Underhood applications in automotive are increasingly using electronics systems for safety and critical functions. In flip-chip ball grid array (FCBGA) packages, underfill (UF) forms the integral mechanical support between the substrate and die. In addition, underfills protect the chip against shock, vibration, moisture, and radiation. Underfills provide great mechanical support to the solder interconnects and limit the amount of plastic work during temperature excursions. Delamination is of the significant failure modes observed at chip-UF interfaces. Chip-UF interfaces have not been studied widely under cyclic fatigue loading with sustained high-temperature exposure. Chip-UF bi-material samples are prepared and subjected to long-term high-temperature aging at 100°C and then tested under four-point bend fatigue loading. The specimens have been exposed to isothermal aging for 30 days, 60 days, 90 days, 120 days, and 180 days. The interfacial crack growth rate with respect to the number of fatigue cycles has been determined from the experiment. The steady-state energy release rate and range of mode-I stress intensity values (ΔKI) have been computed for each of the test conditions. Paris power law has been used to establish the relationship between the crack growth rate and the range of stress intensity factors. Paris exponents (A,n) are determined from the relationship to understand the evolution in interfacial fracture toughness with respect number of days of aging under fatigue loading.