The automotive underhood electronics are subjected to temperatures in the range of 150 to 200°C for prolonged periods. The coefficient of thermal expansion mismatch between the chip and the substrate results in the fatigue-failure of solder joints when operating at high temperatures. Underfills provide extra support to the flip-chip bumps, enhancing the fatigue life and reducing the solder joint strains. Models and material degradation data are needed for the underfills exposed to high temperatures. The effect of the evolution of non-linear constitutive behavior of underfills on the solder balls and the study of the evolution of viscoelastic behavior of underfills have not been studied. In this paper, the evolution of underfill properties over 1-year has been measured for two underfills at sustained high-temperature operation. The aging data has been reported at 30, 60, 120, 240, and 360 days at 100°C, 125°C, and 150°C. The effect of non-linear property (Prony series) evolution of underfills on the FCBGA (Flip Chip Ball Grid Array) package reliability has been evaluated. The quarter FCBGA package is modeled from −40°C to 125°C. The results show that the flip-chip plastic work per unit volume of pristine-linear-elastic constitute model underfill FCBGA was much lower compared to pristine-viscoelastic underfill model FCBGA. Results show the importance of considering the non-linear underfill properties instead of linear properties.