Joule Heating Effect on the Electromigration Lifetimes and Failure Mechanisms of Sn-3.5Ag Solder Bump

In this work, the effect of Joule heating on the electromigration lifetimes and damage evolution mechanism of Sn-3.5Ag flip chip solder bump was investigated at highly accelerated electromigration test conditions by using both ex-situ electromigration test in an oven and in-situ electromigration test in a scanning electron microscope, respectively. Highly accelerated electromigration test temperatures and current densities performed in this experiment were 140 ~ 175 degC, and 6 ~ 9times10⁴A/cm², respectively. Mean time to failure of solder bump decreased as the temperature and current density increased as expected. The activation energy and current density exponent were found to be 1.63eV and 4.6, respectively, which are very high due to severe Joule heating effect. In order to understand the fundamental failure mechanism and to investigate the current flow direction effect on the failure mechanism of Sn-3.5Ag solder bump, in-situ electromigration test was performed, which shows that interfacial crack initiated and propagated along Cu₆Sn₅/solder interface irrespective of current flow directions. Kirkendall voids formed at the Cu₃Sn/Cu interface but, not related to the electromigration induced interfacial crack propagation. Current direction effect on the failure mechanism of Pb-free solder bump is also discussed.