Effects of anisotropic conductive film (ACF) viscosity on ACF fillet and moisture-related reliability for flip-chip-on-board (FCOB) packages

In this paper, effects of ACF viscosity on ACF flillet form-ability and how the ACF fillet affects the moisture- related reliability of flip chip assembly were investigated. ACF viscosity was controlled by varying molecular weight of epoxy materials. ACF viscosity increased, while the molecular weight of contained epoxy materials increased. However, the thermo-mechanical properties of the ACFs were not significantly different each other. Also, the ACFs showed no differences in the moisture absorption rate, die adhesion strength and degree of cure. In SEM images, lower ACF viscosity formed more gentle shape of ACF fillet. In pressure cooker test (PCT), ACF flip chip assembly which had more gentle shape of ACF fillet showed better reliability in a viewpoint of contact resistance changes. In cross-sectional images of flip chip assemblies after PCT 130 hours, flip chip assembly which had more gentle shape of ACF fillet showed less delamination between ACF/chip interface.