<p>Thermally induced dynamic warpage is a critical reliability issue for large-size and multi-level package structures. Reduction of soldering temperature with low temperature solder (LTS) is a potential solution to reduce the impacts of package warpage. The LTS process is implemented by using a low temperature solder paste screen-printed on the board to bond the ball-grid-array (BGA) solder balls attached on the package side. In this study, eutectic SnBi solder (15.8&#xa0;mg) was used to bond with Sn-5 wt.% Sb solder ball (22&#xa0;mg) at 160&#xa0;°C, 180&#xa0;°C, and 200&#xa0;°C for 3&#xa0;min, after which the microstructural evolution of mixed solder was investigated using microscopic observation and phase diagrams. The mixed solder joined to the Cu substrate was heated at 125&#xa0;°C for up to 480&#xa0;h to observe the growth of intermetallic compounds at the joint interface. Fully mixed solder with a composition of Sn-24.3 wt.% Bi-2.9 wt.% Sb was observed at the soldering temperature of 200&#xa0;°C, producing a two-phase microstructure of bulky (Bi) phase dispersed in the continuous (Sn) matrix in the solidified alloy, with Sb dissolving in the two phases. The Cu₆Sn₅ and Cu₃Sn phases were formed at the solder/Cu interface, while the Cu₃Sn phase exhibited a very sluggish growth rate compared to that of Cu₆Sn₅. Solid-state aging at 125&#xa0;°C increased the solubility of Bi in Sn, reducing the quantity of bulky (Bi) particles while producing many fine (Bi) precipitates in the solidified alloy.</p>