Temperature soak reliability of laser-sintered Ag pads for wire bonding

A small amount of Ag nanoparticles (5 nm in average diameter) was supplied to the tips of Cu lead-pins by ink-jet printing, and then metallized by laser irradiation to form circular wire-bonding pads between 2 and 3 μm thick and φ150 μm. A continuous-wave Nd:YAG laser (1064 nm in wavelength and 150 W in maximum power output) was employed in an argon atmosphere. A φ25 μm gold wire was bonded between the adjacent pads using an ultrasonic wire-bonder. The wire-bonded parts were placed in an atmospheric electric furnace for temperature soak tests in dry conditions at 150°C for up to 1000 h. Metallographic changes in the junctions were investigated by SEM, FIB, and EDX, while mechanical strength was examined by pull tests. No metallographic changes were observed in the 500 h specimen, whereas a slight solid-phase diffusion took place at both Cu/Ag and Ag/Au interfaces in the 1000 h specimen. The Ag pad formed on the Cu lead, above 2 μm in thickness, also prevented the degradation of pull strength; an average of 8.8 cN was maintained at both soak times. No separation from the first (ball) and second (wedge) bonds occurred, but the wire fractured close to the hook of the tester.