Automotive underhood electronics are subjected to high operating temperatures in the neighborhood of 150–200 °C for prolonged periods in the neighborhood of 10 yr. Consumer grade off-the-shelf electronics are designed to operate at 55–85 °C with a lower use-life of 3–5 yr. Underfill materials are used to provide supplemental restraint to fine-pitch area array electronics and meet the reliability requirements. In this paper, a number of different underfill materials are subjected to automotive underhood temperatures to study the effect of long time isothermal exposure on microstructure and dynamic mechanical properties. It has been shown that isothermal aging oxidizes the underfill, which can change the mechanical properties of the material significantly. The oxidation of underfill was studied experimentally by measuring oxidation layer thickness using polarized optical microscope. The effect on the mechanical properties was studied using the dynamic mechanical properties of underfill with dynamic mechanical analyzer (DMA). Two different underfill materials were subjected to three different isothermal exposures, which are below, near, and above the glass transition temperature of the underfills. The dynamic mechanical viscoelastic properties like storage modulus, loss modulus, tan delta, and their respective glass transition temperatures were investigated. Three-point bending mode was used in the DMA with a frequency of 1 Hz operating at 3 °C/min.