Underfill flow as viscous flow between parallel plates driven by capillary action

Epoxy underfill is often required to enhance the reliability of flip-chip interconnects. This study evaluates the flow of filled epoxy underfill materials between parallel plates driven by capillary action. An exact model was developed to understand the functional relationship between flow distance, flow time, separation distance, surface tension, and viscosity for quasi-steady laminar flow between parallel plates. The model was verified experimentally with a typical underfill material. The measured values of flow distance agreed well with the exact model. A new material parameter, the coefficient of planar penetrance, is introduced. This parameter measures the penetrating power of a liquid between parallel plates driven by capillary action. The effectiveness of gravity and vacuum as flow rate enhancements are explored.