The influence of underfill material properties on the fatigue life of Ball Grid Array (BGA) packages that are subjected to thermal cycling is investigated in this study. A finite element model is created using Ansys by assuming the existence of an infinite array of solder interconnects, cylindrical in shape, surrounded by underfill material. Axial stresses in the interconnects are determined as a temperature loading is applied. The results show that these normal stresses are on the same order of magnitude as the hydrostatic compressive stresses induced in the solder upon underfill curing. Therefore it is concluded that for the range of underfill properties tested, these Mode I cyclic stresses need to be considered in the development of a fracture-based fatigue life model. In addition, a guideline is provided to aide researchers in designing experiments that will replicate loads on fractured specimens that are consistent with those seen in aerospace applications.