Electromigration induced ductile-to-brittle transition in lead-free solder joints

The effect of electromigration on ductile-to-brittle transition in flip chip solder joints has been studied using one-dimensional bamboo-type samples of eutectic 95.5Sn-3.8Ag-0.7Cu solder joined by Cu wires at two ends. Both electrical current and tensile stress were applied to the samples either in serial or in parallel. In serial tests, the strain rate was 6×10^-3. In parallel test, the creep stress was 7MPa. The current density applied was (1-5)×10³A/cm². The working temperature was 100-150°C. In both tests, the authors observed the ductile-to-brittle transition in which the fracture migrates from the middle to the cathode interface of the joint with increasing current density and time. The transition is explained by the polarity effect of electromigration, especially the accumulation of vacancies at the cathode interface.