Some Fundamentals of Tin-Bismuth Metallurgy and Electromigration 

Estimated reading time: 1 minute

Near-eutectic tin-bismuth alloy is a strong contender as the solder of choice for low-temperature electronic assembly applications. This article 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 Pb-Sn 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. This paper will review the recent research by the iNEMI project team on Sn-Bi electromigration justifying a more physics-based study of the solder alloy. When results are summarized on Arrhenius plots, the prediction of electromigration life under any give field condition becomes straightforward.

To read this entire article, which appeared in the September 2024 issue of SMT007 Magazine, click here.