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I've been studying the Arduino Uno layout and noticed that there are about 40 vias that connect the GND polygons on the top and bottom. Here's a picture with the vias circled:

enter image description here

Is this a standard practice? I'm a hobbyist and a novice when it comes to PCB design, but I've never seen this before..

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It's called stitching. You want your copper pours (polygons) to be well connected. Any non-connected islands are referred to as "dead copper" and should be removed.

Arduino example is a rather moderate one. On multilayer boards, you can have hundreds of stitching vias.

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It ensures that the copper pour areas can't become unintentional radiators, which would be bad from an EMC point of view.

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    \$\begingroup\$ Curious: How exactly is it the vias prevent this from happening? \$\endgroup\$ – boardbite Sep 13 '12 at 18:21
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Yes, it is standard. A PCB usually has an even number of layers, so if you have a spare layer around, it never hurts to have another ground. I have also seen a ground layer for every 2-3 routing layers to help reduce crosstalk. Another thing I've done in the past with very high speed differential pairs is to have them sandwiched with a ground layer above and below to help reduce interference from any other clocks in the area.

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    \$\begingroup\$ "it never hurts to have another ground" is false. If you are dealing with very small signals, all these additional grounds introduce capacitance which changes the behavior of your circuit. \$\endgroup\$ – Ktc Mar 18 '12 at 4:13
  • \$\begingroup\$ Did you backdrill the vias to eliminate stubs when getting your high-speed pairs to a middle layer? It's often better (if possible) to route such signals on the top layer and avoid any vias at all. If you do have to transition it's better to go all the way through to the bottom layer, or backdrill the vias which is often pretty expensive. \$\endgroup\$ – Chiggs Apr 29 '14 at 14:06

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