Signals and plane on the same layer

[Eyal78]

Hi, I’m working on a PCB layout, which is only about the second board I’ve ever designed. I’d appreciate some help with a few basic things. I’ll split my questions into separate posts.

This board has two layers:

• The top layer is used for signals and power planes.
• The bottom layer (shown in the images) is intended to be a ground plane.

I’ve tried to minimize the number of traces on the ground layer, but in some cases, routing constraints leave no choice.

My question is: When placing traces on this ground plane, is it better to:

1. Keep them as tightly packed as possible, grouping them into a concentrated "island" within the ground pour (Image 1)?
2. Space them out slightly, allowing the ground pour to flow between them, thereby increasing the overall ground area surrounding the traces (Image 2)?

Additionally, a large power polygon runs above these signals on the top layer, meaning the traces are effectively crossing underneath it.
Maybe these images don't demonstrate the issue very well, but I’d love to hear your insights on the best approach anyway.

Image 1

Image 2



Thanks in advanced for any answer,
Eyal

 

[ahsrabrifat]

I go for the second arrangement. Closely packed traces may lead to crosstalk, especially if they carry high-speed signals.

 

[augustinetez]

And where possible, do not let signal traces run in the same direction under power traces.

If they have to run in the same direction, space them away from the power trace (preferably shift the power trace if possible) and if they cross, do it at right angles to minimise crosstalk.

 

[eTech]

A good ground plane will fill all areas of the board including between pads and traces.
There are a lot of areas on the board that did not get filled by the plane. The most likely reason is that the clearances and/or the pads/traces are too big. Keep in mind that pad/traces sizes and clearances as provided with PCB design tools "out-of-the-box" are usually conservative and have a lot of room for adjustment.