Electronics > Beginners

Top GND Fill on 4 Layer PCB, Yey or Ney?

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ssashton:
Quick question, I hope!

I'm laying out a switching DC-DC converter on a 4-layer PCB. There is a solid GND plane on the 1st inner layer. Should I also stitch a GND fill on the top layer?

I've read (somewhere) that it is not good practice to connect components directly to a top GND fill if you have an inner plane. So each component has a via directly to the main GND plane and not connected to the top fill. However I'm uncertain if it's a good idea to add GND fill on the top layer around the outside of the converter to 'catch' EMI?

In my image I have highlighted the GND nets. The grey colour is the inner GND plane.

Thank you!

Siwastaja:
Properly stitched top fill - best
No top fill - OK
Unstitched top with long antennae between tracks etc. - worst

If feeling lazy, better leave unfilled. I usually do fill the top, it doesn't take long to add stitching and selectively remove areas where you can't fit the vias, such as narrow passages (adjusting the pour parameters takes care of most narrow fingers, selectively add a few pour cutouts and job done).

What is shown in the image is good.

ssashton:
I did the 3rd one on my last prototype. Top fill and only connected to a single GND point  :--

So now I'm a bit nervous.

spostma:
Use only one ground plane!
Explanation in attachment...

Years ago, I got an excellent EMC expert advice session at the DARE! CE testing by  senior test engineer there.
They also design multi-GHz test equipment themselves, and really know their part in HF and CE compliant design.
I made notes for myself of this valuable session, and attached it FYI.

I hope this helps you pass the CE testing!

ssashton:
Thanks for your replies!

I was using large power fills but after reading the notes from Spostma I have changed to using only traces.

Attached is my current design. What do you guys think?

As before, the grey layer you can see under the audio line level parts is the main internal ground plane that is across the entire PCB.

You can see that I did not put any  top or bottom layer ground fill around the line level audio parts, that is mainly because there is not very much space for via stitching. So I thought it's best to not fill that area.

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