Microstructure evolution in microbumps for 3D-IC packaging

In this study, we would analyzed the solid state reaction between Sn2.5Ag solder bump and Cu/Ni under-bump-metallization (UBM). After 150°C thermal aging, we observed that the intermetallic compounds (IMCs) at chip side and interposer side both were Ni₃Sn₄ IMCs. It indicated that the solder did not react with Cu and the Cu layer was completed. As thermal aging time increased, the thickness of Ni₃Sn₄ IMCs and the Ag₃Sn grain size increased. In addition, the dispersed Ag₃Sn compound would aggregate to form plated Ag₃Sn compound as the thermal aging time increased. The growth kinetics of Ni₃Sn₄ was volume diffusion (n=0.5) domination the diffusion process of Sn and Ni atoms. We calculated the Ni₃Sn₄ IMCs growth rate constant was 0.067 μm/hr^½ at 150°C aging. As the aging time increased, the concentration of Ag in the remaining solder bumps also increased. When the concentration of Ag was over 3.5 wt.%, the probability of plate-like Ag₃Sn compound appeared in the solder bumps would increased. We used the Ni₃Sn₄ IMCs growth rate constant to define the critical volume of Sn2.5Ag solder was 1621.0 μm³. If the volume of the Sn2.5Ag microbumps were 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 1000-hr thermal aging at 150°C.