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How to design pcb

Date:August 2012

Printed circuit board design was initially a fully manual process, where an initial schematic diagram was converted into a layout of parts, then traces were routed between package terminals to provide the required interconnections. Pre-printed non-reproducing mylar grids assisted in layout, and rub-on dry transfers of common arrangements of circuit elements (pads, contact fingers, integrated circuit profiles, and so on) helped standardize the layout. Traces between devices were made with self-adhesive tape. The finished layout "artwork" was then photographically reproduced on the resist layers of the blank coated copper-clad boards. Modern practice is less labor intensive since computers can automatically perform many of the layout steps. The general progression for a commercial printed circuit board design would include

    1:Schematic capturethrough an Electronic design automation tool.

    2:Card dimensions and template are decided based on required circuitry and case of the PCB. Determine the fixed components and heat sinks if required.

    3:Deciding stack layers of the PCB. 4 to 12 layers or more depending on design complexity. Ground plane and power plane are decided. Signal planes where signals are routed are in top layer as well as internal layers.

    4:Line impedancedetermination using dielectric layer thickness, routing copper thickness and trace-width. Trace separation also taken into account in case of differential signals. Microstrip, stripline or dual stripline can be used to route signals.

    5:Placement of the components. Thermal considerations and geometry are taken into account. Vias and lands are marked.

    6:Routing the signal trace. For optimal EMI performance high frequency signals are routed in internal layers between power or ground planes as power plane behaves as ground for AC

    7:Gerber filegeneration for manufacturing.

In layout of the board, a power plane is the counterpart to the ground plane and behaves as an AC signal ground, whilst providing DC voltage for powering circuits mounted on the PCB. Where possible it is good to have a power plane for each ground plane on a board (known as a "plane pair"), as this reduces power supply impedance to the components on the board. In electronic design automation (EDA) design tools, power planes (and ground planes) are usually drawn automatically as a negative layer. Adding primitive layout shapes (for example, a donut pad) on such a layer automatically produces a negative of those primitives, placing copper wherever there is no track or via. (Provenance: Wikipedia)



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