| Summary: | 碩士 === 國立清華大學 === 工程與系統科學系 === 102 === Electromigration induced failure has been regarded as a serious reliability issue in the Pb-free electronic packaging application. Due to the unique bump-to-line configuration in solder joints, the dominant failure mechanism has been reported as pancake-type void formation in the current crowding region and propagation at interface between solders and intermetallic compound, finally leading to open-circuit of devices. In this study, we employ a unique composite structure consisting of alternating Cu/Sn column with different spacings to enhance the electromigration resistance in Pb-free solders. Pure Sn structure and Cu/Sn composite structures with 200 μm and 20 μm of Cu columns were current stressed by 1 x 104 A/cm4 at test temperature of 80 ℃. The results show that the pure Sn structure was found to fail due to solder melting after current stressing for 46 h. However, the samples with Cu/Sn composite structure remained superior conductivity without significant electromigration induced damage after current stressing for 1600 h, suggesting that the Cu/Sn composite structure exhibited better electromigration resistance than that with pure Sn structure. In addition, there were no void formation and no hillock appeared in the Cu/Sn composite structures with 20 um of Cu columns specimen after electromigration test for 1600 h. The lifetime of samples significantly increased as reducing the spacing of composite samples. The experimental observation of microstructure evolution was consistent with the simulated distribution of current density results. We proposed that this improved electromigration resistance in the composite samples is mainly because the high current density was averagely spread and the rate of void nucleation in the composite samples was effectively hindered by the introduction of Cu columns. In addition, the void propagation rate was lowered due to the better electromigration resistance of Cu columns.
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