Viscoelastic Analysis of IC Package Warpage

Package warpage was studied previously using thermoelastic finite element analysis. Epoxy Molding Compound (EMC) was assumed to be thermal-elastic with temperature dependent mechanical properties (usually assigned values above and below the glass transition temperature, Tg). The effect of thermal mismatch of components inside the package was examined without considering the time history of the curing process. To account for this, this study will examine the effect of viscoelastic properties of EMC with different degree-of-cure on package warpage during the encapsulation process. In this study, warpage of a 160 leads Plastic Quad Flat Package (PQFP) was evaluated by employing a three-dimensional viscoelastic finite element model. Dynamic mechanical properties of the EMC for different degree-of-cure were measured in a Rheometrics RMS-800 analyzer at different temperatures: 160°C, 165°C, 170°C, 175°C, 180°C and 185°C to determine the viscoelastic shear modulus of G′ and G″. The test data were then used to formulate the Maxwell model for viscoelastic analysis using a commercial finite element package ABAQUS. The analysis examined the effect of degree-of-cure of EMC on warpage prediction. The predicted warpage values were compared with laboratory measurement using a Coordinate Measuring Machine (CMM).