| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系 === 101 === With the trend of miniaturization, the electronic device becomes smaller and smaller. In addition, the electronic device require some more features, such as small volume of solder bump, better electrical performance and higher I/O density. With all these demands, the size of solder bump shrinkage into micro scale inevitably. Microbumps have been adopted for interconnects in 3D IC packaging. The bump height decreases to about 10-20 μm. Due to the small volume of the solder, some problems may occur. First, the current density inside the bump increased rapidly due to the reduced cross section area. Second, the joule heating effects occur severely. Once the current density and temperature gradient reach a critical value, electromigration and thermomigration effects occur and endanger the microbumps. Therefore, the electromigration performance of the micro-bump becomes a critical issue.
In this study, three different under-bump-metallizations (UBMs) are used, which are Cu-Cu, Ni-Cu and Ni-Ni UBMs with the Sn2.3Ag solder between. There is a copper column on the chip side with 50μm height. It had been reported that copper column had a better performance in resisting the electron wind force and it can also make the current flow uniformly. Electromigration tests were performed at a current density of 2 x 104 A/cm2 at 150 oC. The bump resistance was measured by using Kelvin structure, and defined the bump resistance to increase 10%, 20%, 50% of its initial bump resistance as failure stages. We found that Voids formed in the interface of under-bump-metallization and intermetallic compounds. Besides, the joule heating of the solder joints during electromigration tests was measured by using Kelvin structure and temperature coefficient resistivity of copper . This study provides better understanding of electromigration behavior in flip-chip solder joints with low bump heights.
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