Microstructural Analysis of Reworked Ball Grid Array Assemblies Under Thermomechanical Loading Conditions

Plastic ball grid array packages (Sn3.0Ag0.5Cu) were assembled with Sn3.0Ag0.5Cu and Sn37Pb solder paste to form Pb-free and mixed BGA assemblies. The Pb-free and mixed assemblies were subjected to one (1X) rework cycle and three (3X) rework cycles. All the reworked Pb-free and mixed assemblies were then subjected to a temperature cycling test with a temperature range of -55 degrees C-125 degrees C. The cycles to 0.1% failure of 1X reworked Pb-free assemblies was 18% smaller than that of nonreworked Pb-free assemblies, and the cycles to 0.1% failure of 3X reworked Pb-free assemblies was approximately 100% smaller. However, the cycles to 0.1% failure 1X reworked mixed BGA assemblies was approximately 200% larger than that of the nonreworked mixed BGA assemblies, and the cycles to 0.1% failure 3X reworked mixed assemblies was 35% larger. Detailed microstructural analysis and geometry analysis were provided to explain the temperature cycling reliability differences between the nonreworked and reworked assemblies, as well as the differences between the Pb-free assemblies and the mixed assemblies. The increase in the temperature cycling reliability of reworked mixed assemblies was found to be related to more homogenous Pb-rich phase distribution in the bulk solder.