Study of Interfacial Phenomena Affecting Thermosonic Wire Bonding in Microelectronics

Thermosonic ball bonding is the method of choice for many interconnections to integrated circuits. This study investigated the effects of bonding parameters on bonding strength using a thermosonic bonding machine and a shear tester. Theoretical analyses were conducted to relate bonding strength to interfacial contact phenomena. Our results show that bonding strength of thermosonic wire bonds can be explained based on frictional energy intensity and real contact area. When the ultrasonic power is small, the bonding strength increases with increasing real contact area mainly caused by the bonding force. Increasing the bonding force can hardly increase the frictional energy intensity, but it can increase the real contact area, thus increasing the shear force. For larger ultrasonic power, the ultrasonic power plays an important role in increasing the bonding strength at the interface between the wire and the pad. Increasing the ultrasonic power increases both the frictional energy intensity and the real contact area, thus increasing the shear force until before the frictional energy intensity reaches a critical point. Moreover, increasing the welding time increases both the frictional energy intensity and the real contact area, thus increasing the shear force before the critical frictional energy intensity is attained; this is far smaller than the critical frictional energy intensity when the ultrasonic power is varied.