Abstract: A novel capacitor foil and printed circuit board intermediate made using that foil are disclosed. The capacitor foil is a three-layer construction having a conductive layer, a partially-cured high dielectric constant layer, and a partially-cured bonding layer. The high dielectric constant and bonding layers are formed with epoxy or other polymer, however, the high dielectric constant layer is loaded with capacitive ceramic particles or pre-fired ceramic forming particles. The bonding layer may or may not be filled with ceramic particles or prefired ceramic-forming particles. The resulting capacitor foil may be applied to a laminate having copper patterns thereon to define a PCB intermediate containing at least one buried capacitor device. Multiple layers of capacitance material also can be used to increase the overall capacitance of the board.

Abstract: This invention involves a prepreg composition and procedure for use in forming printed circuit boards with reduced and uniform dielectric constant, improved thermal expansion characteristics, and uniform appearance. The prepreg composite is a resin impregnated substrate and has a reduced dielectric constant as low as 3.5. It contains 5-30% by resin volume of hollow inorganic microsphere filler whose diameters are less than 40 micrometers, and the uniform distribution of the hollow inorganic microspheres is controlled by the presence of hydrophobic fumed silica or other similar flow modifier.