Copper fill necessary when your board has no GND or Vcc?


I’m in the process of designing a small (3cm x 3cm) PCB into which I want to mount 14 LEDs to create two 7 LED chase patterns for the Ghostbusters PKE Meter replica I’m building.

This video shows a successful test of the main PCB I designed and had fabricated by OSH Park:

The chase of green LEDs near the PCB represent one of the chase patterns. The prop has two, with the mode selected by the DPDT switch visible under the display hood.


Here’s the schematic:

This board would plug into the main PCB via a Molex Picoblade harness I’d solder to the connectors on the bottom. I’ve worked out the trace routes in PCBNew. The LEDs are crammed very closely together, so this was a bit of a challenge. (The 3mm LEDs are bigger than indicated in the above image.)

(BTW, when I run the DRC it complains that I have a bunch of overlapping courtyards, but that’s all. It’s my understanding that this won’t prevent the board from being successfully fabricated).

The question is should I add copper fills to this as this board doesn’t have direct connections to V+ or GND. Pins 2-8 of the connector tie to the outputs of a ULN2803A which gives the outputs of my Teensy 3.2 enough oomph to drive three LEDs (two wing LEDs and a display LED) at once without damaging it. Pin 9 will go to one end of the DPDT switch and Pin 10 will go to the other. The common of the switch passes through an 82 ohm resistor to V+ on the main PCB. An off board LED will be connected to Pin 1 here.

If copper fills are always suggested would it make sense to tie one to the common anodes of the LED chase that connects to pin 9 and the other to tie into the common anodes that tie into pin 10?

Or do I just not use fills/pours in this type of “outboard” application?

Thanks for reading!

Shawn Marshall

Having GND or Vcc is not necessarily a reason to have copper fill either. AFAICT it doesn’t matter either way for your design. Turn the question around, what do you hope fills will do for you?

Thanks for the reply. As you might have guessed, I’m pretty much a self-taught beginner. Almost all of the tutorials I’ve watched or read include copper fills. It was my understanding that fills are common practice that improve the reliability or performance of boards. I don’t remember quite why, or whether fills would have any impact of any kind on a simple, low-power design like this.

Hence, my asking others with more expertise in this area for their thoughts.



Using fill is entirely optional, but consider…

  • A PCB with filled copper, removes less copper, and so etching chemicals last longer.
  • Copper fill provides cooling, usually a ‘good thing’ for LEDs

With SMD leds, you will see quite extensive copper and cooling, especially if you push the light output.

In your case, yes, it would be sensible to make 2 planes, one per anode group, and then route to ‘maximise copper’ of both planes.

You might also want to increase the trace widths, as they do not need to be so fine here.
Also check the physical spacing of the 4 closest LEDs, they may splay given the outlines overlap.

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These are the reasons I know of to use fills:

  1. To reduce supply resistance
  2. To reduce copper etching
  3. To have a ground plane
  4. To have a heatsink

Your design doesn’t look like it puts more than a few mA through the LEDs so not 1. 2 is not your problem. You don’t have high frequencies so not 3. You are not driving a lot of power and they are THT LEDs so not 4. So I would say it’s optional. But it can’t hurt to use a couple of planes because of 1. I don’t know if fabs recover the copper so I don’t know if they prefer to have the copper back or use less etchant.

I would not bother with unconnected fill, but would make your tracks much thicker

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Thanks for all the replies, I appreciate it. There seems to be differing opinion as to whether a copper fill would be of benefit, so I guess I’ll think about it.

I used the PCB calculator to see what width it recommends for .02A (20 mA for an LED), and it says 2.1 mil. I made my tracks/traces 8 mil to give me plenty of overhead, but now I’ve bumped them all up to 12 mils which I think would be plenty for this application.

The 3mm LEDs I bought don’t have a flange, so some might be touching but I don’t think they’ll impinge on each other.



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Modern LEDs glow very bright with a few mA compared to years ago. You may not have to put that much current through it.

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Thanks for the info. The spec sheets for my LEDs specify values for 20mA, so I’ve used that value and my voltages to calculate my resistors. They’ll be nice and bright, which is what I’m going for.



That’s probably the max rating. Worth spending a couple of minutes wiring it up quickly to check the brightness before you solder any resistors.

Tracks do more than just carry the current.
They also provide cooling to the LEDs

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Good to consider, thanks.


Thanks. The LEDs in my video above are the brightness I plan to use with the resistor value calculated based on a voltage about halfway been their typical and max voltages, usually 2.1V, and the 20mA current spec.



Modern LEDs are bright enough for indication at just 1mA.
10mA is a torch.


That’s fine, I want it to stand out at the conventions and events I appear at.



20mA considered to be a flame thrower. :sunglasses:

What can I say? When I fire this bad boy up I want to be the center of attention. Take a gander at the blue power cell lights on my scratch-built Proton Pack:

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  1. To avoid PCB warping in oven (not know - I just read somewhere about it),
  2. To make internal layer the same average thickness everywhere. I also just read somewhere that if half of layer is filled and the next half not then the resulting PCB can be not well glued because press will press effectively only half of it. So even not electrically needed the second half should get the fill too.

I’m vaguely remembering from the dim past that a faster etch produced cleaner edges and reduced undercutting of thin tracks.
So copper fills are good for many reasons.
In your case, you could have top as LED anode, bottom as LED cathode.

Well, I submitted the board without fills to OSH Park last night, but I also started a support ticket with them to see what they recommend in this circumstance. I might as well ask the fabricator directly what they prefer.