Thick film technology covers films from 2 to 50 microns thick, although they are typically between 10 and 50 microns. This is compared to 1 micron or less for thin films. The thick film process uses relatively simple and cheap equipment, meaning that design changes can be made much more readily and smaller production runs can be accommodated. Each layer, possibly containing resistors, capacitors and inductors, is built up from successive printing, drying and firing operations for conductive then insulating layers, and typically up to four such operations are needed for a single layer. Although it can be arranged that more than one printing can be fired together, there are still a lot of steps in the process that can make it rather expensive. Using photolithography and photosensitive inks exposed through artwork, lines less than 40-micron wide can be achieved. More recently there has been a jump in the technology with the development of special pens and inks to write directly onto the substrate.

Thick film hybrid circuits can be single or double-sided, with circuitry on the top and bottom of the ceramic substrate, although single sided is usual.

The ceramic substrates are processed in very clean conditions and the inks carefully specified. In practice the multi-layer hybrid starts with the fired ceramic substrates. Typically 25 of the 6” by 4” substrates are mounted on a carousel to be sequentially screen-printed. Each layer is dried at 150-180 ºC and fired at 850 ºC for 45 minutes, building up the three dimensional structure. This process is repeated for up to 30 separate prints, taking several days overall.  Connections are made by “vias” or holes in the ceramic filled with conductive material. Many circuits can be made on a large ceramic substrate and diced singly using a laser. A laser can also be used to trim thick film resistors to obtain precise values. An overglaze of lower temperature glass can be used to protect chips and conductors and helps to avoid solder bridging.

17.1.3 Co-Fired Ceramic Circuitry

Modern ceramics-based packages are a sophisticated multi-layered combination of ceramics, glasses and metals, providing a cost-effective solution for a variety of applications. These ceramic circuits use a number of processed ceramic substrates sandwiched together. The high compressive strength, dimensional stability and chemical inertness make ceramics ideal for these structures. Because of the manufacturing and sintering process a wide range of dielectric, resistive, conductive and magnetic materials can be incorporated into these multi-layer ceramic bodies providing high-density circuitry that can withstand temperatures up to 1000 ºC. The materials have to be carefully selected for the right thermal and electrical properties, so that in operation the correct temperature is maintained and there is no electrical breakdown.

Co-fired hybrid circuit prior to fitting
semiconductor devices - source Wikipedia
via Konstantin Lanzet

High and low fired, co-fired ceramics are such laminated structures. They provide interconnection, insulation and advanced ceramic devices to support the unpackaged semiconductor chips mounted on their surface. However, the significant difference is that the co-fired multi-level ceramic structure is fired only once. Manufacture starts with unfired or “green” ceramic tape, around 0.1 mm thick and 10 to 40 cm wide, which is formed from ceramic powders and a binder and dried. It has the consistency of dried putty and is formed in rolls typically 50ft (15m) long. The ceramic is often based on alumina with silica and alkaline fluxes to help sintering. The green ceramic tape is cut, punched and screen-printed, producing 70-micron lines and spaces. It is then assembled into a multi-level three-dimensional structure, then the entire structure of ceramic and metallisation is fired simultaneously.

The organic additives are removed at 600 ºC, the carbon at 1200 ºC and sintering takes place at 1600 ºC. There is some 14 to 17% shrinkage in the overall process. Multilevel co-fired structures can be complex with around ten ceramic layers, including some high permittivity ones. It can include filters, buried resistors and coupling capacitors. Surface mounted silicon chips, printed resistors and other surface mounted devices are added to make a sophisticated signal processing system.