With the dramatic advances that have taken place in microelectronics over the past three decades, ball-grid array (BGA) packages are increasingly being used in microsystems applications. BGA packages with area-array configuration have several advantages: smaller footprint, faster signal transmission, testability, reworkability, handling easiness, etc. Although ceramic ball grid array (CBGA) packages have been used extensively in the microsystems industry, the use of plastic ball grid array (PBGA) packages is relatively new, especially for automotive and aerospace applications where harsh thermal conditions prevail. This thesis work has developed an experimental and a theoretical modeling program to study the reliability of two PBGA packages. The physics-based theoretical models take into consideration the time-dependent creep behavior through power law creep and time-independent plastic behavior through multi-linear kinematic hardening. In addition, unified viscoplastic constitutive models are also taken into consideration. The models employ two damage-metrics, namely inelastic strain and inelastic strain energy density, to predict the solder joint fatigue life. The theoretical predictions have been validated through air-to-air in-house thermal cycling tests carried out between 55 and #61616;C and 125 and #61616;C. In addition, laser-moir interferometry has been used to determine the displacement contours in a cross-section of the package at various temperatures. These contours measured through moir interferometry have also been used to validate the thermally-induced displacement contours, predicted by the models. Excellent agreement is seen between the experimental data and the theoretical predictions. In addition to life prediction, the models have been extended to map the field-use conditions with the accelerated thermal cycling conditions. Both linear and non-linear mapping techniques have been developed employing inelastic strain and strain energy density as the damage metric. It is shown through this research that the symmetric MIL-STD accelerated thermal cycles, currently in practice in industry, have to be modified to account for the higher percentage of creep deformation experienced by the solder joints in the field-use conditions. Design guidelines have been developed for such modifications in the accelerated thermal cycles.