Thick Film Circuits for Telecommunications Equipment

The use of thick film passive networks and hybrid integrated circuits by the Bell System is described in this paper. Included is a discussion of how attributes of thick films such as a large resistance range, high power handling capability, a bipolar capacitor and multilevel interconnection capability are used to provide a range of circuit properties that supplement the properties available in a well-established thin film technology. Design aids permitting a rapid transition from an electrical schematic to a finished circuit, special laser trimming procedures and material blending procedures are described for resistor networks. Also included are assembly and packaging techniques suitable for high reliability and low cost manufacturing. Several hybrid integrated circuits which benefit from the use of thick films are described in terms of their particular electrical or structural requirements. Included is a family of power converter circuits illustrating the advantages of a modular approach to circuit design in which functional resistor trimming provides a number of different circuit characteristics. Several applications involving the use of thick films in combination with thin films to provide special hybrid circuit properties not available from thin films alone are described. These and other possible uses of the two technologies in the same circuit are discussed in terms of combining the best properties of each technology to provide higher reliability and lower circuit cost. Finally, the future for the use of thick films in the Bell System is discussed.