The effect of impurities in the Cu film on the void formation at the interface of the Sn-rich solder joint has been extensively studied. However, this has rarely been studied for In solder joints. In this study, Sn-3.0Ag-0.5Cu (SAC305) and In-solder/ Cu films with high impurities were aged at 150 °C to analyze the films at various aging times. A metastable CuIn2 phase appeared and transformed into a stable Cu11In9 phase. Noticeable voids and cracks were observed at the SAC305/Cu interface during the solid-state aging. In contrast, there was a void-free interface between In and Cu-CP. This was primarily due to the difference in diffusion behaviour between In/Cu and SAC305/Cu. The mechanism of the above phenomenon was revealed.
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