Relationship between mechanical properties and microstructure of ultra-fine gold bonding wires

The ultra-fine gold bonding wires, having diameter of 24 μm and consisted with nano-sized grains, show much higher strength and modulus than those of wrought and annealed gold. The stress–strain curves of ultra-fine Au wires have been characterized by using the micro-tensile testing technique. The ultimate tensile strength of ultra-fine Au wires was measured from 190 MPa to 407 MPa, and decreased with increasing the annealing temperature. The elastic modulus was measured from 79 GPa to 97 GPa, which was also higher than that of wrought and annealed gold. The grain size of ultra-fine Au bonding wire was characterized using high-resolution scanning electron microscope. The grain size was measured from 230 nm to 1650 nm, and the grain growth was not observed up to an annealing temperature of 500 °C. The texture of ultra-fine Au bonding wire was characterized using X-ray diffractometer to characterize the orientation distribution function. Theoretical elastic moduli of ultra-fine Au wires have been calculated by using the Voigt model. The theoretical elastic moduli show good agreement with the measured elastic moduli using the micro-tensile testing technique. The tensile strength of ultra-fine Au bonding wire can be controlled by controlling the grain size and texture.