A printed circuit board (PCB) refers to the bare unpopulated board without components. The aim of the electronic manufacturing industry has long been to achieve a reliable printed circuit board design with repeatable electrical characteristics, good mechanical properties and to be of an acceptable appearance. Up until the 1950s, electrical printed circuit boards and systems were assembled using individual wires to connect each of the components. The components were then mounted on what were known as tag strips and sockets.
In response to the needs of the consumer for repeatable performance, smaller sizes and above all lower costs, it was necessary to develop assembly schemes that would allow for greater manufacturing efficiency. One method that proved very successful was the use of printed circuit boards to provide the contact between components. These were made from a laminate of an insulating material and were typically about 1.6 mm thick. One side had a layer of copy foil fixed onto it.
The foil was then selectively removed to leave a pattern that interconnected the components in the desired manner. Holes were then drilled through the laminate material to enable components to be fixed to the non-copper side. The components had flexible leads as their connection points and these were passed through the laminate. Electrical connection was achieved by soldering these to the remaining foil. The foil provided the required electrical connection between the components.
The process met the needs of volume manufacture in that it could be relatively easily automated and created a final product which gave repeatable electrical performance and had sound mechanical strength.
Early printed circuit boards were simple designs comprising a small number of components and limited interconnections. Layout level design took place by manually constructing the artworks (or interconnection patterns) for each layer using tape on transparent sheets. Due to only the one layer of connection available to the printed circuit board designer, no connections could be permitted to corss, otherwise a short circuit would occur. These patterns were then photographed to produce the masks for fabrication. As printed circuit board densities began to increase it was necessary to allow for more and more layers of interconnect to enable the complexity of design. This resulted in a more and more intricate design problem and it became apparent that some degree of automation would be needed to manage the increasing difficulty inherent in the design process.
The design and manufacture of modern electronic equipment has taken these principles to the extreme. There are now up to 32 layers of interconnect possible between any two points on a printed circuit board, and depending on the manufacturing process, even more may be possible. The numbers of layers of interconnect means that the surface area on the printed circuit board can be much reduced and more components fitted onto the smaller area. The type and shape of components being used in the design are also continually reducing and new features are being added. The printed circuit board design process must itself be adaptable and able to keep up to date with these changes. Finally, the level of complexity of printed circuit boards now means that they can only be effectively designed by using computer-based design tools.
