Warpage Modeling of Ultra-Thin Packages Based on Chemical Shrinkage and Cure-Dependent Viscoelasticity of Molded Underfill

Molded underfill (MUF) is an essential component in ultra-thin flip chip packages to ensure their long-term reliability and mechanical integrity. However, the warpage evolution of package during the curing process of MUF has a critical effect on both the SMT yield and package reliability. Therefore, a thorough understanding of the MUF physical behavior under curing is essential. The present study therefore successfully establishes a novel process modeling approach based on finite element method to predict the final warpage of an ultra-thin flip chip scale package (fcCSP) in accordance with the chemical shrinkage and cure-dependent viscoelastic behavior of MUF. In describing the cure-dependent behavior of the MUF, the chemical shrinkage, curing kinetics and time-domain viscoelasticity are characterized by the pressure-volume-temperature (PVT) method, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), respectively. The predicted package warpage results are shown to be in good agreement with the experimental thermal Shadow Moiré data. It is additionally shown that the accuracy of the warpage predictions is seriously degraded if the mechanical properties of MUF is modeled using a simple temperature-dependent elastic assumption.