Influence of underfill filler settling on the thermomechanical reliability of the flip chip on board package

Abstract

A Flip Chip on Board package (FCOB) is analyzed under thermal cycling loading to study the effect of underfill filler settling on the thermo-mechanical reliability of the package. The Mori-Tanaka method, a micromechanics based formulation, is utilized to model the property gradation caused by filler settling to obtain the effective material model for underfill. The modeling of the underfill material depends on the filler settling assumed. A total of five different underfill material models are used to assess their impact on the reliability. The equivalent underfill material properties calculated are subsequently used in finite element simulations to evaluate the FCOB assembly reliability. The reliability issues investigated in this work are solder joint fatigue and die cracking. This work aims to predict solder joint fatigue lifetime and also determine an acceptable pre-existing flaw size in the die backside to prevent die cracking. The results show that, when the underfill filler settling is gradual (instead of homogeneous), the corresponding solder joint has the minimum fatigue lifetime among the models assessed. However, the filler settling configuration has very little influence on the critical flaw size in the die and the board warpage.