| Summary: | 碩士 === 國立中正大學 === 化學工程研究所 === 107 === The amount of the IMC would influence the reliability a lot. Usually, alloying the solder with additional elements to decrease its thickness is a great method. This research put emphasis on the interfacial reaction between solders containing Ge and metal substrates, particularly Co substrate. Due to the Co UBM has received more attention, our study mainly investigated the correlated interfacial reaction of Co substrate.
Both liquid-state and solid-state reactions were conducted. First, in the liquid-state reaction, with addition of less than 0.05wt% Ge, one continuous CoSn3 layer was formed. More Ge adding in the solder, the better suppression effect on the CoSn3 phase. Most important of all, the CoSn3 growth was decreased by 50% with 0.05wt% Ge addition. When the Ge content increased to 0.07wt%, the IMC phase changed to the new ternary Sn-Ge-Co intermetallic compound, which was never found in the literature (named as X phase). The X phase was stably present at the interface. Even after aging for 1 day, it had no obvious growth. Furthermore, with 0.3wt% Ge addition, the dominant reaction phase change to the irregular Co5Ge7, and a thin layer of X phase was observed between the Co5Ge7 phase and Co. The Co5Ge7 phase became unstable and drifted into the solder. Instead, the X phase became dominant. To discuss the mechanism of inhibition, multi-stage reaction with different solders was also examined. The observed results indicated that the inhibition of IMC growth was possibly attributed to the minor Ge doping in the CoSn3 phase. In the solid-state reaction, Ge also suppressed the CoSn3 phase effectively. With only 0.1wt% Ge addition, the IMC thickness is thin no matter how long the aging time is; it seemed that the growth of IMC was not sensitive to temperature as well. The IMC growth kinetics were investigated as well (under 0.05wt% Ge), the growth of CoSn3 showed a linear relationship with aging time at the temperature range between 165oC to 200oC. Besides, in the Sn-Ag-Cu-Ge/Co system, because of the extra effect of Cu element, the formed CoSn3 phase was greatly suppressed no matter what the reaction happened. Although adding Ge in the solder had significant effect on Co-related system, it is found that no special influences on the substrate like Cu and Ni, except the PdSn4 formed with Pd substrate.
To confirm whether the ternary X phase was an equilibrium phase, the partial isothermal section in the ternary Sn-Ge-Co system at 250oC was also experimentally studied. Four three-phase regions were determined, including L-CoSn3-X, L-Co5Ge7-X, L-Co5Ge7-(Ge) and CoGe2-Co5Ge7-(Ge). The XRD results from powder of Sn-Ge-Co alloy and IMC at interfacial reaction were cross-comparison. The X phase was demonstrated to be an equilibrium phase, and the reaction paths of Sn-Ge/Co reaction couples were illustrated on the phase diagram simultaneously.
With the trend of miniaturized semiconductor package, the solder joint would be destroyed easily in the case of current passing. Using the Sn-Ge/Co/Sn-Ge structure to observe the current effect. The formed phase at the interface is as the same as solid-state reaction, and still retain the suppression effect on CoSn3. In the system of Sn-0.1Ge/Co, the metastable CoSn4 phase would appear in the solder.
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