17.1 Ceramic Thick Film and Hybrid Circuits

17.1.1 Introduction

Multi-layer printed circuit boards using
polymer boards - courtesy Welwyn Electric
and ERL Phase

Ceramics have several roles in electronic integrated circuitry. It can be the insulating substrate the circuits are built upon, the conductors on the substrate, the dielectric layers for capacitors and for insulation, and as advanced signal processing elements. Conventional printed circuit boards have layers of copper conductor on polymer boards that are photo lithographically etched to form the conductive circuit. Boards may be sandwiched together to form a multi-layer configuration.

However, the complexity of computing, mobile communications and satellite systems increases the need for miniaturisation of circuitry. This can be effected by using similarly fabricated conductive circuits, but by putting them on a ceramic rather than a polymer substrate.

The simplest structure is where the processed ceramic substrate only supports the interconnection between semiconductor devices attached to its surface. Alumina is the most popular ceramic used. In the late 1960s high quality alumina ceramic substrates were usually 2” by 2”, but by the 1990s had grown to 6” by 4”. Lasers are used to machine and cut the substrates that can be from 0.5 to 3mm thick. An early application in the Second World War was as a miniature radio proximity fuse for munitions.

Ceramic printed circuit board - courtesy
Welwyn Electric

Alumina substrate with holes and cut-outs
- courtesy Welwyn Electric

Advanced ceramic materials that can be applied directly onto the surface of the substrate make it possible to carry out signal processing directly on the substrate as well. Films of polycrystalline ferroelectrics were developed in the 1970’s and 80’s to include barium titanate, PZT, PLZT, barium strontium titanate (BST) and lithium niobate. The whole circuit can then be sintered (fired) to produce ceramic devices, insulation and conductors.

The most common form of ceramic thick film application is the hybrid circuit, where there is a combination of electronic circuitry made by the thick film process and discrete add-on semiconducting devices, so that the combination performs the required electronic function. Compared with polymer substrates, ceramic circuits can be smaller, more resistant to hostile environments and be used at higher frequencies.