In this study, effect of various moisture condition on underfill interfacial adhesion loss were examined using C-SAM, 4-point flexural bend test, and cross sectional analysis. In addition, weight gain analysis was used to determine packages moisture absorption at preconditioning level. In order to understand mechanical properties degradation of underfill material, 4-point flexural bending test was used. Maximum flexure load to bend the FC packages was recorded. Failure mode then was categorized to 3 mode; mode 1 for die cracking, mode 2 for underfill cracking and mode 3 for the combination of mode 1 and mode 2. Selected units for every condition were cross sectioned and analyzed with their bend test graph to further understand FC packages breaking mechanisms. It was interesting to found that maximum flexure load was degraded after every moisture condition .The failure mode has changed from die cracking to underfill cracking. Furthermore, cross sectional analysis shows that underfill cracking failure mode has propagated from cohesive failure at fillet area to adhesive failure at underfill to substrate/die interfaces. As a conclusion, packages flexural strength degraded after moisture stressing with failure mode shows the underfill mechanical properties degradation is the one of the factor that degrades the flexure strength and decreases underfill interfacial adhesion.