Electromigration in Eutectic Tin-Bismuth Bottom-Terminated-Component Solder Joints

Eutectic tin-bismuth solder has been identified in an iNEMI (International Electronics Manufacturing Initiative) roadmap as the solder of choice for 2nd level electronic packaging because of its desirable property of low melting temperature causing less warpage of the assemblies being soldered. Bismuth in Sn-Bi solder however has a high propensity to electromigrate leading to its segregation at the anode end of the solder joint, increasing the electrical resistance and potentially the brittleness of the joint. This paper reports on the electromigration testing of 60-µm high eutectic Sn-Bi bottom-terminated-component (BTC) solder joints on OSP (organic surface preservative) and ENIG (immersion gold on electroless nickel) surface finishes in 60, 80, and 100 °C ambients subjected to current densities in the 1-3 kA/cm
²
range. Electromigration rates were determined by measuring the rates of increase in solder joint electrical resistances. The results were captured on Arrhenius plots that can be used to predict electromigration lifetimes under any application condition. The rates of electromigration were found to be similar for BTC solder joints on OSP and ENIG surface finishes.