Effect of underfill filler settling on thermo-mechanical fatigue analysis of flip-chip eutectic solders

Underfills containing filler particles exhibit filler settling during the (capillary-based) wicking and curing processes, thus causing the reliability estimation to deviate from that of the presumed base of no filler settling. This paper examines the thermo-mechanical responses of the solder joints in flip-chip packaging to various conditions of filler settling. We built five y-dependent profiles for describing the uniform, bilayered, and gradual settling of filler spheres in the through-depth direction of the underfill and used the Mori–Tanaka method to calculate the effective material properties of the filler–resin underfill compound by considering a linearly elastic, temperature-dependent resin with a glass transition temperature range of 70–130 °C. For each settling profile we analyzed the fatigue indicators, referred to as the inelastic shear strain ranges and the inelastic shear strain energy densities of the solder joints, and compared their magnitudes against the extent of filler settling. The results show that the fatigue indicators depend on the extent of filler settling. A greater extent of bilayered filler settling produced larger (in magnitude) fatigue indicators. The fatigue indicators associated with gradual filler settling, however, were almost always smaller, on average, than those associated with no filler settling, indicating that some types of filler settling might favor a longer solder fatigue life. This preliminary but intriguing finding may be partially explained by considering the asymmetric thermal mismatch in the through-depth direction of the underfill; a comparatively good thermal match near the bottom side of the solder joints may compensate for the thermal mismatch at the top side, thus contributing to an overall better thermal match in the solder joint.