Characterization of Interfacial Hydrothermal Strength of Sandwiched Assembly Using Photomechanics Measurement Techniques.

The reliability issues have been converted to the underfill adjacent interfaces since the introduction of the underfill to flip chip package in the 1990s. Both thermal cycling and hygrothermal conditioning severely attack the interfaces to delaminate. 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 chapter, interfacial reliability of a sandwiched assembly exposed at 85°C/85%RH was studied using photomechanics measurement techniques, i.e., moiré interferometry (MI) and digital image correlation (DIC). 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 it increased the strains induced by hygrothermal swelling. It indicated that the time effect is not negligible when the assembly is subjected to moisture conditioning; otherwise, the deformation induced by the swelling could be overestimated. The DIC technique was applied to measure the critical interfacial fracture toughness of the interface. The results showed that the moisture could significantly reduce the interfacial strength due to the break of hydrogen bonding. By combining the MI and DIC 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 (SEM). Remarkable changes of the failure mode were observed. [ABSTRACT FROM AUTHOR]