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A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. PCBs can be ''single sided'' (one copper layer), ''double sided'' (two copper layers) or ''multi-layer'' (outer and inner layers). Multi-layer PCBs allow for much higher component density. Conductors on different layers are connected with plated-through holes called vias. Advanced PCBs may contain components - capacitors, resistors or active devices - embedded in the substrate. FR-4 glass epoxy is the primary insulating substrate upon which the vast majority of rigid PCBs are produced. A thin layer of copper foil is laminated to one or both sides of an FR-4 panel. Circuitry interconnections are etched into copper layers to produce printed circuit boards. Complex circuits are produced in multiple layers. Printed circuit boards are used in all but the simplest electronic products. Alternatives to PCBs include wire wrap and point-to-point construction. PCBs require the additional design effort to lay out the circuit, but manufacturing and assembly can be automated. Manufacturing circuits with PCBs is cheaper and faster than with other wiring methods as components are mounted and wired with one single part. Furthermore, operator wiring errors are eliminated. When the board has only copper connections and no embedded components, it is more correctly called a ''printed wiring board'' (''PWB'') or ''etched wiring board''. Although more accurate, the term printed wiring board has fallen into disuse. A PCB populated with electronic components is called a ''printed circuit assembly'' (''PCA''), ''printed circuit board assembly'' or ''PCB assembly'' (''PCBA''). The IPC preferred term for assembled boards is ''circuit card assembly'' (''CCA''),〔IPC-14.38〕 and for assembled backplanes it is ''backplane assemblies''. The term PCB is used informally both for bare and assembled boards. The world market for bare PCBs reached nearly $60 billion in 2012.〔http://www.ipc.org/ContentPage.aspx?pageid=World-PCB-Market-Grew-in-2012 IPC World PCB Production Report 2013〕 ==Design== Initially PCBs were designed manually by creating a photomask on a clear mylar sheet, usually at two or four times the true size. Starting from the schematic diagram the component pin pads were laid out on the mylar and then traces were routed to connect the pads. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout. To fabricate the board, the finished photomask was photolithographically reproduced onto a photoresist coating on the blank copper-clad boards. Nowadays PCBs are designed with dedicated layout software, generally in the following steps:〔http://www.cs.berkeley.edu/~prabal/teaching/cs194-05-s08/cs194-designflow.ppt Printed Circuit Board Design Flow Methodology〕 # Schematic capture through an electronic design automation (''EDA'') tool. # Card dimensions and template are decided based on required circuitry and case of the PCB. # The positions of the components and heat sinks are determined. # Layer stack of the PCB is decided, with one to tens of layers depending on complexity. Ground and power planes are decided. A power plane is the counterpart to a ground plane and behaves as an AC signal ground while providing DC power to the circuits mounted on the PCB. Signal interconnections are traced on signal planes. Signal planes can be on the outer as well as inner layers. For optimal EMI performance high frequency signals are routed in internal layers between power or ground planes.〔(See appendix D of IPC-2251 )〕 # Line impedance is determined using dielectric layer thickness, routing copper thickness and trace-width. Trace separation is also taken into account in case of differential signals. Microstrip, stripline or dual stripline can be used to route signals. # Components are placed. Thermal considerations and geometry are taken into account. Vias and lands are marked. # Signal traces are routed. Electronic design automation tools usually create clearances and connections in power and ground planes automatically. # Gerber files are generated for manufacturing.〔(【引用サイトリンク】title=The Gerber File Format Specification )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Printed circuit board」の詳細全文を読む スポンサード リンク
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