Suppression of the height deviation on metal bumps manufacturing by an ultrasonic vibration control

On-demand droplet printing based on piezoelectric micro-jet device (PMJD) is considered a flexible and high-precision method to generate metal droplets directly for flip-chip bonding in industrial electronics. However, the quality of flip-chip bonding is closely related to the height deviation of the solidified droplets (the metal bumps), which is influenced by the complicated hydrodynamics of impacting and oscillation of the droplet with oxide film. Here, the numerical and experimental investigations are first conducted to study the effect of the liquid bridge and deposition parameters on the height deviation of the solidified droplets. The rapid oxidation of the liquid bridge and under-oscillation during the deposition process are the main reasons for height deviation. In addition, the undamped oscillation with high-speed impact and instantaneous solidification also deteriorates the height deviation. To this end, an oscillation control strategy based on ultrasonic-assisted metal droplet deposition (UAMDD) is proposed and verified to be a reliable regulation strategy to suppress the height deviation of the printed bumps. The effective regulating range of height deviation is studied experimentally by changing the ultrasonic vibration amplitude. Finally, a 10 × 10 array composed of 100 solidified metal droplets is printed with the UAMDD, which has the height deviation of 554 ± 6 μm. And the dimensionless height deviation (Δh/h) of solidified bumps is stayed below 2.1 %.