1
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What are the main reasons why one would choose one of the following options:

  • sig
  • (1") space
  • gnd
  • (4") space
  • pwr
  • (1") space
  • sig

comparing with

  • sig
  • (1") space
  • gnd
  • (1") space
  • sig/pwr
  • (1") space
  • sig

I know the impedance would be better controlled and the loopback would be smaller. But what if I want to use layer 3 for signal and local power planes. Can the GND layer be my reference one? Should I even consider that option?

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  • \$\begingroup\$ Are you asking about adding a four inch space versus a one inch space or combining the signal and power on the third layer? Are all the signals/power/ground assumed to be homogenous in the other two dimensions? \$\endgroup\$ – Samuel Jun 24 '15 at 17:25
  • \$\begingroup\$ No sorry. I was thinking about using 4 times bigger spacing between planes than between signal and plane. \$\endgroup\$ – Bip Jun 24 '15 at 17:36
3
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Well hopefully you don't have 1" spacing between your layers because that's a heck of a thick board :) You can use layer 3 for small local power planes and routing but the consequences I can think of would be the following:

  • Traces on layer 4 will reference the reference plane on layer 2 if there is no reference plane below them on layer 3. As you say the distance is greater, you won't be able to get reasonably sized traces and maintain the same impedance you have on layer 1. Well unless the over all thickness is very small, but in a 4 layer board it won't be.
  • Your radiated emissions will increase because your loop area is increased
  • Traces on layer 3 will reference layer 2
  • Places where your traces on layer 4 cross your power planes will result in impedance discontinuities and reflections. Depending on your speeds this may or may not be an issue for you. I'd advise avoiding this, and where possible only cross traces on layer 3 and 4 orthogonally (like a plus sign).
  • Traces from layer 3 and 4 can couple to one another easily so avoid long parallel runs between the layers just as you would on a single layer.
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  • \$\begingroup\$ Yeah sorry for using inches. I am not familiar with metric signs. Thank you, I didn't think of the last one :) \$\endgroup\$ – Bip Jun 24 '15 at 17:33
  • \$\begingroup\$ Great answer, but even high speed signals crossing each other may cause trouble if they reference to the same plane in the same direction. The risk and potential harm is smaller, but with very high-fidelity signals it's always better to never cross other signals without reference in between. Of course, long parallel wires can be a lot harder to quickly recognise and are the devil's work when you suddenly get cross talk, so it's good to mention those first and foremost :-). \$\endgroup\$ – Asmyldof Jun 24 '15 at 17:38

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