Effect on Filling Time for a Non-Newtonian Flow During the Underfilling of a Flip Chip

In the flip-chip packaging, an underfill encapsulant is used to fill the gap between the chip and the substrate around the solder bumps. The underfill can reduce the thermal stress in solder bumps but increase the stress in the silicon. Epoxy with silica fillers is usually used as the underfill materials. These materials exhibit the non-Newtonian effect during the underfill flow and this type of effect needs to be included in the simulation of the underfilling process. In the analysis, the underfill flow can be treated as a flow through a porous medium. This paper provides the formulation of the effective permeability in the porous flow domain of the flip-chip encapsulation for a non-Newtonian fluid. An average viscosity of the encapsulant is also proposed for the flow in the porous flow domain. It is found the non-Newtonian flow will affect the filling time largely while the filling pattern is similar. The non-Newtonian flow model must be employed in order to accurately predict the underfilling flow.