Three-dimensional Integrated-circuit (3DIC) needs coatings with both high thermal conductivity and high electrical insulation for isolating electronic devices and interconnects while spreading heat generated by stacked integrated circuits effectively. Single crystalline diamond possesses excellent electrical insulation and thermal conductivity, which is a perfect candidate for the need by 3DIC. However, a large-area coating of single crystalline diamond is difficult to achieve. So we use polycrystalline diamond films instead. But for polycrystalline diamond films with many grain boundaries, the severe phonon scattering and electrically conductive graphitic carbon contents in grain boundaries cause the electrical insulation and the thermal conductivity to decrease. The smaller the grain size is, usually the decrease is more severe. A good compromise is to retain the high thermal conductivity of diamond crystals while minimizing the electrical conductivity of polycrystalline diamond coatings by removing the charge-transfer doping mechanism enabled by hydrogen termination on diamond grains and minimizing graphitic carbon in the grain boundaries. This paper reports a large-area tri-layer diamond coating structure to achieve sustainable 1010 μcm electrical resistivity in the ambient atmosphere. A nanodiamond base layer provides a high-density diamond seeding layer for the polycrystalline diamond film to contain few voids and graphitic carbon in the grain boundaries. The second nanodiamond film is used to encapsulate the de-hydrogenated microcrystalline diamond film to prevent degradation of electrical resistance due to the ambient atmosphere.