Thermosonic flip-chip bonding using longitudinal ultrasonic vibration

Flip-chip assembly is an important technology for first level electronic packaging. Among different assembly approaches, thermosonic bonding is becoming an attractive choice because it is cost-effective. To increase the bonding capability for high I/O assembly, a novel longitudinal bonding system is developed in this study. This bonding system has two advantages over the transverse bonding system: it overcomes the planarity problem and simplifies bonding tool configuration. During the development, an end-effector is designed with rigorous considerations of ultrasonic wave propagation, bonding force and co-planarity between the chip holder and hot stage. The bonding system is then characterized to make sure that it is suitable for flip-chip assembly. This bonding technology is proven successful by mechanical bonding tests as well as a functional CMOS/SRAM module demonstration. During the bonding process, the chip is under longitudinal ultrasonic "hammering". However, there is no apparent damage because the impact stress is low due to the low ultrasonic vibration amplitude.