Modeling and Characterization of Cure-Dependent Viscoelasticity of Molded Underfill in Ultrathin Packages

Warpage control of an ultrathin package is critical in the semiconductor industry because of the large volume of molded underfill (MUF) encapsulated in such packages and because of the low stiffness of the entire package structure. The viscoelastic relaxation behavior that is observed during the MUF curing process results in a major warpage change and thus should be investigated. First, the material model of the cure-dependent viscoelasticity of MUF in the time domain and the curing kinetics model of MUF were successfully established based on the characterization results. Then, a finite-element (FE) model integrated with the material model for package warpage prediction was successfully developed. The final prediction results of package warpage obtained using a novel process modeling approach are in agreement with the experimental data measured using thermal shadow Moire. Furthermore, the effects of MUF volume change observed by changing the die size and thickness in an ultrathin package were studied using the warpage prediction model. The results indicate that package warpage can be effectively reduced by decreasing the die size and increasing the die thickness.