The effect of the concentration gradient lies in the solder on the interfacial reaction was investigated in this paper. Sn-9Zn/Sn double layers solders were reacted with Cu at 250°C for 1–5min. The concentration gradient and its direction affect the composition and the morphology of the reaction products. When the reflow time was less than 3min, the reaction products were Cu5Zn8 and CuZn5, only Cu5Zn8 was found when the reflow time was 5min. Solder pieces with different thicknesses were reacted at 250°C for 5min. Only Cu5Zn8 was found at the interface and the size and the amount of the reaction product changed as the thickness varied. When the thickness of the solder piece was 300µm, bulk type Cu5Zn8 with large faceted surfaces was observed in the fine grains Cu5Zn8 phase next to the Cu substrate.
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