Thermomigration in SnPb composite flip chip solder joints

We have used composite flip chip solder joints to study thermomigration. The composite solder joint has 97Pb3Sn on the Si side and eutectic 37Pb63Sn on the substrate side. Redistribution of Sn and Pb occurs in thermomigration, and we found that Sn has moved to the Si side (the hot end) and Pb to the substrate side (the cold end). We estimate the driving force that a temperature gradient of 1000 degrees C/cm or a temperature difference of 10 degrees C across a solder joint of 100 mu m in diameter is sufficient to induce the thermomigration. (c) 2006 American Institute of Physics.