Underfill viscous flow between parallel plates and solder bumps

Underfill is used to improve the reliability of the flip-chip interconnect systems. A conventional underfill is filled to the gap between the chip and substrate around the solder joints by the capillary flow. A modified Hele-Shaw flow, that considered the flow resistance in both the thickness direction and the restrictions between solder bumps, was used. This model estimates the flow resistance induced by the chip and substrate as well as the solder bumps, and provides a reasonable flow front prediction as compared to the experimental observations. A method to simulate the edge effects during the underfill process was also proposed, which gave a close flow front prediction in a chip assembly with full array solders.