A number of drive-critical functions are enabled by electronics, including lane-departure warning systems, collision-avoidance systems, driver-alertness monitoring, park-and-drive assist systems, adaptive cruise control, and semi-autonomous navigation. Electronics on the automotive platform may reside underhood and be mounted on-engine, transmission, firewall, or wheel well where the temperature may be in the neighborhood of 150-200°C. FCBGAs that are used to enable a lot of the advanced functionality require the use of underfills to provide supplemental restraints for the flip-chip bumps to achieve the needed thermo-mechanical reliability. Failure of the chip-underfill interface or the substrate-underfill interface of the FCBGA is often the precursor for the failure of the flip-chip solder joints. There is a scarcity of research on the effect of sustained thermal exposure on the interface integrity internal to the FCBGA. Damage models are needed to capture the interface damage mechanics and pursue life prediction under monotonic and fatigue loads. This paper examines the effect of the sustained high-temperature operation on the interfacial fracture toughness of the chip-underfill and substrate-underfill interface under both monotonic and fatigue loads. Bi-material specimens have been fabricated to study the interfacial fracture toughness of the interfaces after sustained high-temperature exposure. The measurements have been used to extract the fracture toughness values as a function of the duration of sustained operation at high temperatures. Paris's Power Law parameters have been extracted for the substrate-UF interface and the chip-UF interface. The results reported in the paper can be used for predictive models and assess reliability in the end application.