Recent iNEMI Research on Electromigration in Tin-Bismuth Solder

Near-eutectic tin-bismuth alloy is a strong contender as the solder of choice for low-temperature electronic assembly applications. This paper will describe the recent significant developments by the iNEMI project team on the understanding of the electromigration behavior of Sn-Bi solder which has a unique physical metallurgy and electromigration behavior quite different for that of Sn-Ag-Cu (SAC) solder and other solders in common use today. The high rate of Bi electromigration and its high homologous temperature sets the near-eutectic Sn-Bi solder apart from the SAC and the Sn-Pb solders. The latter have lower homologous temperatures and have no one element dominating the electromigration phenomenon that can be readily tracked to monitor their electromigration behavior. In Sn-Bi alloys, the rates of Bi electromigration are clearly reflected in the rates of electrical resistance increase making it more convenient to use the fundamental physics-based electromigration equations rather than Black’s somewhat empirical equation that deals with the mean time to failure of solder joints. The high homologous temperature of Sn-Bi alloys makes in-situ study of electromigration in a scanning electron microscope more convenient, resulting in a much deeper understanding of the physical metallurgy and electromigration of the Sn-Bi alloys. For example, recent in-situ studies have shown that the root cause of decreasing solder electrical resistance in the early stage of electromigration, very evident at low temperatures and low current densities, is due to the microstructural coarsening of the alloy. When results are summarized on Arrhenius plots, the prediction of electromigration life under any given field condition becomes straightforward.