Electromigration in Solder Joints for High Temperature Flip-Chip Application

Abstract

The electromigration effects in chip metallization and wire bonds are well known and detailed investigated. Current density could be extremely high because of the small size of the cross sectional area of conductors. This can cause a migration of metal atoms toward the electrical field, so current densities up to 106 A/cm2 are possible. In comparison with chip structures are the usual solder joints of flip chips relatively thick. But the homologue temperature of solder alloys, typically based on tin, is also much higher than for gold or aluminum wires. For instance a SAC solder alloy is naturally preheated up to 0.6 homologue temperature, for high temperature application with 125 °C operating temperature even more than 0.8. This means, that atoms are very agile and a directed movement needs only lower field strength. Additionally is the specific resistance of solder alloys tenfold higher than for aluminum, copper or silver. So is the self-heating of solder joints not negligible. This contribution shows the test results of flip-chip assemblies, loaded with different current densities and stored at 125 °C ambient temperature. At the end of life of a significant number of test chips, a metallographic analysis shows the causing failure effects and weak spots of assemblies. Accompanying simulations help to explain the interaction between current density and migration effects.