Effects of additional Co atoms on microstructural evolution in Sn-Ag-Bi-In solder under current stressing

Electromigration behavior of Sn-3.0wt%Ag-3.0wt%Bi-4.0wt%In (SABI) and SABI+Co solder joints with respect to the crystallographic orientation of Sn grains was investigated. The test sample had direct contact on Cu substrate with Cu dummy chip and the applied direct current was 20 A at 125 °C solder joint temperature. Before current stressing SABI solder solidified to form large Sn grains. After current stressing, failure of the solder joint was caused by the dissolution of Cu electrode on the cathode side. Fast dissolution of Cu atoms occurred along the c-axis of Sn in the SABI solder joint when the c-axis parallel to the electron flows. On the other hand, SABI solder joint with a small amount of Co shows a different microstructural evolution as compared with SABI, i.e., randomly orientated smaller Sn grains compared to SABI. Also, slight Cu dissolution was observed at the cathode side under same test condition since the Co addition has a substantial effect on the microstructural evolution on SABI solder.