Feasibility and bonding strength for gold stud bumps thermosonic flip-chip onto flex substrates with nonconductive pastes

The goal of this work is to verify the process feasibility and to improve the bonding strength of a flip-chip-on-flex (FCOF) assembly achieved by thermosonic bonding with a nonconductive paste. Prior to flip-chip bonding, a non-conductive paste was deposited on the surface of copper electrodes over a flex substrate, and a chip with eight gold bumps was then flipped and thermosonically bonded onto copper electrodes on flex substrate. The ultrasonic power plays an important role in removing the nonconductive paste beneath the gold bumps during bonding process, so that gold stud bumps could be directly bonded onto copper electrodes to guarantee perfect electrical connections between chips and the flex substrate. The ultrasonic power provided necessary energy to form a metallurgical bonding between gold bumps and the copper electrodes. The bonding strength of chip on the flex substrate increases with increasing ultrasonic power to an appropriate amount. Selecting an appropriate curing temperature is essential in improving bonding strength between chips and flex-substrates and has been achieved in this work. The suggested bonding parameters for gold stud bumps flip-chip bonded to flex substrates using thermosonic bonding process with nonconductive adhesive are: 500 gf in bonding force, 40 s in curing time, 140degC in curing temperature and 14.46 W in ultrasonic power. The suggested process can be potentially applied to packaging of IC-chips to flex-substrates with copper pads, such as packaging of TFT LCD driver IC.