Effects of Anisotropic Conductive Film Viscosity on ACF Fillet Formation and Chip-On-Board Packages

In this paper, the effects of anisotropic conductive film (ACF) viscosity on ACF fillet formation and, ultimately, on the pressure cooker test (PCT) reliability of ACF flip chip assemblies were investigated. The ACF viscosity was controlled by varying the molecular weight of the epoxy materials. It was found that the ACF viscosity increased as the increase of molecular weight of the epoxy materials. However, there was little variation of the thermomechanical properties among the evaluated ACFs with different viscosites. Also, the results showed that the ACFs have no differences in moisture absorption rate, die adhesion strength, and degree-of-cure. In scanning electron microscopy images, the lower ACF viscosity resulted in the smoother ACF fillet shape and the higher fillet height. From the results of PCT, the ACF flip chip assembly with the smoother fillet shape showed better reliability in terms of contact resistance changes. After 130 h of PCT, the flip chip assembly with lower ACF viscosity also showed a lesser degree of delamination at the ACF/chip interface.