Thick film ceramic capacitors and resistors inside printed circuit boards

The high level of current interest in embedded passives in printed circuit boards is driven by the tremendous pressure to pack more circuitry into smaller spaces. However, adoption has been limited, partly due to design, prototyping and infrastructure issues, but mainly due to the lack of component density, stability and tolerances necessary for widespread replacement of discretes. Thick film ceramic capacitors and resistors are the technology of choice for most passive devices due to performance, stability, tolerances and availability in many sizes and grades. The focus of this work has been to develop a process and materials to reliably incorporate thick film ceramic resistors and capacitors inside printed wiring boards, thereby providing options not previously available. The resistor materials are based on standard nitrogen firing thick film pastes that have been used in automotive applications for 15 years. Sheet resistivities range from 10 ohms/square to 10 K ohms/square. ne capacitor material is a nitrogen-firing barium titanate composition. The dielectric constant is > 1000, has X7R characteristics, and yields capacitance densities of 150 nF per square inch. The compositions are printed and fired on a conditioned copper foil using a standard nitrogen thick film furnace at 900degreesC This is followed by lamination of the copper foil, components face down, to an FR4 board using a standard pre-preg. Photoresist is applied to the copper and the remaining steps - expose, develop, etch and strip - are carried out under conventional PWB processing. The result is copper circuitry with ceramic components inside an FR4 matrix. This inner layer can then be incorporated inside a multilayer PWB. This paper describes the process, presents the performance, and discusses preliminary design guidelines for the embedded passives.