Comparing Electromigration in Tin-Bismuth Alloys using planar geometry solder joints

Electromigration in planar solder joints has been shown to be comparable to that in bottom-terminated component (BTC) solder joints of interest to the electronics industry. The planar solder joint approach to studying electromigration has the advantage that the bismuth electromigration and the microstructural evolution can both be tracked nondestructively while monitoring the electrical resistance of the solder joints. The planar solder joints are also easy to fabricate. The electromigration behavior of eutectic Sn-58Bi alloy was compared with that of Sn-58Bi-0.4Ag and Sn-37Bi-0.5Cu-0.5Sb alloys at 2 to 6 kA/cm
²
current densities in an oven at 80 °C. Also, the electromigration and the thermally driven microstructural evolution were studied using a homemade hot stage in a scanning electron microscope. A key finding was that microstructural coarsening led to decreasing solder joint resistance in the early stage of electromigration stressing. Later when the bismuth accumulation on the anode side of the solder joint became sufficiently continuous, the resistance began to rise in quite a linear manner. The three solder alloys tested were found to suffer similar electromigration rates with the possibility that the rate in the Sn-37Bi-0.5Cu-0.5Sb alloy may be somewhat lower.