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I'm working on a design of a 4-layer PCB. The stackup is like this:

  1. Top: signal
  2. Inner 1: power distribution traces and copper fill (VCC) in some areas
  3. Inner 2: signal
  4. Bottom: ground plane

There is a lot of unused space on the signal layers. I'm tempted to fill this to give the board fab a break. But, should I leave these unconnected? Or connect them to ground? Or should I just not fill these spaces?

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    \$\begingroup\$ Look up "copper thieving pcb", probably already here somewhere. It helps the fab house prevent over etching. That said a lot depends on your layout and circuit requirements. Poorly connected planes are sometimes worse than no planes, and large pour regions can create resonant antenna structure, which is why the thieving uses small geometrical patches. \$\endgroup\$ – isdi Oct 18 '18 at 18:23
  • \$\begingroup\$ electronics.stackexchange.com/questions/85633/… \$\endgroup\$ – Voltage Spike Oct 18 '18 at 18:34
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As a matter of course, I always fill all unused areas with ground - just make sure you scatter vias around so that all the layers are well connected to each other, so that they essentially look like part of the main ground plane. DON'T fill with ground pours but only have them connected to the main ground plane at one point.

This gives you ground plane closer to signals and power in many areas of the board.

Also, as a rule of thumb, I'd put the ground layer next to the VCC layer. But, with most designs, you can get away with a heck of a lot (unless you have high speed digital, RF, or analog signals), and often it's not worth worrying about it too much.

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    \$\begingroup\$ There is a sdram chip on the pcb that has it's signals sandwiched between the vcc and ground layers. I'd thought it was a good idea to keep the generated emi "inside" the pcb, since there is also an audio part on the pcb. This is my only option to do that, changing the VCC layer to be a ground plane to won't work. \$\endgroup\$ – Tim Walther Oct 18 '18 at 22:52
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There is a lot of unused space on the signal layers. I'm tempted to fill this to give the board fab a break. But, should I leave these unconnected? Or connect them to ground? Or should I just not fill these spaces?

Depends on the results you want. High frequency signals will take the path of shortest induction back to the source. If you fill the planes and connect them to ground high frequency signals are more likely to return back to the source or ground. Thus by creating planes it can create a better way to suppress potential EMI.

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