printed circuit(redirected from Printed circuit boards)
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printed circuit,electric circuit in which the conducting paths connecting circuit components are affixed to a flat, insulating base board. The base is typically of plastic, glass, ceramic, or some other dielectricdielectric
, material that does not conduct electricity readily, i.e., an insulator (see insulation). A good dielectric should also have other properties: It must resist breakdown under high voltages; it should not itself draw appreciable power from the circuit; it must have
..... Click the link for more information. , and the conducting paths may be placed on it by a variety of methods. An etched circuit is a subtractive method in which a metallic foil is bonded to the base, the circuit pattern drawn on the foil with an acid-resistant wax, and the remainder of the foil then etched away with acid, leaving the desired conducting pattern. For a multilayered circuit electroplating was frequently used in the past, but today silk screen printing with conducting polymer inks is commonly used. The circuit components themselves—resistors, capacitors, and other devices—are mounted on the finished base afterward, either with their leads being inserted through holes drilled through both the conducting pattern and the base and soldered to the conducting strips or, increasingly, by directly soldering leadless components to the circuit.
See C. F. Coombs, Jr., Printed Circuits Workbook Series (1990).
a subassembly of electric or electronic apparatus that is constructed on a single board in the form of a system of printed electric or electronic components interconnected by printed wiring. Many passive components are made by printing. Such components include resistors and capacitors, inductors and transformers, connectors and switches, and super-high-frequency components for operation at frequencies of 500–2,000 megahertz, such as strip lines, directional couplers, band-pass filters, and attenuators.
Resistors are produced either by applying a resistive ink through a mask to separate strips or areas on the board, with an accuracy of 20–40 percent of the rated resistance value, or by thermovacuum deposition on the board of a thin layer of carbon, metal (tantalum or niobium), metal oxide (tin dioxide), or alloy (nichrome), with an accuracy of 5–10 percent. Capacitors are produced by forming metallized areas on one or both sides of the board. Low capacitance (up to several dozen picofarads) and the high values of the tangent of the dielectric loss angle limit the use of such capacitors. Inductors in the form of single or multiturn spirals are produced by etching on metal-clad boards or by baking-on silver on ceramic boards. The inductance usually does not exceed 7–10 microhenrys (μH), and even in the case of very fine conductors it is not higher than 50 μH. Transformers are produced in a similar manner. To produce connectors with spring contacts, a series of printed strips with a wear-resistant coating of rhodium or platinum, which act as plugs, is formed at the edge of the board.
The contact portion of switches that have a complicated switching system, such as the code disks for digital equipment, is formed in similar fashion. Single-layer or multilayer connecting cables in the form of flat multiconductor systems are produced by etching flexible metal-clad tapes. The size and weight of such cables are substantially less (by a factor of 7–10) than, for example, ordinary radio-frequency cables. The printed components of superhigh-frequency circuits, and sometimes also the passive components of intermediate frequency and audio amplifiers, are produced in a single procedure on a large board (up to 500 × 500 mm) made of a nonpolar dielectric. A printed circuit is usually coated with a moisture-resistant and heat-resistant lacquer, after which it is a finished product.
The passive components of hybrid and thin-film integrated microcircuits are manufactured in essentially the same manner.
The use of printed circuits has substantially increased the circuit density, production efficiency, and reliability of the subassemblies in radio and electronic equipment, such as computers and television and radio receivers; it is the basis for their microminiaturization and integrated miniaturization, especially for large-scale production.
REFERENCEPechatnye skhemy ν priborostroenii, vychislitel’noi tekhnike i avtomatike. Moscow, 1973.
B. P. LIKHOVETSKII
printed circuit[′print·əd ′sər·kət]
A conductive pattern that may or may not include printed components, formed in a predetermined design on the surface of an insulating base in an accurately repeatable manner. Printed circuits are fabricated by any of several graphic art processes. They greatly simplify mass production and increase equipment reliability. Their most important contribution, however, is the tremendous reduction achieved in size and weight of electronic devices and equipment. Printed circuits are used in practically all types of electronic equipment: toys, radio and television sets, telephone systems units, electrical wiring behind automobile dashboards, computers, and industrial control equipment.
