Effects of bonding frequency on Au wedge wire bondability

This paper studies the effect of bonding frequency on bondability of Au wire on a PCB bond pad. The wire bonding was performed at two different frequencies, 62 kHz and 138 kHz, and at varying bond pad temperatures between 60 and 110 °C. It is shown that the bond strengths of the wires bonded at a high frequency were generally higher than those bonded at a low frequency for all temperatures studied. Two distinct wire failure modes were observed for the wires bonded at the high frequency: the wires with high pull strengths failed at the bond neck, while those with low pull strengths failed mainly within the bonded wire body. This resulted in a large pull strength data scatter, which was explained by the high Q factor of the high frequency transducer. The bondability obtained for the high bond power end was much higher for the high frequency, giving rise to a wider process window in terms of bond power for the high frequency bonding. The wire bonding performance was compared between the “open” and “closed” loop bonding systems. The minimum bond powers required for successful bonds in the closed loop system were significantly lower than those required in the open loop system (e.g. 20–40 mW vs. 110–135 mW at 90 °C). The closed loop system was able to correct the resonance frequency shift, thus maintaining an almost constant bond power during bonding. In the open loop system, in contrast, a high bond power needs to be continuously supplied because of the power drop. This causes an excessive energy to be transmitted to the bonded wire, resulting in weakened wire due to excessive deformation.