temperature, and the enhanced brittle mechanical behavior. In addition, the challenges for a better understanding of electromigration in Pb-free solder microbumps are discussed. As the dissolution rates of Ni and Cu in Pb-free solders are much higher than those in Pbcontaining solders, fast dissolution of Cu and Ni from the under-bump metallization into the solder bump under electromigration may endanger the solder bump reliability because of the anisotropic nature of Sn microstructure in Pb-free solder microbumps. © 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.