printed circuit boards perform two functions. They provide mechanical support for electronic devices and connect all the devices through conductive traces. Today, they are nearly universal in electronic devices. It wasn’t always so. Early electronics were hand-soldered by skilled technicians. Some industrial devices utilized wire wrap, a technique that involved wrapping a solid wire around a vertical post with the assistance of a hand tool or power wire wrapping tool. This is still widely used for prototypes. A few devices used stitch-wiring, with solid wires tack-welded to pads on a printed circuit board. Fortunately this method was short-lived as performing repairs or troubleshooting was difficult.
Printed circuit boards (PCB), also called printed wiring boards, or circuit card assemblies, replaced the other processes because they’re less costly to manufacture, and they permit the automated assembly of electronic circuits. They range from simple, “home-brewed” boards to complex, multi-layer versions that require computer aided design and precision assembly methods.
Thomas Edison experimented with conductive traces on linen paper in 1904. Other researchers performed similar experiments in the 1920s and 1930s, some of them resulting in patents. The advent of the Second World War brought a need for mass-produced PCBs for proximity fuses in artillery shells. The process used a ceramic plate with metal traces screen printed onto its surface. Production was kept secret until after the war when the process was offered for commercial use.
The underlying board material is called the substrate. A rigid board may be composed of phenolic, a kind of resin impregnated paper, fiberglass, low-loss plastics for high power radio circuits, or ceramic. Flexible PCBs are available for applications that require bending or the ability to fit into a confined space.
Boards fall into three broad types: single-sided, double-sided, and multi-layer. Single and double-sided have conductive traces on their outer surfaces and use through-holes to mount parts. Multi-layer boards are constructed like a sandwich, with several layers of conductors connected by plated “vias”. Multi-layer construction is often less costly because it is entirely automated.
PCB layout uses silkscreen or photographic printing methods to apply the “artwork” to the substrate. The subtractive process starts with a copper plated board, masks the traces, and etches the remaining copper away. The additive method uses the artwork as a pattern for applying chemicals that will allow copper to bond to the substrate.
