Rheology of the underfill flow process in a flip chip package

Underfills that flow have traditionally shown superior reliability performance compared to no-flow underfills. This has ensured the necessity for flowable underfills in the IC market. Flow speed has been an issue, as faster flow would reduce cycle time, but should not be at the expense of package reliability. Schwiebert and Leong (1996) established the basic theory of underfill flow. Others have modified this theory, but in all these models, the effect of change in underfill viscosity during the flow process was not considered. This has rendered the theories incomplete and has in part led to errors in flow modelling. In the first half of this study, the Schwiebert and Leong model was expanded to include the underfill viscosity change at flow temperature. The resulting derivation is a more general form of the flow equation. The paper shows that the new equation can be reduced to the Schwiebert and Leong equation when underfill viscosity is considered to be constant. Experimental verification was done by first characterizing the viscosity change over time of three different underfills. The flow distances versus time, the change in rheology over flow temperatures and time, and the change in underfill contact angle to a laminate substrate and polyimide die are also measured over time and temperature. The effect of change in contact angle over time at underfill flow temperatures and other parameters that influence underfill flow are discussed. The experimental results are then compared with assumptions made in theoretical prediction of underfill flow. Finally, experimental flow results are compared with the theoretical model.