In this study, the annealed effect (at 150 °C ∼ 250 °C for 1 h) on the tensile mechanical properties of thin copper wires with φ = 25 μm (1 mil) was investigated. The microstructural characteristics and the mechanical properties before and after an electric flame-off (EFO) were also studied. Results indicate that with annealing temperatures of more than 200 °C, the wires possessed a fully annealed structure, the tensile strength and the hardness decreased, and the elongation was raised significantly. Through recrystallization, equiaxed grains and a few annealed twins formed in the matrix structure. The microstructures of the free air ball (FAB) of the various wires after EFO contained column-like grains. The column-like grains grew from the heat-affected zone (HAZ) to the Cu ball, and the preferred orientation was <100>. According to Weibull's reliability analysis, the failure rates of all the specimens were the modus of wear-failure. The tensile strength and the reliability of both the 200 °C and 250 °C annealed wires in the HAZs showed the highest values of all.
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