In this study, we study the metallurgical reactions at liquid state for different thickness Sn2.5Ag solder microbumps with Cu/Ni UBM. After 260 °C reflow, we observed that the intermetallic compounds (IMCs) at chip side and interposer side both were Ni3Sn4 IMCs. As reflow time increased, the thickness of Ni3Sn4 IMCs and the concentration of Ag in the solder increased. We calculated the Ni 3Sn4 IMCs growth rate constant was 0.45 μm/min 1/2. In addition, the dispersed Ag3Sn compound agglomerate to form plate-like Ag3Sn compound as reflow time increased. We used the Ni3Sn4 IMCs growth rate constant to define the critical volume of Sn2.5Ag solder was 1088 μm3. If the volume of the Sn2.5Ag microbumps was below the critical volume, the concentration of Ag in the remaining solder would over 3.5 wt.%, and the plate-like Ag3Sn would appear in the remaining solder after 10 min of reflowing at 260 °C.