Size effects on the interfacial reaction and microstructural evolution of Sn-ball/Sn3.0Ag0.5Cu-paste/Cu joints in board-level hybrid BGA interconnection at critical reflowing temperature

The effects of solder joint size on the interfacial reaction and microstructural evolution of Sn-ball/Sn3.0Ag0.5Cu-paste/Cu joints in board-level hybrid BGA interconnection during reflow at two critical temperatures of 217.2 and 227.0 °C were investigated comprehensively. For the joints reflowed at 217.2 °C, which is slightly higher than the melting point (217.1 °C) of Sn3.0Ag0.5Cu solder, the Sn ball does not melt, but coalesces with the melted Sn3.0Ag0.5Cu paste, regardless of change in solder joint size, and the microstructure of the mixed part of the Sn ball and Sn3.0Ag0.5Cu paste does not change significantly. In contrast, for the joints reflowed at 227.0 °C, which is nearly 5 °C lower than the melting point (231.9 °C) of Sn ball, with decrease in joint size, the Sn ball exhibits premelting behavior gradually, and the microstructure of the mixed part of the Sn ball and Sn3.0Ag0.5Cu paste varies obviously. The thickness of the Cu₆Sn₅ layer at the Sn3.0Ag0.5Cu-paste/Cu interface increases with decrease in size of joints reflowed at 217.2 and 227.0 °C, and the growth of the interfacial Cu₆Sn₅ is significantly influenced by Cu concentration distribution near the interface between the solder and intermetallic compound layer in the molten solder. The experimental data and two-dimensional finite element simulation results indicate that the Cu concentration in small joints increases faster than that in large ones, and the relatively long time influence of high Cu concentration in the small joints leads to the formation of large Cu₆Sn₅ grains at the interface.