Recrystallization Effect and Electric Flame-Off Characteristic of Thin Copper Wire

Abstract

Compared with gold wire, copper wire is cheaper and possesses lower cost, greater strength and better electrical conductivity, and therefore is becoming more commonly used. But the lower ductility and oxidation undermine the reliability of copper wire bonding, making the wire susceptible to breaking. In the present study, the annealed effect (at 150-250°C for 1 hour) on the tensile mechanical properties of copper wires with Φ = 25μm (1 mil) was investigated. In addition, the microstructural characteristics and the mechanical properties before and after an electric flame-off (EFO) process were also studied. Experimental results indicate that with annealing temperatures of more than 200°C, the copper wires possessed a fully annealed structure, the tensile strength and the hardness decreased, and the elongation was raised significantly. Through recrystallization, the matrix structure transferred from long, thin grains to equiaxed grains and a few annealed twins. The microstructures of the free air ball (FAB) of the variation annealing wires after an EFO process were column-like grains. The column-like grains grew from the heat-affected zone (HAZ) to the Cu ball, and the preferred orientation was (100). Under the thermal effect of EFO, the necks of the Cu balls underwent recrystallization and the grain growth was induced. Additionally, the decreased hardness and the strength of the HAZ resulted in the breakage sites of the EFO wires being in the HAZ near the Cu balls.