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Is it a good idea to fill free space on Top and Bottom layers in a multilayer (4-layer or more) PCB?

I usually use external layers for signal routing and internal layers for power supplies and ground.

* Top - Signals, High speed signals (if there are any)
* Ground Plane
* Power layer
* Bottom - signals

In the stackup above there's a whole layer dedicated to ground with no traces on it. So, that means there's no need to fill the free space of external layers with copper stitched to ground (using polygon pours) but it's something I tend to do. I also see many manuacturers follow that design (e.g. Raspberry Pi 3 b+). What is the main reason of that? Is it harmfull for the design?

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Yes. It reduces waste since less copper is etched so less waste chemicals are produced and is therefore more environmentally friendly. It's also faster to etch.

In the old days when PCB quality was not as high, boards would warp or twist if the copper was imbalanced on both sides of the PCB. Copper fills prevented this from happening.

Also it makes the PCB look cheap when so much copper is etched away because fiberglass is ugly.

It also helps heatsinking. Don't leave the fills floating. Via stitch them (just one via is good for low frequencies) to the ground plane or else they can reflect and cause RFI issues.

If you have internal signal layers, then the copper fills on the outside serve as shielding as well, as well as serving as a second ground plane on both sides of the internal trace.

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    \$\begingroup\$ "In the old days when PCB quality was not as high, boards would warp or twist if the copper was imbalanced on both sides of the PCB. Copper fills prevented this from happening." This is not necessarily an indication of PCB quality. Anyone, even today and with the proper effort, can cause PCB warping to happen due to imbalanced copper - especially after a reflow or two. \$\endgroup\$ – Chris Knudsen Mar 4 at 21:36
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I also see many manufacturers follow that design (e.g. Raspberry Pi 3 b+). What is the main reason of that? Is it harmful for the design?

Filling planes and space creates extra capacitance. For boards that may be radiators (of RF) this is intentional.

Sometimes it is done for thermal reasons, a larger ground plane can dissipate more heat.

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  • \$\begingroup\$ Removing more copper from a board doesn't "save" copper anyways. If anything, it wastes etchant. \$\endgroup\$ – duskwuff Mar 4 at 23:34
  • \$\begingroup\$ Good point, How dumb can I get? \$\endgroup\$ – laptop2d Mar 5 at 0:56

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