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.