Design of PCB

  tmliir   Jun 25, 2010   PCB   0 Comment

Design of PCB

Printed circuit board artwork generation was initially a fully manual process done on clear mylar sheets at a scale of usually 2 or

4 times the desired size. The schematic diagram was first converted into a layout of components pin pads, then traces were

routed 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 capture through 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. 1 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 impedance determination 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 traces. For optimal EMI performance high frequency signals are routed in internal layers between power

or round planes as power planes behave as ground for AC.
7. Gerber file generation for manufacturing.

In the design of the PCB artwork, a power plane is the counterpart to the ground plane and behaves as an AC signal ground,

while providing DC voltage for powering circuits mounted on the PCB. In electronic design automation (EDA) design tools,

power planes (and ground planes) are usually drawn automatically as a negative layer, with clearances or connections to the

plane created automatically.

(from Wikipedia)

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