Modeling and Simulations of the Underfill Filler Settling Effect on the Interfacial Stresses of Flip Chip Packaging

The influence of filler settling on the interfacial stresses in the die/underfill interface was examined by using a typical flip chip packaging model considering the solder's viscoelasticity and plasticity. Since filler settling may result in uncountable configurations of filler distribution in underfill, this paper considers three underfill models which span the extreme configurations of filler settling, namely, a macroscopically homogeneous underfill model, a bilayered underfill with one layer having fillers most densely settled, and a underfill with fillers gradually settled. The Mori-Tanaka method was employed to calculate the effective material properties of the three underfill models. Thermal stresses were analyzed using finite element method. It found that the filler settling likely has no apparent influence on the distribution of the interfacial stresses. There exhibits local variation in the interfacial peeling stresses near the soldering region and the free end of the die. Similar variation is also seen in the distribution of the axial stress in the die along the die/underfill interface. For both, filler settling surprisingly helps reduce the stresses, but only to a minor degree.