FCBGAs are finding applications in automotive underhood environments where they may be subjected to sustained temperatures of 125–200°C for sustained periods during operation. While, FCGBAs have been previously used in consumer applications where operating temperatures typically range in 55–85°C, relatively little is known on methods to design damage-tolerant packages in automotive underhood environments. There is insufficient information on plastic encapsulated electronic components capable of surviving high temperatures for long periods (> 100,000 hours). In this paper, four different types of underfills has been cured and aged. Mechanical tests have been performed on all the four types of underfills too understand the degradation in properties under extended high temperature operation. Uniaxial tensile tests are conducted to study the elastic modulus, ultimate tensile strength and percentage elongation of the underfills. After uniaxial tensile tests, Optical Microscope, SEM and EDS are applied to study the microstructure behaviors of the cross-section area of the underfills. The experimental results are compared before and after aging tests, including pristine, 30 days, 60 days, 90days, 120 days and 240 days. The research focuses on microstructure-property-processing-performance relationships, building the relation between the microstructure evolution and macro-mechanical properties. Reliability physics of high temperature degradation of packaging material is studied.