Warpage Modeling and Characterization of the Cure-dependent Chemical Shrinkage and Viscoelastic Relaxation for Cured Molding Process

Warpage has always been an important issue in electronic packaging, especially for fan-out and flip-chip chip scale packages (fcCSP). The molded underfill for encapsulation plays a key role in warpage control. It is critical to establish a fundamental understanding of the properties of cure-induced shrinkage and viscoelasticity, and the effects of pressure, temperature, volume variation, and degree of cure on package warpage.In this study, the cure-dependent shrinkage and cure-dependent viscoelasticity of a molded underfill are carefully characterized during in-molding and post molding cure (PMC) thermal processes. A plunger dilatometer is used for shrinkage characterization, and the volume change is described by pressure, volume change, temperature, and degree of cure relations. In addition, the cure-dependent properties are characterized by other two techniques, including cure kinetics by differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). It was found that the volume change after the gel point is the dominant factor for package warpage. The package warpage evolution was modeled by finite element analysis. Our predictive models are not only in good agreement with the experimental thermal shadow moiré result, but also effectively evaluate the effect for shrinkage and CTE/modulus. The results indicate that warpage at high temperature is highly sensitive to the CTE/Modulus of MUF.