Microstructure evolution in microbumps for 3D-IC packaging

Ruo-Wei Yang,Chih Chen,Yuan-Wei Chang,Jin-Ye Juang,Tao-Chih Chang,Chau-Jie Zhan

doi:10.1109/impact.2010.5699610

Abstract

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 Ni 3 Sn 4 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 Ni 3 Sn 4 IMCs and the Ag 3 Sn grain size increased. In addition, the dispersed Ag 3 Sn compound would aggregate to form plated Ag 3 Sn compound as the thermal aging time increased. The growth kinetics of Ni 3 Sn 4 was volume diffusion (n=0.5) domination the diffusion process of Sn and Ni atoms. We calculated the Ni 3 Sn 4 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 Ag 3 Sn compound appeared in the solder bumps would increased. We used the Ni 3 Sn 4 IMCs growth rate constant to define the critical volume of Sn2.5Ag solder was 1621.0 μm 3 . 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 Ag 3 Sn would appear in the remaining solder after 1000-hr thermal aging at 150°C.

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