Flow properties of liquid underfill encapsulations and underfill process considerations

The underfill encapsulation process and corresponding underfill flow behavior are studied numerically and experimentally. A 2-D volume of fluid (VOF) FLUENT model was used to better simulate one-side underfill flow compared to a pre-existing analytical model. For the two-side underfill flow, a 3-D flow model based on the Hele-Shaw approximation was created to calculate the process time. A 2-D analytical model was also modified by including a body force to simulate inclined underfill flow. Experiments were performed to verify the numerical model, and good agreement was observed for the flow time and flow distance relationship for different processes. Experiments at varying temperatures allowed the inference of the encapsulant's strong temperature dependent viscosity and of an optimal temperature for the underfill process. Experimental results also revealed that dispensing from more than one side of the chip changed the underfill flow behavior and thus changed the process time. We also noted that the addition of center bumps increases the fill time compared to just edge bumps. However, for two different densities of center bumps, no significant difference on the total underfill time was observed.