Printed circuit board: Difference between revisions
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A '''printed circuit board''' or '''PCB''' interconnects [[electronics}electronic]] components. The simplest PCB is a layer of copper foil glued to a sheet of plastic. The unneded foil is removed, and components are attached to the remaining foil. |
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'''Printed circuits''' are a method of interconnecting [[electronics]] components without discrete wires. Printed circuits were first used commercially in the 1950s. Printed circuit technology uses [[photolithography]] or mechanical deposition to attach conductive layers to an insulating substrate. A printed circuit is also called a printed circuit board. Printed circuits can be multi-layer, meaning that multiple coplanar layers of interconnecting wires are laminated together. Layers may be connected together through electroplated or mechanically inserted interconnects called "vias". Single-layer boards are least expensive and are commonly employed in low-cost consumer products. Multi-layer boards are needed for complex digital circuits such as computers. Ground and power planes are rectangular sheets of conductor that occupy entire layers to provide better power distribution in multilayer circuit boards as well as thermal distribution of heat generated by the components on the board. Components leads are inserted in "holes" or mounted on the surface "pads" and electrically and mechanically fixed to the board with a metal solder. A solder mask keeps islands of solder from running together and protects conductor layers from abrasion. A [[silkscreen]] legend on the top or bottom surface of the board provides readable information about component part numbers and placement that aids in manufacturing and repair. |
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Practical PCBs have between one and sixteen conductive layers. In more complex PCBs, two of the layers are dedicated to providing ground and power. One, two, four and six layers are favored compromises. |
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'''Printed Circuit Manufacturing Guides''' |
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The usual method of forming a layer is to expose photoresist on the copper, and then etch away the uneeded copper chemically. The photomask is usually prepared with a [[photoplotter]] from data produced by a skilled technician who uses a specialized computer program called a "PCB design program." |
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* http://www.ee.washington.edu/circuit_archive/text/design.html |
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* http://www.precisioncircuits.com.au/cid/hm_cid.html |
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* http://amscourseware.com/guidelines.htm |
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* http://www.filtranmicro.com/design.html |
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* http://www.goldengategraphics.com/pcgloss.htm |
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* http://www.analog.com/publications/magazines/Dialogue/archives/30-2/bibliography.html |
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* http://www.xilinx.com/products/virtex/handbook/ug002_ch4_pcb.pdf |
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* http://www.elchempub.com/files/electroc2.htm |
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* http://www.pcbprotech.com/Dh3/DH3right.htm |
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PCBs are rugged and inexpensive, however they are somewhat less reliable and harder to repair than [[wire wrap]] boards. They require much more design than [[point-to-point construction|point-to-point]] equipment. |
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* [[Printed_Circuits/Conductor Current Capacity|Conductor Current Capacity]] |
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Usually an electrical engineer designs the circuit, and the technician designs the PCB. I trequires specialized skills to design a PCB that is easy to manufacture, with the parts all spaced according to certain minimum distances, and yet keep the PCB as small and as inexpensive as possible. |
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Layers are interconnected with plated-through holes. Basically, a hole is drilled, a thin conductive layer is placed, then copper is electroplated onto that layer. Good designers minimize the number of holes to keep the cost of drilling down. |
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On older, two-layer PCBs, it was common to solder a wire through the hole. |
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Originally, every electronic component had wires coming out, and the PCB had holes drilled for each wire of each component. |
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However, the wires and holes are wasteful. The holes have cost money to be drilled, and the wires are merely cut off. |
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In the 1990s, a new technique called "surface mount" was developed. Components were mechanically redesigned to have small metal tabs or pads that could be directly soldered to the PCB. An automated machine reoved the parts from reels, and sticks them to the PCB. A silk-screened application of sticky soldering flux holds the parts. They are soldered to the PCB by passing them through a ripple, or "wave" of molten solder. After that, they are cooled at a controlled rate by passing them under heat-lamps. The parts and pads of the PCB are designed so that the surface-tension of the molten solder centers the parts on their copper pads. |
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The result was components that were one quarter to one tenth of the size and weight. and half to a quarter of the cost of wire-mounted parts. |
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See also [[electronics]], [[wire wrap]]. [[point-to-point construction]]. |
Revision as of 03:57, 30 July 2002
A printed circuit board or PCB interconnects [[electronics}electronic]] components. The simplest PCB is a layer of copper foil glued to a sheet of plastic. The unneded foil is removed, and components are attached to the remaining foil.
Practical PCBs have between one and sixteen conductive layers. In more complex PCBs, two of the layers are dedicated to providing ground and power. One, two, four and six layers are favored compromises.
The usual method of forming a layer is to expose photoresist on the copper, and then etch away the uneeded copper chemically. The photomask is usually prepared with a photoplotter from data produced by a skilled technician who uses a specialized computer program called a "PCB design program."
PCBs are rugged and inexpensive, however they are somewhat less reliable and harder to repair than wire wrap boards. They require much more design than point-to-point equipment.
Usually an electrical engineer designs the circuit, and the technician designs the PCB. I trequires specialized skills to design a PCB that is easy to manufacture, with the parts all spaced according to certain minimum distances, and yet keep the PCB as small and as inexpensive as possible.
Layers are interconnected with plated-through holes. Basically, a hole is drilled, a thin conductive layer is placed, then copper is electroplated onto that layer. Good designers minimize the number of holes to keep the cost of drilling down.
On older, two-layer PCBs, it was common to solder a wire through the hole.
Originally, every electronic component had wires coming out, and the PCB had holes drilled for each wire of each component.
However, the wires and holes are wasteful. The holes have cost money to be drilled, and the wires are merely cut off.
In the 1990s, a new technique called "surface mount" was developed. Components were mechanically redesigned to have small metal tabs or pads that could be directly soldered to the PCB. An automated machine reoved the parts from reels, and sticks them to the PCB. A silk-screened application of sticky soldering flux holds the parts. They are soldered to the PCB by passing them through a ripple, or "wave" of molten solder. After that, they are cooled at a controlled rate by passing them under heat-lamps. The parts and pads of the PCB are designed so that the surface-tension of the molten solder centers the parts on their copper pads.
The result was components that were one quarter to one tenth of the size and weight. and half to a quarter of the cost of wire-mounted parts.
See also electronics, wire wrap. point-to-point construction.