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I was looking a raspberry pi zero board, and I think it is four layer pcb, but I dont understand now why the designer used a ground plane on top and bottom layer?

It is assumed the four layer pcb has ground and power internal plane, why external plane added, why not only leave the signal plane without ground planes or I can added a polygon plane no connected.

Sorry if I am wrong. I hope somebody can explain in a short answer pls

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There are a lot of copper pours and only 4 layers on this open Github source design for the Rasp Pi

enter image description here

You can fork the design and make your own changes

All pours are AC grounds when microvias and low ESR Caps are used.

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  • \$\begingroup\$ Oh gosh whats is a AC ground, where can I learn to do this complex design \$\endgroup\$ – asterix May 24 '19 at 21:33
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Two possibilities:

  1. It's not a plane. It's copper pour to fill in areas that have no traces so it's not just blank FR4. Then it's grounded because floating copper can cause noise issues. Less copper to etch saves time and produces less waste chemicals which means faster, cheaper, and more environmentally friendly. Also keeps tension balanced on both sides of the PCB which prevents warping. Don't call it a plane if this is the case because that's wrong.

  2. It's more than 4 layers and the outer planes are used as a shield for the inner layers.

The pictures of the PCB I can find show lots of traces on the outer layers so that means #1.

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  • \$\begingroup\$ thanks for your reply very useful for me, really. Now so copper pour grounded is not like ground loop in a two layer PCB routing ground path.. I hope you can understand that I mean \$\endgroup\$ – asterix May 24 '19 at 1:32
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    \$\begingroup\$ I don't understand what you mean. But ground loops aren't a concern here, nor are ground loops always a problem. There are often multiple paths for ground if you look hard enough. \$\endgroup\$ – DKNguyen May 24 '19 at 1:34
  • \$\begingroup\$ thanks a lot, so ground loop appear when I dont have a ground plane :| \$\endgroup\$ – asterix May 24 '19 at 1:41
  • \$\begingroup\$ Ground loops are only a problem when they are big enough to act as an effective loop antenna or have currents circulating around inside the loop itself. \$\endgroup\$ – DKNguyen May 24 '19 at 1:43
  • \$\begingroup\$ :| okay I mean you know a lot of pcb thanks :) \$\endgroup\$ – asterix May 24 '19 at 1:49

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