The configuration in which electronic circuit elements are located and the routing of conductor paths between the circuit elements establish the precise circuit pattern. Location of the circuit elements can depend on a number of factors, including the form factor (outline of a printed wiring board in a piece of electronic equipment), signal criticality, and the power dissipation of the circuit elements. Conductor path routing is a function of the circuit element location, signal criticality, width and spacing of interconnection conductors, number of wiring channels per layer of interconnect structure, and number of interconnect layers allowed.
As a result of increased circuit complexity, sophisticated computer-aided engineering (CAE) programs have been developed to automate the design of printed circuits. Output from the computer-aided engineering database includes a circuit element parts list and schematic diagrams of the circuit interconnections. This computer-aided engineering database can be used as input to a computer-aided design (CAD) program that optimizes the location of circuit elements within the given form factor and automatically performs the conductor routing between circuit elements. See Computer-aided design and manufacturing, Computer-aided engineering
Artwork masters are used to fabricate the screens and masks for the application of photoresistive materials in the actual formation of the required patterns on the finished parts. The computer-aided design database is also used in the preparation of numerous types of tooling, for example, drill templates, tapes for operation of numerical-tape-controlled drilling equipment, routing templates and dicing fixtures for trimming printed circuits or integrated-circuit dies to final configuration, laminating and holding fixtures, and string lists to drive automated test equipment. Numerous processes, including etching, screening, plating, laminating, vacuum deposition, diffusion, and application of protective coatings, are used in combination to produce various types of printed circuits. Completed printed circuits are inspected visually and dimensionally by using such techniques as microsectioning and infrared photospectrometer measurements in determining thicknesses of critical materials; in addition, they may be x-rayed and electrically tested to assure conformance to requirements.
Printed wiring is undoubtedly the most common type of printed circuit. The printed wiring board (PWB) is a copper-clad dielectric material with conductors etched on the external or internal layers. Printed wiring boards can be subdivided into single-sided, double-sided, and multilayer boards.
Single-sided boards contain all the interconnect structure on one of the external layers and are the least expensive to manufacture. Double-sided boards contain circuitry on both external layers. Plated through-holes and occasionally eyelets are used to provide electrical continuity between the sides. Double-sided boards are used in those applications in which the maximum number of interconnections (conductors) in a given area are required for minimum cost. Both single- and double-sided boards are commonly used in such commercial applications as automotive equipment, radio and television sets, and toys.
Multilayer boards contain circuitry on internal layers throughout the cross section of the board as well as on the external layers. Because of the reduced size of miniaturized microelectronic parts, these boards accommodate the increasing complexity and density of circuitry used in applications such as high-speed computers and signal processors. Multilayer printed wiring boards are manufactured by using two different methods: subtractive (print and etch) technology and additive (plate-up) technology.
Thick-film circuits consist of such passive elements as resistors, capacitors, and inductors deposited on wafers or substrates of such dielectric materials as ceramic, glass, quartz, sapphire, and porcelain-coated metal. They are used for mass fabrication of passive networks for inclusion in linear microcircuits and large-signal digital and analog modules. Thick-film design and manufacture are usually based on film thicknesses of approximately 0.0005–0.0015 in. (12–38 μm).
The deposition of thin films was the first application of printed circuit technology to microelectronics. The most important advantages to thin-film circuits are the following: (1) films with a uniform thickness in the range from 5 × 10-6 mm to 5 × 10-3 mm can be vacuum-deposited and controlled by measuring the resistance across a test pattern during deposition to ensure that final thicknesses are within design limits; (2) patterns formed during deposition or by selective etching afterward are much more precisely controlled than those which are printed, as in thick-film circuits; (3) more stable resistive materials can be used; and (4) thin films have less porous surface metallization, enabling faster rise times. Because of this precision and stability, thin-film circuits are frequently used in radio-frequency applications in avionics and industrial electronics.
A multichip device, often referred to as a hybrid, is a combination of two or more electronic components mounted and interconnected via a substrate. The multichip device serves a customized electronic function and is packaged as a single device.
A multichip device serves the same function as a circuit card assembly; however, all the components are packaged together in a single hermetic case. Unlike printed wiring boards where all components are individually packaged and then mounted to the board, multichip devices may use bare, unpackaged dies. The advantages of multichip devices are the vast reduction in volume, area, and weight; improved thermal management; and increased functional densities, frequencies, and electrical performance. The disadvantage is the increased cost over that of equivalent printed wiring board assemblies. Multichip devices can be digital, analog, or a combination of both.