Dissolution Behavior of the Cu-2.0 wt% Be Alloy (Alloy 25) in Molten Sn, Sn-3.0 wt% Ag-0.5 wt% Cu, and Sn-58 wt% Bi Lead-free Solders

This study investigated the dissolution behavior of Cu-2.0 wt% Be alloy (Alloy 25) in molten Sn, Sn-3.0 wt% Ag-0.5 wt% Cu (SAC), and Sn-58 wt% Bi (SB) lead-free solders at 240 °C, 270 °C, and 300 °C for 5–100 min. The dissolution rate of Alloy 25 in each molten solder system increased with the soldering temperature. The sequence of dissolution rates was as follows: Sn > SAC > SB. In addition, first-principles calculation revealed that adding Be to Cu makes it more difficult for Cu to incorporate into the Sn bulk. Notably, the linear C1–C2 path where the transition occurs directly between two adjacent lattice sites exhibits the lowest diffusion energy barrier for Be (0.24 eV), which increases to 0.48 eV in the presence of Bi. The Cu₆Sn₅ phase, with minor solubility of the Be atom, was formed at all solder/Alloy 25 interfaces. In the SAC/Alloy 25 system, the Ag₃Sn phase precipitated within the Cu₆Sn₅ grains, decreasing the dissolution rate of Alloy 25 in the molten SAC solder. Notably, a Cu₆Sn₅/liquid/Cu₆Sn₅ structure was observed in the SB/Alloy 25 system. The formation of a liquid phase at the SB/Alloy 25 interface effectively reduced the dissolution rate of Alloy 25.