Combining thick-film and printed wiring board processes allows thick-film ceramic resistors and capacitors to be embedded in printed wiring boards (PWB). The resistor materials are based on lanthanum boride and cover the range of 10 ohm/square to 10 Kohm/square resistivities. The capacitor material is based on doped barium titanate. Both systems are designed to be “thick-film” printed on copper foil in the locations desired in the circuit and the foil is then fired in nitrogen at 900°C to form the ceramic component on the copper foil. The foil is then laminated, component face down, to FR4 using standard prepreg. The inner layer is then etched to reveal the components in a FR4 matrix. The resistors can be trimmed to tight tolerance at this stage and the components tested. The inner layer can then be laminated into a multilayer PWB. The process is described and the influence of board design, PWB processing and materials are presented and discussed. Examples of circuits using embedded thick-film passives are shown and results of reliability studies are presented.
Factors Controlling the Surface Morphology of Copper Foil for Printed Wiring Board Electrodeposited at High Speed.
