Stress-induced atom diffusion at thermosonic flip chip bonding interface

The thermosonic flip chip bonding was realized with a lab TSFC bonder, and 60–80 g/bump bonding strength was obtained. With the scanning electron microscope (SEM), core-like bonding interface, ductile dimples fracture were observed on the pads bonding interface, dislocation density incensement and dislocation nets were found at the pad. What's more, slip bands and crossed slip lines were observed on the surface and in the inner of bump. Then a finite elements (FE) model was used to simulate the stress concentration at the bonding interface. After that, the relationships among stress concentration, dislocation multiplication and propagation, driving force for Au and Ag atom inter-diffusion at bonding interface were discussed. Experiment results and theory analysis indicate that the ultrasonic vibration induces stress concentration at the bonding interface edge, which causes dislocation multiplication and propagation on bumps and pads, causes high density dislocation net and provides short-circuit diffusion channel and driving force for Au and Ag atom inter-diffusion. Enhancing the stress concentration effects, such as increasing ultrasonic frequency, may be helpful for better atom diffusion and bonding strength. This may be one of the reasons why higher ultrasonic frequencies (such as 132, 125, 108 kHz) thermosonic bonding is better than lower ultrasonic frequencies (such as 60 kHz) thermosonic bonding in industry for better reliability and bonding speed.