An Examination of Underfill Flow in Large Dies With Nonuniform Bump Patterns

In this paper, numerical modeling and experimental results are presented for underfill flow in a large die with a nonuniform bump pattern in a flip-chip packaging configuration. Two different 2-D flow models coupled with the volume-of-fluid method are applied to track the underfill flow front during the simulation of the flip-chip encapsulation process. The first model employs the modified Washburn model and uses a time-dependent inlet velocity to account for the flow resistance across the gap direction in the presence of bump interconnects. The second model introduces a momentum source term in the Stokes equation to represent the gapwise flow resistance. Rheological properties, surface tension, and dynamic contact angles for commercial underfill material and the effect of flux residue on underfill wetting properties are experimentally determined. Simulation results based on the two models are compared with in-situ flow visualization conducted using bumped quartz dies. The modified Stokes model yields better predictions of the underfill penetration length as a function of time and the total flow-out time. This model is then used to investigate the effects of dynamic contact angles and temperature-dependent underfill viscosity on underfill flow in a large die with a nonuniform bump pattern.