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Comments on Signals and plane on the same layer

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Signals and plane on the same layer

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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 the following issue:

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:

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

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

pcb-layout ground-plane

posted about 1 month ago

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2 comment threads

Do you have any PDN ripple specs? Load Transient specs?, Step load current? (1 comment)

Your question is vague and unanswerable without knowing risetime, signal impedance, thus tolerance t... (1 comment)

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My answer from another place: -

It's best to use a 4 layer board looking at the tracks you have on your ground-plane side.

But, if you are really insistent on using only two layers (why?) then do whatever you can to minimize the track lengths on the ground side. I would say from experience in answering this type of question on this site that you have almost certainly (a probability thing) not minimized the track lengths on the ground side so, please do so.

Work on the top layer tracks and do what you can. If necessary go from top to bottom then back to top then, back to bottom and then back to top to keep the ground-plane as uncluttered as possible.

If you are going to group tracks, keep the group small and leave a good amount of ground plane between adjacent track-groups.

posted about 1 month ago

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Why not use a 4-layer board? (5 comments)

Hi Andy, You mentioned: "**If** you are going to group", But this is exactly my question: Shall... (2 comments)

Eyal78‭ wrote about 1 month ago

Thank you all for your responses and for the time you’ve dedicated to this discussion. However, I’ve noticed that many of you are strongly insisting on using a 4-layer board, even though that’s not what I was asking about.

In this post, my question was whether it’s better to route the sneaked-in traces placed on the ground plane layer or to leave more spacing between them. Maybe the title is misleading, but I wasn’t asking whether my board should be 2-layer or not.

In the linked post, I mentioned in a comment (and later asked in a separate post) whether my signals are considered high-speed or not. I would appreciate it if someone could address this comment itself. Additionally, I actually answered the question that seemed to intrigue you.

I’ve posted quite a few questions here recently, and I’d truly appreciate it if someone could focus on addressing my actual questions.

Lundin‭ wrote about 1 month ago

Eyal78‭ The thing is: it's often much more work to route a 2 layer board than a 4 layer board. Particularly when you have a QFP or similar fine pitch IC with lots of pins. Your example is all about that: instead of just grounding what needs to be grounded on a ground later and placing signal traces on one of the layers dedicated to that, you have to spend time and effort creating some manner of hybrid between the two. So the 2 layer board is more complex, more expensive in terms of development time and the same price in raw 1.6mm FR4 PCB costs. So there's not really any arguments in favor. Whereas 4 layer PCB is the most common industry standard (for SMD).

One of the primary duties of an engineer is to question if the requirements given make sense. So we aren't just to blindly answer questions but also point out better ways if they exist.