Thermosonic flip- chip bonding for stud bumps onto copper electrodes with titanium and silver layers

Thermosonic flip-chip bonding of chips with gold stud bumps onto copper electrodes of an alumina substrate was studied. Copper electrodes were deposited with silver as the bonding layer and with titanium as the diffusion barrier layer. Deposition of these layers on copper electrodes serves to improve the bonding quality between the gold stud bumps and copper electrodes. With appropriate bonding parameters, 100% bondability was achieved. Bonding strength between the stud bumps and copper electrodes was much higher than that converted from the JEDEC standards. The effects of process parameters including bonding force and ultrasonic power on bonding strength were investigated by design of experiment (DOE). The experimental results showed that bonding strength increased with increasing bonding force and ultrasonic power. The appropriate bonding parameters ultrasonic power of 4.52 W in, bonding force of 1000 gf, bonding time of 0.5 s and bonding temperature of 200degC. Deposition of these two layers on the copper electrodes not only provides an effective wry and direct way of thermosonic flip-chip bonding of chips with gold stud bumps onto the substrate, but also ensures excellent bonding quality. Potential applications such as flip- chip bonding of chips with low pin counts or LED packaging are expected.