Study of critical factors influencing the solidification undercooling behavior of Sn-3.0Ag-0.5Cu (SAC) lead-free solder and SAC/Cu joints

The solidification behavior of Sn-3.0Ag-0.5Cu solders and Sn-3.0Ag-0.5Cu/Cu joints was investigated by using differential scanning calorimetry (DSC) incorporated into the reflow process. The critical factors influencing the undercooling behavior of the solders and joints, such as the solder size, composition and substrate size, were examined. The results show that the type of the primary solidification phase and its volume fraction are the dominant factors affecting the undercooling of the solders and joints, and the degree of undercooling of the solders and joints increases inversely with the solder volume (joint size). For the same solder size (volume), when Cu₆Sn₅ is the primary solidification phase, the degree of undercooling of Sn-3.0Ag-xCu(x≥0.5wt.%)/Cu joints increases with increasing the substrate size and decreases with increasing the volume fraction of Cu₆Sn₅. The factors influencing the undercooling of solder joints are complicated, and the composition of the solder matrix is the dominant factor affecting the undercooling of the Sn-3.0Ag-xCu/Cu joints made of the solders with hypoeutectic compositions, while the substrate size plays the dominant role when the Cu content exceeds its eutectic composition.