Effect of hygrothermal aging on interfacial reliability of flip chip on board (FCOB) assembly

The moisture migrating into underfill decreases the adhesion strength, swells to deform the assembly, and weakens underfill mechanical and thermal properties. In this study, interfacial reliability of FCOB exposed at 85 degC/85%RH was studied using Moire interferometry and mu-DiSC system developed. A thermal aging study was simultaneously performed to understand the time effect on long duration of hygrothermal aging. The results showed that the thermal aging relieved the stresses induced by hygrothermal swelling mismatch between dissimilar materials involved, which indicated the time effect is not negligible in the moisture conditioning. Subsequently, a mu-DiSC technique was applied to measure critical interfacial fracture toughness of Si/UF interface. It is shown that moisture could reduce the interfacial strength significantly due to the breaking of hydrogen bonding. Finally, the morphologies of fracture were studied under the SEM