More about Printed Circuit Board Assemblies (PCB)

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Anyone who has ever opened up a piece of electronic equipment has seen a printed circuit board, or PCB. They are thin, flat, and often green rectangular chips that are covered with a maze of fine copper lines and silver buttons. They look like something right out of the movie Tron. They are also the heart and soul of most electronic equipment.

Printed circuit boards replaced point-to-point construction in most electronics during the 1950s. Essentially, point-to-point construction used wires soldered to “terminal strips,” which are boards stamped with metal (usually tin-plated copper) loops. In a point-to-point operated device, small electronic components and their wires were directly soldered to the terminals, as were wires from the larger parts, such as transformers. As you might imagine, this system involved a confusing tangle of wires. It was also very difficult to use in mass-production, since each wire and each part had to be looped and soldered to the correct part on the terminal strip. Enter the PCB, which did away with almost all of the wiring used in point-to-point construction and facilitated mass production.

In some ways, a printed circuit board is a highly organized collection of wires. Only instead of wires, a PCB uses extremely thin copper tracks to conduct current to the proper components. These thin copper tracks are threaded across a substrate, which is usually either a thin sheet of phenolic resin-infused paper, or an epoxy resin-infused fiberglass mat. A single printed circuit board can hold up to sixteen layers of these conductive sheets glued together. In a multi-layer PCB, holes known as vias connect the various layers. Component leads are attached to holes in the board using solder. The small silver dots you see on a PCB are typically solder points that link through-hole components on the other side.

Printed circuit board assemblies are relatively inexpensive to manufacture, especially in long production runs. One interesting note is that the copper tracks you see on a board are the result of etching. They are typically not attached as thin strips. Instead, the board is covered with a thin layer of copper and the proper patterns the result of excess copper being scraped away. As you would expect, the most difficult and costly part of the PCB is not its manufacture, but its design. A number of factors play into the maze-work design operation that is PCB engineering. Components must be properly mapped, copper to board ratios kept even to reduce waste and prevent warping, distances between tracks and components placed so that current does not jump, and track widths must accord with the frequency of the passing current. In other words, PCB engineering is a highly specialized craft, and layout is often the most expensive aspect of PCB production.