Warpage prediction of integrated circuits packages

Warpage is formed as a result of the thermal mismatch between the IC package components. The warpage problem becomes a critical issue particularly in large and thin packages such as thin quad flat package (TQFP). A thorough understanding of the molding process parameters and material properties is necessary in order to minimize the problem of warpage and the associated stressing issue. Previous research works have focused on stress analyses without full consideration of the thermal history of the package and time-temperature dependence of the properties of the epoxy molding compound (EMC). In the present investigation of IC package warpage and molding residual stresses, special emphasis has been placed on: (a) the characterization of epoxy molding compound properties; (b) the construction of degree-of-cure based EMC material property models; and (c) the study of the variation of package warpage and residual stress against molding process parameters such as molding temperature, time and post-cure conditions.; The EMC properties were obtained by various thermal analysis techniques: degree-of-cure by differential scanning calorimetery (DSC); modulus by dynamic mechanical analysis (DMA); and coefficient of thermal expansion (CTE) by thermomechanical analysis (TMA). The glass transition temperature {dollar}Tsb{lcub}g{rcub}{dollar} was shown to be a direct measure of degree-of-cure. Experimental data of shear modulus {dollar}Gspprime{dollar} and {dollar}Gsp{lcub}primeprime{rcub}{dollar} measured by DMA were used to formulate the shear modulus and time shift factor models.; Mold filling analyses wcre conducted to predict the cure content within a 160 leads PQFP package. The G and CTE models, expressed as a function of {dollar}Tsb{lcub}g{rcub},{dollar} were used in 3-dimensional numerical analyses to predict the formation of warpage and stress after the encapsulation. Predicted warpage and stress values were compared with the laboratory measurements using a Coordinate Measuring Machine (CMM) and test chip, respectively. The numerical and experimental results agreed quantitatively.; The viscoelastic model of EMC was found to provide a more accurate result than the elastic model. Less warped and stressed packages could be obtained if the packages were molded at a lower temperature for a fixed time or at a fixed temperature for a longer time. Less warpage was also found in package with copper leadframe but this resulted in a more stressed package. Fast cooling could make the warpage and stress problems more seriously. Post-curing provided an efficient way for warpage reduction and stress relaxation.