printed circuit board

[′print·əd ′sər·kət ‚bȯrd] (electronics) A flat board whose front contains slots for integrated circuit chips and connections for a variety of electronic components, and whose back is printed with electrically conductive pathways between the components. Also known as circuit board.

Printed Circuit Board

a board whose surfaces have printed current conductors with contact areas, which are used to connect components mounted on the boards according to the circuit diagram of a functional subassembly for electric or radio apparatus, and also have plated circuit holes and nonplated mounting holes. There are about 200 methods for making printed circuit boards; among the most important are the photochemical, photoelectrochemical, and offset-electrochemical methods. The methods differ in the means of producing the conductive coating or the form in which the pattern of the printed conductors is realized.

The dimensions of printed circuit boards, the position of the hole centers, and the hole diameters are standardized. The boards are 10–360 mm long, 10–240 mm wide, and 0.05–3.0 mm thick (depending on the rigidity required). The spacing of the coordinate grid for marking the holes is 2.5 mm or, less frequently, 1.25 mm, and the hole diameters range from 0.2 to 3 mm. A distinction is made among one-sided, two-sided, and multilayer boards (up to 15 layers). Multilayer boards are made by using extended leads, by metallizing the walls of through holes, by molding two-sided boards in pairs, or by layer-by-layer buildup. The material most often used for printed circuit boards are metal-clad and plain Micarta and fiberglass laminates, reinforced fluorine plastic, polystyrene, polyethylene terephthalate, polyamide, polycarbonate, and radioceramics.

REFERENCE

Konstruirovanie i tekhnologiia pechatnykh plat. Moscow, 1973.

B. P. LIKHOVETSKII

printed circuit board

(hardware)(PCB) A thin board to which electronic componentsare fixed by solder. Component leads and integrated circuitpins may pass through holes ("vias") in the board or they maybe surface mounted, in which case no holes are required(though they may still be used to connect different layers).

The simplest kind of PCB has components and wires on one sideand interconnections (the printed circuit) on the other. PCBsmay have components mounted on both sides and may have manyinternal layers, allowing more connections to fit in the sameboard area. Boards with internal conductor layers usuallyhave "plated-through holes" to improve the electricalconnection to the internal layers.

The connections are metal strips (usually copper). Thepattern of connections is often produced using photo-resistand acid etching. Boards, especially those for high frequencycircuits such as modern microprocessors, usually have one ormore "ground planes" and "power planes" which are largeareas of copper for greater current carrying ability.

A computer or other electronic system might be built fromseveral PCBs, e.g. processor, memory, graphics controller, diskcontroller etc. These boards might all plug into amotherboard or backplane or be connected by a ribbon cable.

printed circuit board

A rigid, flat board that holds chips and other electronic components. The board is made of layers, typically two to 10, that interconnect components via copper pathways. The main printed circuit board (PCB) in a system is called a "system board" or "motherboard," while smaller ones that plug into the slots in the main board are called "boards" or "cards." See 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. The finished layers are then glued together. A similar process creates the microminiaturized circuits on a chip (see chip).

Starting in the 1940s
Printed circuits were first used in the 1940s to connect discrete components together. By the 1960s, PCBs were widely used in all electronic systems, but still mostly connecting discrete components. Integrated circuits (chip) were emerging and added to the boards, and by the 1980s, PCBs were holding large quantities of chips. Today, printed circuit boards typically connect mostly chips with only a few discrete components, each chip containing from a few thousand up to hundreds of millions of transistors. See surface mount, via, discrete component, chip, card and motherboard.

Motherboard and Expansion Cards

This Baby AT board is an example of an early PC motherboard that used an ISA bus. It accepted both ISA and PCI cards containing peripheral controllers.

They Can Be Very Small

This WOW HD board from SRS, which has an audio jack for headphones on the end of the cable, creates a more dynamic audio experience for music listeners. The complex processing that takes place on this tiny board was unthinkable in the early days of computing. (Images courtesy of SRS Labs, Inc., www.srslabs.com)