Effect of compressive stress on microstructural evolution of Sn-Bi-Ag/Sn-Ag-Cu hybrid BGA solder joints assembled by low-temperature soldering

The effect of compressive stress on the microstructure of low-temperature Sn-49Bi-1Ag/Sn-3.0Ag-0.5Cu hybrid BGA solder joints was investigated. The diffusion behavior of Bi atoms was determined by the competition of chemical potential gradient and strain-assisted effect. At the low temperature of 358 K, irregular Bi phases coarsened and the degree of coarsening increased with increasing compressive stress. Bi atoms in the mixed zone migrated along the strain direction, accumulated at the solder/Cu interface, resulting in the decreasing mixed percentage. At the high temperatures of 383 & 393 K, Bi phases gradually dissolved in ß-Sn matrix. The compressive strain inhibited the growth of interfacial IMC layer and retarded the Bi atoms diffusion from the mixed zone towards the un-melted Sn-Ag-Cu solder at all test temperatures.