Microstructure evolution in a sandwich structure of Ni/SnAg/Ni microbump during reflow

The microstructure evolution in the sandwich structure of Ni/Sn2.3Ag/Ni microbumps with 4 μm and 12 μm in solder thickness during reflowing process at 260 °C was revealed in this study. When the microbumps with 4 μm in solder thickness were subjected to a 4-min reflow, the intermetallic compounds (IMCs) formed at the interface of the Ni under-bump-metallization (UBM) and the solder were Ni3Sn4, and Ag3Sn IMCs were formed dispersedly in the solder matrix. After the reflowing time reached 34 min, the Ni3Sn4 IMCs almost occupied the entire interface, and there were some voids remained in the Ni3Sn4 IMCs. The out-flowing of solder and the “side-attack” effect were responsible for the void formation. Finally, the remaining solder solidified through the out-flowing process, and the plate-like Ag3Sn IMCs were formed at the edge of the microbumps. Thus, the mechanical strength of the heterogeneous phase boundaries between Ag3Sn and Ni3Sn4 will play a crucial role in affecting the reliability of the microbumps in three dimensional integrated circuit (3D IC) applications. By comparison, the growth rate of Ni3Sn4 IMCs in the microbumps with 12 μm in solder thickness was somewhat slower than that of the case with 4 μm in solder thickness. However, the solder thickness just have little influence on the growth rate of the Ni3Sn4 IMCs in the microbumps, since the parabolic constant, k, in both cases are of the same order.