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The effect of underfill on various thermomechanical reliability issues in super ball grid array (SBGA) packages is studied in this paper. Nonlinear finite element models with underfill and no underfill are developed taking into consideration the process-induced residual stresses. In this study, the solder is modeled as time and temperature-dependent, while other materials are modeled temperature and direction-dependent, as appropriate. The stress/strain variations in the package due to thermal cycling are analyzed. The effect of underfill is studied with respect to magnitude and location of time-independent plastic strain, time-dependent creep strain and total inelastic strain in solder balls. The effect of copper core on the solder ball strains is presented. The possibility of delamination at the interposer-underfill interface as well as substrate-underfill interface is studied with the help of qualitative interfacial stress analysis. Results on SBGA packages indicate that the underfill does not always enhance BGA reliability, and that the properties of the underfill have a significant role in the overall reliability of the BGA packages. The predicted number of thermal cycles to solder joint fatigue are compared with the existing experimental data on similar nonunderfilled BGA packages.