Predicting the fatigue life of solder interconnections is a challenge due to the complex nonlinear behavior of solder alloys and the load history. Long experience with Sn-Pb solder alloys together with empirical fatigue life models such as the Coffin-Manson rule have helped us identify reliable choices among package design alternatives. However, for the currently popular Pb-free choice of SnAgCu solder joints, designing accelerated thermal cycling tests and estimating the fatigue life are challenged by the significantly different creep behavior relative to Sn-Pb alloys. This study is divided into two parts: In the first part, a hybrid fatigue modeling approach inspired by nonlinear fracture mechanics is discussed. The hybrid fatigue model has been shown to predict the crack trajectory and fatigue life of a Sn-Pb solder interconnection subjected to both isothermal accelerated thermal and anisothermal power cycling conditions. In the second part, results of experimental characterization via fatigue testing of microelectronic packages with SnAgCu solder interconnections subjected to anisothermal power cycling conditions are described. Packages of different geometries were tested to study the effects of these variations on the estimation of fatigue life. The afore mentioned hybrid model relies on the estimation of two fracture parameters which are to be determined experimentally. In this study, a novel technique involving tracking crack fronts in solder interconnections as a function of number of fatigue cycles is proposed to estimate these fracture parameters that can be then used to model fatigue crack growth using the hybrid modeling technique.