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The reliability issues have been converted to the underfill adjacent interfaces since the introduction of the underfill to flip chip package in 1990's. Both thermal cycling and hygrothermal conditioning severely attack the interfaces to de- laminate. The moisture migrating into the underfill decreases the adhesion strength, swells to deform the assembly, and weakens the mechanical and thermal properties of the material. In this study, interfacial reliability of a silicon/underfill/FR-4 assembly exposed at 85degC/85%RH was studied using moire interferometry and micro-digital image speckle correlation (mu-DiSC) techniques. A thermal aging study was simultaneously performed to understand the long-term reliability of the assembly. The results showed that the thermal aging relieved the stresses induced by hygrothermal swelling mismatch between dissimilar materials involved, whereas increased the strains induced by hygrothermal swelling. It indicated the time effect is not negligible when the assembly is subjected to the moisture conditioning, otherwise, the deformation induced by the swelling could be overestimated. The mu-DiSC technique was applied to measure the critical interfacial fracture toughness of the silicon/underfill interface. The results showed that the moisture could significantly reduce the interfacial strength due to the break of hydrogen bonding. By combining the moire and mu-DiSC results, it was concluded that the hygrothermal loading could increase the possibility of interfacial delamination in a flip chip package. Finally, the morphologies of the fractured surface were studied with the aid of scanning electron microscope. Remarkable changes of the failure mode were observed.