An evaluation of electromigration performance of SnPb and Pb-free flip chip solder joints

As the I/O count and power requirement of flip chip packages continue to grow, a decrease in solder joint dimension, and an increase in current density are expected. At the 90nm technology node, the current density through solder joints is predicted to increase to ∼2.7 × 10³/cm². At this level of current density, the electromigration (EM) reliability of solder joints becomes a concern, particularly for Pb-free solder alloys. In this study, flip chip packages with high-Pb and Pb-free solder joints are tested at a series of temperatures and current densities. Different designs of under bump metallization (UBM) are also included for each solder material. Statistical lifetime data are collected. Failure analysis is done utilizing focus ion beam (FIB), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX). The lifetime data suggest that the Pb-free solder has a much shortened EM life compared with the high-Pb solder joints. Additionally, even with the same solder material, different failure rates and mechanisms are observed for packages with different UBM designs. This difference is believed to be caused by the differences in the dissolution rate of the UBM materials into the solder and the formation rate of intermetallic compounds at the solder/UBM interface. These findings suggest that improvement of EM performance is possible through redesigning the UBM stack. A Black´s equation based analytical model is formulated using the experimental data collected. Failure data reported in the literature are also discussed and compared with those from the current study.