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Definition: printed circuit board


A rigid, flat board that holds chips and other electronic components. The printed circuit board (PCB) is made of layers, from two to a dozen or more, that interconnect components via copper pathways. The main board in a computer is called the "system board" or "motherboard," while smaller ones that plug into slots on the main board are called "boards" or "cards." See motherboard, expansion card and flexible circuit.

Etched Circuits
The "printed" circuit is an etched circuit. A copper foil is placed over a fiberglass or plastic base of each layer and covered with a photoresist. Light is beamed through a negative image of the circuit paths onto the photoresist, hardening the areas that will remain after etching. When passed through an acid bath, the unhardened areas are washed away, and the finished layers are then glued together. The etching process is also used to create integrated circuits (chips).

Starting in the 1940s
Printed circuit boards (PCBs) were first used in the 1940s to connect discrete components together. By the 1960s, they were widely used in all electronic systems, and as integrated circuits emerged in the 1970s, chips were increasingly mounted on the boards. Today, PCBs hold a few discrete elements but mostly chips, and each chip contains from thousands to billions of transistors (see chip). See surface mount, via and discrete component.




Motherboard and Expansion Cards
This Baby AT board is an example of an early PC motherboard. It accepted both ISA and PCI cards containing peripheral controllers. See ISA and PCI.






Boards Can Be Very Small
This earlier WOW HD board from SRS Labs created a more dynamic music experience. The complex processing that takes place on tiny PCBs such as this was unthinkable in the early days of computing. (Images courtesy of SRS Labs, Inc.)