Experimental investigation on the height deviation of bumps printed by solder jet technology

Solder jet technology is considered as a flexible and low cost method to print bumps directly for flip-chip packaging. However, the height deviation of solder bumps, which decides the quality and reliability of the packaging, is significantly influenced by the fluid dynamics and solidification behaviors during solder droplet impact. Here, an experimental investigation was first conducted to understand the influence of impact parameters on the height deviation of solder bumps. The results showed that the underdamped oscillation of droplets before complete solidification was the main reason for the bump height deviation, because this oscillation resulted in different bump shapes under different solidification rates. A clear threshold, dividing the regions of the large and small deviation of the solder bump height, was found by estimating the droplet total solidification time scale. “Deposition and reheating” process was presented and proved to be an effective method to reduce the height deviation. A solder bump array with the height of 223 ± 2 μm, was printed through this process. The dimensionless height deviation (Δh/h) of printed bumps was remarkably less than 1%. The present work provided a method of printing uniform height solder bumps.