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Design007-June2022

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34 DESIGN007 MAGAZINE I JUNE 2022 ere are three reasons for using copper pours on multilayer PCBs: 1. PCB fabrication: e primary reason is to provide a uniform copper distribution across a layer to make etching and plat- ing more predictable, and to reduce the amount of etching fluid used (hence cost) during PCB manufacturing. 2. RF and microwave design: Ground pours contain the electromagnetic (EM) radia- tion in a localized area thus reducing spurious coupling, radiation, and dispersion. 3. Digital design: A ground plane improves noise immunity, gives much greater grounding uniformity, and provides a low inductance current path to devices. Please note the distinction between cop- per pours and ground (GND) pours/planes. A copper pour per se is an isolated copper plate. is is not a good idea since it buys you little benefit, whereas a ground plane is connected to the distributed ground net. During the PCB pre-fabrication process, the CAM engineer will generally add copper pours to individual layers of a multilayer PCB. is is referred to as copper thieving and involves placing a pattern of repeated simple geometri- cal shapes or cross-hatched regions into large void areas of a PCB. But this is not ground— it is floating copper. Copper thieving is best placed outside the board outline to avoid influ- encing the functionality of the design. Some designers advocate leaving the copper thieving decisions up to the PCB fabrication shop, but this may not necessarily be the best design decision. While the fab shop will cre- ate thieving that will best fit the plating needs of the board, they will not necessarily have all the tools available to determine the electrical impact of adding metal to the design. Floating (unconnected copper) is a real hazard for sig- nal integrity. All copper pours must be connected to ground or power net. A ground pour that is not accompanied by grounding vias can become a conduit for crosstalk between the traces on either side of the ground shape. e CAM engi- neer could inadvertently be creating antennae that radiate with the application of short wavelength signals. It is crucial that any metal changes to the Copper Pours in High-speed Design Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Unnecessary copper GND pour on outer layers.

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