Large-Scale Hybrid Integrated Circuitry: A Case Study

The continuing demand for greater circuit density has extended beyond the monolithic integrated circuit domain into the hybrid level,creating unique mechanical and thermal problems that exceed normal electrical concerns. The largest area consumer used in hybrid circuits is the filter capacitor chip. This chip, used fornoise suppression by decoupling from the power supply lines, is usually of a fairly large value and is liberally dispersed throughout the circuit design as a conservative practice. An attempt to reach a higher level of integration would, therefore, require the removal of this surface-mounted component. Obviously, this must be accomplished without electrical compromise. The effect of the physical proximity of the capacitor on the integrated circuit element requiring it is discussed. Two methods of achieving this objective were explored and are described in the case history of a data-retrieval circuit that was constrained to fit in a smallvolume hybrid configuration. In the initial phase of the investigation, capacitors were buried within the thick-film multilayer structure by introducing high K-value dielectric materials between large conductor areas serving as counterelectrodes. Connection to the appropriate monolithic integrated circuit wire bond/die pads was provided by standard blind-via techniques. Several dielectric materials were investigated; the experimental plan, processing, and electrical results are discussed as is an overall assessment of the parameters used for selecting the circuit fabrication material. In a subsequent phase of the investigation, a different approach was considered for use as buried capacitors to.obtain comparative data and provide an alternative postion during production. CAPSTRATER, a recently introduced substrate format containing integral capacitor elements, was tested as the base for a conventional thick-film multilayer structure. The initial results of this test are presented. Functional electrical testing of the resulting hybrid microcircuits was carried out to system requirement, and the performance data are presented. The results of a thermal analysis, conducted to ensure reliable operation, are also given. The hybrid packaging scheme that evolved from this effort yielded a component density of alm- ost 100 components per square inch (including analog/linear monolithic integrated circuits and filter capacitors). The electrical characteristics of the data retrieval device were within the required specifications, and junction temperatures were within reliability limits.