Substrates are the important components in electronic packaging systems. Their functions mainly contain equipping and fixing electronic devices, forming circuitous pattern on their surface or interior, as well as insulation and electronic interconnection with external chips. With the development of consumer electronics and mobile products toward lightweight, thinner and intellectualization, organic substrates have became the preferred substrates, due to their low dielectric constant, high moisture resistance, low density, and easy process. However, as modern electronic devices continue to move toward faster, higher integration, miniaturization and higher performance, the properties of the organic substrate cannot meet the requirement. It’s important to develop high-performance organic substrates.The present dissertation will focus on the fabrication of the organic substrate, based on the analysis of present research progress and the trend of development for the organic substrates. The excellent mechanical, thermal and dielectric properties of the obtained organic substrates were investigated based on the relationship between microstructure and interfacial or macroscopical properties. The main results can be summarized as follows:1. In view of the toughness-deficiency issue for organic substrates, we demonstrate a new method that can simultaneously improve strength and toughness of the glass fiber reinforced bismaleimide-triazine (BT) resin composites by using polyethylene glycol (PEG) to construct flexible bridge at the interface. The mechanical properties, including the elongation, ultimate tensile stress, Young’s modulus, toughness, and dynamical mechanical properties were studied as a function of the length of PEG molecular chain. The incorporation of PEG produces an increase in elongation, ultimate tensile stress and toughness, due to the “bridge effect” of PEG between glass fiber and BT resins. Furthermore, the glass transition temperature and dielectric properties are slightly reduced to 225.1 ℃ and increased to 4.4 at 1.0 MHz, respectively. Although the addition of PEG leads to the deterioration of glass transition temperature and dielectric properties, they still meet the requirement of dielectric packaging.2. In view of the issue that the addition of fillers leads to brittleness, we have prepared multiwall carbon nanotubes (MWCNTs)@SiO2 core-shell hybrids by coating MWCNT with insulating silicon oxide (SiO2). The reason we chose MWCNT is that it possesses excellent mechanical and thermally conductive properties. The BT resins/glass fibers/ MWCNT@SiO2 substrate materials were prepared using MWCNT@SiO2 as the fillers. We demonstrate that the MWCNT@SiO2 is more effective than the raw CNT and commercial spherical SiO2 in improving the mechanical properties of the composites. The tensile strength and Young’s modulus increase with the increase of SiO2 thickness and CNT@SiO2 loading, and reach their maximum values (tensile strength, 248.4 MPa and Young’s modulus, 10.8 GPa) with 4.0 wt% MWCNT@SiO2 (10 nm of SiO2 thickness).The enhanced properties are mainly attributed to the intrinsically nature of MWCNTs, homogeneous dispersion of the hybrids, as well as improved interfacial interaction. Meanwhile, the composites remain high electrical insulation (9.63 ×1012 Ω cm) because of the existence of SiO2 layer on CNT surface. We believe that this research has provided a guidance to construct composites with high-performance and will open new applications of CNTs.3. In view of the development of organic substrates toward thinner, MWCNT@SiO2 based polymer nanocomposites without glass fibers which have high mechanical strength and electrical insulation have been fabricated. The MWCNT@SiO2 is more effective than the raw CNT and commercial glass fibers in improving the mechanical properties. The mechanical properties increase with the MWCNT@SiO2 loading and SiO2 thickness. The volume resistivity of the nanocomposites containing MWCNT@SiO2 shows high electric insulation (2.36×109 Ω.cm). Addition of MWCNT@SiO2 hardly affects the dielectric properties and optical transparency of BT resin. Furthermore, the values of Td5% increase with increase of MWCNT@SiO2, and reach 353.7 °C for the nanocomposite containing 1.6 wt% MWCNT@SiO2, which is 18.5 °C higher than that of pure BT resin. These excellent properties are attributed to MWCNT@SiO2’s strong interfacial interactions with BT resin. The BT/MWCNT@SiO2 nanocomposites have been successfully demonstrated as an organic substrate, on which a frequency “flasher” circuit and the electrical components work well. The concept of using MWCNT@SiO2 as fillers in this study will provide a solid theory foundation for the thinness of organic substrate.4. Inspired by the nano/microscale hierarchical structure and precise inorganic/organic interface of natural nacre, we have fabricated artificial nacre-like papers based on noncovalent functionalized boron nitride nanosheets (NF-BNNSs) and poly(vinyl alcohol) (PVA) via vacuum-assisted self-assembly technique. The mechanical properties of the artificial nacre-like papers increase with the PVA loading, and reach maximum value when the PVA loading is 6.0 wt%. The artificial nacre-like papers exhibit excellent tensile strength (125.2 MPa), Young’s modulus (15.7 GPa) and toughness (2.37 MJ m-3). The resulting papers also render high thermal conductivity (6.9 W m-1 K-1). These excellent mechanical properties result from an ordered ‘brick-and-mortar’ arrangement of NF-BNNSs and PVA, in which the long-chain PVA molecules act as the bridge to link NF-BNNSs via hydrogen bonds. We also used the paper as organic substrate to prepare a simple electronic device. The f-BNNSs/PVA paper exhibits higher ability to remove the heat, leading to the spot temperature of the light-emitting-diode chips (LED) is only 34.6 °C, which will protect the LED performance and extended service life. The combined mechanical and thermal properties make the materials highly desirable as flexible substrates for next-generation commercial portable electronics.5. In order to prepare composites with high thermal conductivity at low filler loading, we have demonstrated ice-templated assembly strategy to construct three dimensional BNNS (3D-BNNS) network in the polymer composites in order to improve thermal conductivity of the polymer composites. The 3D-BNNS network was infiltrated with the epoxy resins to obtain 3D-BNNS composites. The obtained 3D-BNNS composites exhibit anisotropic and enhanced thermal conductivity, compared with that of the composites containing random distributed BNNSs (RD-BNNS). At the highest BNNS loading (9.29 vol%), thermal conductivity for 3D-BNNS composites in parallel and perpendicular to the ice direction reach 2.80 and 2.40 W m-1 K-1, respectively, whereas the RD-BNNS composite has a modest thermal conductivity of 1.13 W m-1 K-1. The remarkable improvement results from the unique hierarchical structure of 3D-BNNS, which leads to smaller BNNS-BNNS interfacial thermal resistance (5.9-7.7×10-9 m2 K W-1) in the 3D-BNNS composites than BNNS-epoxy interfacial thermal resistance (9.21 ×10-7 m2 K W-1) in the RD-BNNS composites. Furthermore, the 3D-BNNS composites present a smaller CTE (24-32 ppm/K) and a higher glass transition temperature (130-136 °C). Our work provides a new insight into the fabrication of composites with high performance for versatile applications in advanced organic substrates. 6. We have fabricated fibrous epoxy films with aligned fibers by electrospinning technique using liquid crystal epoxy resins as the raw materials. The single fibers are found to be a degree of chain orientation, thereby improving their thermal conductivity. The thermal conductivity of the fibrous epoxy films can be tuned by varying the diameter of fibers and can reach values as high as 0.8 W m-1 K-1, more than three times higher than the bulk epoxy value. In addition, the porous microstructure renders the fibrous epoxy films to have low dielectric constant and loss (1.8 and 0.075 at 1.0 MHz, respectively) and excellent flexibility. In order to demonstrate the fibrous epoxy film can be used as the flexible substrate, we have fabricated a simple flexible electronic device. We further demonstrate that the fibrous epoxy substrate has more effective in heat removal of power device (LED) than the bulk epoxy by using the infrared thermography. We believe that the fibrous epoxy substrate can replace conventional plastic and paper in some applications in the future.