Influence of filler settling on the analysis of solder reliability of flip chip packaging

In the capillary-flow underfilling process many practical evidences suggest that the distribution of fillers is not uniform as is usually assumed. It is likely caused by filler sedimentation, variation of the flow pattern, and rheology of uncured underfill flow, etc. Such a nonuniform distribution of fillers requires that we modify the reliability estimates of flip chip packages that are based on an ideal, isotropic underfill. This paper reports an analytical study of the influence of filler settling on the flip chip solder reliability analysis. By taking 35% for the volume fraction of silica fillers, we have described the y-dependent filler settling by three settling scenarios and calculated the thermomechanical properties of the underfill by considering three types of temperature-dependent, linearly elastic epoxy resins for each settling profile. In total, nine underfill materials were individually introduced in a 2D finite element flip chip package model to comparatively study the thermal response of flip chip solder joints that are surrounded by the underfill. The FE results showed that the inelastic shear strains steadily accumulate in the flip chip solder joints. The inelastic shear strains increase significantly as filler settling is introduced. We also observed that as the extent of filler settling increases, the average inelastic shear strain energy density associated with the outermost solder joint increases.