Experimental and numerical study of solder bumps impact on the underfill fluid flow under electrohydrodynamic effect

Solder bumps can increase filling time, which is one of the main challenges in electronic packaging reliability. Here, we compare the capillary flow behavior between parallel plates with and without solder bumps to examine how solder bumps affect the length of the underfill fluid flow under the effects of an electric potential. We found that the solder bumps restrained the flow length, while the electric field enhanced it. By enhancing the voltage from 0 to 1000 V, in the case without solder bumps, the flow length increased by up to 30%, and it increased by up to 25% in the case of solder bumps. To determine the optimum bump design, we selected the diameter and pitch size of the solder bumps as the independent variables. The results revealed that larger pitch sizes and smaller diameters show longer fluid flow lengths. The effect of the electric field on varying nozzle positions was also investigated. We found that the fluid flow length increased when the nozzle was between the solder bumps compared to the top of the solder bumps. According to our observations, the nozzle position is also the main factor in determining the fluid flow length compared with the bump diameter and pitch sizes for the design of the underfill packaging process. Numerical simulations were also performed to compare the experimental results, and the average discrepancy between the experimental and numerical results at various time steps for different solder bump parameters was between 5 to 10%. Our findings demonstrate the potential of using electric potential in conjunction with solder bumps to control underfill flow, which can benefit flip-chip packaging applications. Numerical methods can accurately predict underfill fluid flow with solder bumps under the electric field effect.