Thick film technology has historically been an attractive method of producing conductors, dielectrics, and resistors that are rugged and reliable. The technology is well suited for economical production of short production runs. Its ability to be patterned in multilayer configurations has allowed fabrication of devices with extremely high circuit density. The successive levels of conductors in the multilayer structure are separated by insulating dielectric layers and are interconnected by vias through the dielectric layers.
The multilayer approach is more expensive than a single layer approach because it requires painstaking inspection, realignment between layers, and careful processing to avoid blistering and cracking.
The most obvious way to reduce these problems associated with multilayer production is to reduce line and space dimensions, thereby reducing the number of layers in a given structure. The problem with this approach has been the limited resolution capability of thick film screen printing, which limits the size of vias used to connect layers of circuitry to 10 to 15 mils diameter. Likewise, conductors are limited to a narrowest line width and spacing of 5 to 7 mil lines and spaces in production quantities.
Many different approaches have been tried to obtain finer pitch lines and smaller vias. Extremely fine screen mesh and improved emulsion backing have allowed line resolution of as low as four mils line/space to be obtained in limited production. Photoformable pastes have been developed that allow five mil or finer vias, and two to three mil line/space pitch. Thick film metallizations have also been patterned with photoresists and etched to produce fine line patterns and thin film conductors have been plated up to produce fine line patterns with high conductivity.
All the above approaches have associated drawbacks. For example, fine mesh screens typically lay down thinner conductor and dielectric layers than are desirable. Photoformable pastes have a larger amount of organic matter that increases shrinkage during firing, and can produce dirty burnout that may render fired parts useless. Conductors produced with photoformable pastes have an undesirable edge curl that can reduce the reliability of circuits fabricated with them. Furthermore, all processes that require etch, photoresists, or plating, are lengthy, process sensitive, and expensive.