Influence of Stress on the Electromigration Life of Solder

Abstract

The electromigration (EM) of a solder ball under high-current stressing causes it damage and reduces its service life. Previous studies suggest that the EM life of solder can be improved by properly selecting the solder stress property (compressive or tensile) at the corners where the electron current enters and exits, but complete and relevant research on this proposition is absent. In this paper, we investigated this proposition systematically with experiments and finite element simulations. The experimental results show that the mean time to failure of solder under EM is slightly greater if the stresses at the electron current entrance and exit are compressive instead of tensile. The simulation results show that the increasing rate of the vacancy density at the cathode corner will be the lowest if the stress at the cathode is compressive and the stress at the anode is tensile. The reverse case produced the highest rate of increasing vacancy density, while the other two cases of identical cathode/anode stress types were intermediate, as verified in the experiments. However, the differences in the increasing rate of vacancy density under different stress types were not obvious, and the influence of stress on the EM life of a 0.3-mm-diameter solder ball is negligible. Further investigation found that pre-existing stresses at any two corners of solder may have an effect on the EM life of the solder only when its size is under $10~\mu \text{m}$ . In summary, our investigation provides guidance to improve the reliability of solder under EM.