Influence of solder volume and interfacial reaction on the undercooling and solidification microstructure of BGA structure Cu/Sn-3.0Ag-0.5Cu/Cu joints

The solder joint´s volume and interfacial reaction during the soldering significantly affect its solidification behavior and microstructure feature; further, the microstructure of the solder matrix and the intermetallic compounds (IMC) have a direct impact on the performance and reliability of the solder joint. In this study, the effects of the solder volume and interfacial reaction on the undercooling behavior and solidification microstructure of ball grid array (BGA) structure Sn-3.0Ag-0.5Cu/Cu (SAC/Cu) single-sided and Cu/Sn-3.0Ag-0.5Cu/Cu (Cu/SAC/Cu) double-sided joints with different solder diameters (0.76, 0.50 and 0.30 mm) were investigated by reflow soldering process using a differential scanning calorimeter (DSC). DSC reflow results show that the undercooling of both SAC/Cu single-sided and Cu/SAC/Cu double-sided joints decreases with the increase of solder ball diameter. However, there is no big difference in undercooling value between SAC/Cu and Cu/SAC/Cu joints with the same joint size (or solder ball size). Microstructural analysis shows that the primary solidification phase of 0.30-mm-diameter double-sided solder joints is Ag₃Sn, instead of Cu₆Sn₅ in the solder joints with diameter of 0.50 and 0.76 mm, which leads to decrease of the undercooling of 0.30 mm solder joints to a value only a little higher than the undercooling of 0.50 mm solder joints. The undercooling of solder joints is influenced by several factors, mainly including the type of substrate material (UBM), solder dimension and primary solidification phase.