The law of ultrasonic energy conversion in thermosonic flip chip bonding interfaces

In this paper, the vibration characteristics during the flip chip (FC) bonding process were observed by using a laser Doppler vibrometer (LDV), and the atom diffusion features in vertical section of the FC bonding interfaces were inspected by using a high resolution transmission electron microscope (HRTEM). Results show that the vibration velocity of a die was about 500 mm/s during the traditional FC bonding process, and that of a substrate was only about 180 mm/s. It led to the difference of atom diffusion in the FC interfaces. For the given variables, the thickness of atom diffusion at an up-interface (i.e. Au/Al interface) of the FC bonding was about 500 nm where was an inter-metallic compound (i.e. AuAl₂), and that of atom diffusion at a down-interface (i.e. Au/Ag interface) was about 200 nm. Furthermore, the law of ultrasonic energy conversion was found that the ratio of the up-interface to the down-interface in the FC bonding was statistically about 2.21:1. According to this principle, different bonding processes are suggested to improve the performance of two interfaces. The experimental evaluation confirms the effectiveness of the suggested processes on minimizing the inter-metallic compound layer and equilibrating the thickness of atom diffusion at two interfaces.