A study on the tensile fracture mechanism of 15 μm copper wire after EFO process

In this study, 15 μm copper wires were bonded on substrates with thermosonic process, and the tensile fracture characteristics of FAB, as well as bonded samples, were investigated. For electronic packaging applications, all 15 μm wires were fully annealed, and the microstructures consisted of equiaxed grains. After EFO (electric flame-off) process, the microstructure of wire can be divided into three parts: (1) free air ball (FAB) with columnar grains, (2) heat-affected zone (HAZ) with equal-diameter grains, (3) annealing zone with equiaxed grains. According to tensile test results, EFO process simultaneously reduced UTS and elongation of the wire. For both FAB and bonded samples, the tensile fracture zones were either in the region of equal-diameter grains or in coarse grains located within 100 μm from the ball. And it was observed that the breakage sites appeared near the twins and the columnar grains when tensile fracture happened. Meanwhile, the relationship between hardness and microstructure of wires after EFO process were analyzed with nano-indentation. The nano-hardness value of 15 μm wire was 1.2–1.45 GPa.