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The reaction between Cu and the Sn-Ag solders doped with different amounts of Ni is studied. Four different solders with the Ag concentration fixed at 3.5 wt.% and Ni concentrations varied between 0.0 wt.% and 1.0 wt.% are used. In contrast to the reaction between Ni and the Sn-Ag solders doped with different amounts of Cu, the type of intermetallic compound formed does not depend on the Ni concentration. The compound Cu 6 Sn 5 forms for all the Ni concentrations used. For the Ni-doped solders, the Cu 6 Sn 5 phase contains a small amount of Ni. The compound Cu 3 Sn appears subsequently between Cu 6 Sn 5 and Cu as the reaction time increases. The addition of Ni has the effect of substantially increasing the amount of intermetallic compound at the interface. The addition of Ni also produces two distinct Cu 6 Sn 5 regions at the interface. The outer region contains more Ni, and the inner region contains less Ni. This study also finds that, during solid-state aging, the growth of Cu 3 Sn becomes slower when Ni is added to the solder. The findings of this study are rationalized using the Cu-Ni-Sn isotherm.

An electromigration failure mechanism in flip chip solder joints is reported in this communication. The solder joints failed by a very rapid, asymmetrical, and localized dissolution of the Cu metallization on the cathode side. The average dissolution rate was about 1 μm/min. The dissolved Cu included not only the Cu under bump metallurgy but also the on-chip Cu conducting trace. From the location and geometry of the dissolved Cu, it can be concluded that current crowding plays a critical role in the rapid dissolution. The dissolved Cu atoms were driven to the anode side by electromigration, and a large amount of Cu6Sn5 was formed there.

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Wire and arc additive manufacturing (WAAM) produces thin-walled parts superior to other additive manufacturing methods, because of its high efficiency, good compactability, and low cost. However, the WAAM accuracy is limited by its large heat input. Here, 0.8 mm 316L stainless steel welding wire is deposited via speed cold welding to form 30-layered thin-walled samples, with 2 mm thickness, and up to 65 mm height. The effects of three process parameters (the bottom current mode, scanning speed, and cooling time) on the deposition process stability, macro morphology, structure, and mechanical properties are studied. In the experiment, the probability density curves of electrical parameters of sample #GRBC-30 cm/min-10 s on the third and tenth layers were narrower than other samples, which implied a more stable process. The three process parameters mainly affect the deposition morphology and have a minor performance effect. The hardness and tensile properties mainly depend on the deposition direction. Gradual, layer-by-layer current reduction improves the bottom molding and performance, and the deposition efficiency, and stabilizes the process. Scanning speed enhancement or cooling time reduction destabilizes the end formation, reduces the effective deposition rate, and slightly degrades the performance. All deposited samples are distinctly anisotropic, but satisfy the industrial standard. Overall, deposition in speed cold welding mode, with 10 s cooling time, 30 cm/min scanning speed, and gradually reduced bottom current exhibits good stability, and the molding efficiency and mechanical properties are optimal.

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