Flip-Chip Ball Grid Arrays (FCBGAs) are finding applications in automotive underhood electronics for enablement of safety-critical functions. Underfills needed to reinforce flip-chip interconnects in FCBGAs need to operate reliably under sustained high temperature operation. Underfill-to-substrate interface is one of the primary failure locations under wide thermal excursions and usually a precursor to flip-chip joint failure. In order to assess the reliability in the end application, there is need for understanding the damage progression of the underfill-to-substrate interface as a function of operating time and operating temperature. In this study, the Substrate-UF interface was exposed to high temperature and the interfacial fracture toughness quantified. A three-point composite beam specimen of PCB/Underfill was fabricated to study the interface and thermally aged for periods of 10 days, 30 days, 60 days at temperatures ranging from 100°C to 150°C. Quasi-static bending was used to observe and determine interfacial delamination of the sample specimen. A 2D-Digital Image Correlation (DIC) method was also employed to understand the Crack tip opening displacement (CTOD), crack initiation and the fracture toughness, CTOD were compared with the aging schedule and temperature.