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As they say a picture says a thousand words. Consider this extreme example:

Fragmented ground plane example

You have this copper polygon (a ground plane), but due to other signals routed on the same layer, the polygon comes to a choke point (the "strait" between Pad2&3).

I assume this makes it a suboptimal ground plane (comment if I'm wrong).

Is there a tool in Eagle to identify such problematic choke points? (I'm using Eagle 5.11).

For the example it is trivial to see it visually, but the actual board I'm worrying about is much more complicated and it's hard to identify anything by just looking at it. The tools tells me it is bad though, because I had to stitch the GND planes (I have one on both top and bottom, it is a 2-layer PCB) with dozens of vias to avoid empty areas with no copper. In terms of the frequencies involved, the PCB is a mostly digital device that has a few micros, some at 16 MHz, one at 200 MHz.

One way to do it that comes to mind is to temporarily increase the isolation parameter on the ground polygons, and recompute the fill - it would separate on the choke points and probably airwires would emerge, but I don't think this is a particularly elegant way.

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    \$\begingroup\$ HF signals want to return on the ground plane underneath the trace from which it came as that makes for the smallest loop surface. So it's not so much about choke points as it is about making sure every signal can return without detours on the ground layer. \$\endgroup\$ – Unimportant Jan 11 '19 at 8:44
  • \$\begingroup\$ Generally it is not recommended to rely on the copper fill to finish the routing. Explicitly routing provides less of this uncertainty. \$\endgroup\$ – Jeroen3 Jan 11 '19 at 9:34
  • \$\begingroup\$ The tool I use is optical and i have two of them built into my face. \$\endgroup\$ – Andy aka Jan 11 '19 at 13:10
  • \$\begingroup\$ @Andyaka, I believe you're joking, but if you aren't, let me disagree - you can extend the same line of argument about other tools (who needs DRC, when you can visually inspect the clearances?). Yet you have them and they are absolutely crucial. \$\endgroup\$ – anrieff Jan 11 '19 at 13:54
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    \$\begingroup\$ @anrieff Andy's not joking. We do have DRC for line widths, line separations etc, so we don't need to use eyes for those. However, we do not have DRC for the thing you're asking, which is 'where must my lace curtain be stitched together so it's good enough as a ground plane'. It needs an engineer's thought, two eyes plus a brain, plus some experience. Too not enough experience, you don't know whether the ground is OK. With enough experience, you know not to do it this way, you start with a good ground, and refrain from trashing it as you lay other things out. \$\endgroup\$ – Neil_UK Jan 11 '19 at 14:50
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If your PC has a virus, it's best to nuke from orbit and rebuild it clean. Messing around looking for this and that, and hoping you've got everything, just isn't worth it.

Similarly, if you've turned your ground into a lace curtain by routing tracks through it, stop. Stop. Take a step back, several in fact.

If you're going to use a ground plane, keep it whole. Route nothing through it.

You may need to add an extra layer to your stackup. Have you used power planes? They are almost always a waste of time and real estate. Ditch the power planes, and use those layers to move connections out of your ground plane.

If you have too few layers to dedicate one to ground, then don't use a ground plane. Use a ground grid on two layers, Manhattan style. This gives you a consistent way to get 'very good' ground and tracks systematically from anywhere to anywhere without cutting things up.

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  • \$\begingroup\$ A clarification: your suggestion is to use a grid, composed of horizontal lines on e.g. bottom layer, and vertical lines on the top layer? Or a full grid on a single layer? \$\endgroup\$ – anrieff Jan 11 '19 at 14:02
  • \$\begingroup\$ @anrieff I said "use a ground grid on TWO layers". "Manhattan style" (with respect to a two layer grid) means horizontal on one layer, vertical on the other, so that everywhere they cross you have an opportunity to via them together. This allows any other connection to be made, via'ing between layers as required, without disturbing the ground connectivity. \$\endgroup\$ – Neil_UK Jan 11 '19 at 14:46

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