Agglomeration of Ag3Sn compounds in microbumps during reflow for 3D-IC packaging

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 Ni₃Sn₄ 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 μm. 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.