Hybrid circuits - Design

It's very difficult to give a precise definition of a hybrid circuit. There are, however, a few basic characteristics that make them easy to identify. A hybrid circuit is built on an insulating substrate, usually ceramic, on which a complete electrical function is formed using active and passive components. Active components may be bare chips or encapsulated in packages. Passive components are either printed on the ceramic substrate, or in chip form. In all cases, the components are transferred to the surface.

In the spirit of this definition, hybrids have been around since the late 50s.

There are two families of hybrid circuits:

• thick-film hybrids: in this family, specially designed inks are applied to a substrate, then baked in an oven. Application is normally carried out using the screen-printing technique which, by using a screen specific to the circuit in question, avoids masking operations. Several layers (conductive, resistive, dielectric) can be applied sequentially. The name thick film comes from the fact that the films are quite thick: from 10 to 50 µm thick;

• thin-film hybrids: this name applies to hybrids whose conductive, resistive or dielectric layers are produced by vacuum deposition: evaporation or sputtering. The lines are defined using photolithography techniques: masking with photosensitive resins, then etching. The resulting films are thin: from 10 to 100 nm. The circuits have a single conductive layer, with or without a resistive and/or dielectric layer.

Hybrid circuit technology has evolved quite rapidly since the beginning of the decade. The main thrust has been on increasing connection density, achieved either by increasing the number of layers (especially in the thin film), or by improving marking and etching techniques. As a result, a new terminology has emerged for hybrid circuits with high interconnect density: the Multi Chip Module (MCM).

It is important to describe two categories of MCM that are the direct evolution of thick-film and thin-film hybrids:

• MCM-C: hybrid circuits with bare chips on a ceramic substrate and screen-printed layers;

• MCM-D: hybrid circuits with bare chips on ceramic or conductive substrates, whose layers are produced using microlithography techniques derived from semiconductor manufacturing.