| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系所 === 102 === In this study, three types of solder bump samples with Kelvin bump structures were employed to monitor non-destructively the evolution of resistance during electromigration (EM) testing.
The first type of sample was flip-chip bumps. The bump resistance was found to be less than 1 mΩ and increase as a concave-up curve. After the bump resistance increased to more than 10 times its initial value, it started to grow rapidly and then failure. The corresponding microstructure showed void nucleation and propagation. The void first formed near the current crowding spot and then grew along the interface between the intermetallic compound (IMC) and the solder. At the end stage of EM testing, phase coarsening caused by EM retarded the failure, and the void split into two parts. The relation between the remaining contact area and the bump resistance was calculated.
The second type of sample was 6-μm microbumps. The microbump resistance curve was concave-down. It started around 15 mΩ, increased rapidly in the beginning, and then reached a constant value after 400 hr of testing. The increase in the early stage of testing was around 5 mΩ, which was reasonable when compared with the results of finite-element models (FEMs). During EM testing, the cathode-side under-bump-metallization (UBM) reacted with the solder and transformed the entire microbump into Ni3Sn4. Ni3Sn4 has better EM resistance than the solder and caused the bump resistance to remain at a constant value. The bump resistances at different angles indicated that current crowding still took place, but in the Cu UBM and not in the solder. The complete voltage drop across the microbump was the value obtained at 0°. However, the bump resistance obtained at 0° was 7 times larger than that measured at 180°. That is, the RC delay caused by microbump is actually very large. For simplicity of description on the relation between microbump resistances, crowding ratio, and structural dimensions, a numerical model was built. The expressions of microbump resistance and the crowding ratio were also obtained.
The last type of sample was the 10-μm microbumps. The resistance behaved first concave-down and then concave-up because the solder was too much for the interposer-side UBM to consume. The concave-down curve was first observed for the same reason as that of the low-bump-height case. However, the height of the solder was around 10 μm, which was too high for the interposer-sider UBM to react with. When the electrons flow upward (from interposer to chip), the interposer-side UBM, 2-μm Ni, was the cathode side. Driven by EM, the-2μm Ni quickly dissolved into the solder. After the 2-μm Ni ran out, the void was formed, causing the bump resistance curve to become concave-up again.
The solder height affected the failure mechanism. When the solder height was 25 μm, void propagation was the main failure mechanism. When the solder height decreased to 10 μm, the mechanism became the combination of void propagation and IMC growth. When it was 6 μm, the failure mechanism changed to IMC growth only. The FEM described clearly the evolution of current density distribution at various stages of EM and therefore helped predict accurately the failure mechanism. Moreover, the Kelvin bump structure is compatible with the generally used daisy chain structure. Both bump resistance and daisy chain resistance could be obtained at the same time.
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