Anisotropic conductive films (ACFs) composed of polymer adhesive resin and conductive particles have been used as interconnection materials for ultrafine pitch chip-on-flex (COF) packages due to their flexibility, stable electrical properties, and bending reliability. To realize an efficient ultrafine pitch COF package, a new anchoring polymer layer (APL) was recently introduced. However, the chip flexibility in terms of ACFs materials after APL layer integration needs lot of improvement. Herein, we study the order to obtain more flexible and reliable COF packages using ACFs interconnection materials, and the effect of the ACFs material properties on the COF bending reliability was investigated. It was found that when the APL ACFs were used for the COF packages, 100 & x0025; electric insulation rates were obtained due to the suppression of conductive particle movement by the APL structure compared with the conventional ACFs. Furthermore, the polyacrylonitrile (PAN) APL ACFs showed better ACFs joint properties and dynamic bending properties compared with the polyvinylidene fluoride (PVDF) APL ACFs due to its higher adhesion property caused by the chemical reaction between the PAN and the curing agent inside the epoxy polymer adhesive resin confirmed by the Fourier transform-infrared spectroscopy (FT-IR) analysis. In addition, the strain applied to the PAN APL ACFs joints during the bending test was 1.35 & x0025; which was quite smaller than that of the PVDF APL ACFs joints, 2.01 & x0025; resulting in better bending reliability too. For the bending reliability, there was no failure at the PAN APL ACFs joints up to 150 000 cycles, not only because of the excellent mechanical properties of PAN APL ACFs but also because of the lower strain applied to the APL ACFs joints during the dynamic bending test in COF packages.