Microstructure and Property Changes in Cu/Sn-58Bi/Cu Solder Joints During Thermomigration

Thermomigration is very important for electronic packaging. To conduct heat away, it requires a temperature gradient. Just 1 degrees C difference across a microbump of 10 mu m in diameter induces a considerable thermal gradient in solder bumps, known as thermomigration, especially in eutectic Sn58Bi solder joints with a low melting point. However, the study of thermormigration in eutectic Sn-58Bi solder joints is rare,not to mention the effect on thermal-stress distribution in Sn-58Bi solder. In this study, Cu/Sn-58Bi/Cu solder joints are examined between different thermogradients of 1284 degrees C/cm and 890 degrees C/cm. We found the effect of thermogradients on the microstructures in the Sn-58Bi solders. The considerable Bi accumulations at the cold end and smaller grains formation near the hot end are caused by the higher thermogradient. Conversely, the fewer Bi accumulations at the cold end and grain growth at the hot end are induced by the lower thermogradient. Additionally, phase segregation would lead the crack propagation along the boundary due to the thermal-stress crowding on the phase boundaries. The findings provide a new sight of reliability issues for electronic packaging.