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I have a four layer PCB, with a traditional: signal layer, power layer, ground layer and final signal layer.

Due to the set up with the power rails, there are some large spaces on the power plane layer which do not have any copper on them. I have various options for these spaces; leave them empty, add ground pour or increase size of power planes so that their copper pour fills the space.

The first option is my current plan, but I have got a slight concern about the manufacturing issues this may cause. Most people these days say that there is no need to worry about the imbalance in the PCB causing warping or lamination issues.

The second option seems like it would put copper in, but not really gain anything (so just sit there to balance the PCB stack up).

The final option seems the worst, as I would have to be very careful in which pour goes where to try and keep the correct reference planes for the high speed signals.

Does anyone out there have a conclusive answer for what I should do with these spaces?

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    \$\begingroup\$ The power and ground planes form a high quality capacitor so that makes it interesting to maximize that capacitor. You need to be carefull about EMC of course. \$\endgroup\$
    – le_top
    Sep 10, 2018 at 10:09
  • \$\begingroup\$ @le_top yes adding more copper pour to get the capacitance up is good, as long as I'm careful where I put the plane. But the benefits from the slight increase in capacitance is small compared to the the issues caused by not having the correct reference plane for my high speed signals. I am following your advice though, and increasing power pours as much as seems right. I still have large areas of no copper though. \$\endgroup\$
    – Puffafish
    Sep 10, 2018 at 13:16
  • \$\begingroup\$ For the last option, If you're only adding copper to areas that are now empty, how will that change the reference for any signal? Do you have top layer signals referenced to layer 3 ground? \$\endgroup\$
    – The Photon
    Sep 10, 2018 at 14:16
  • \$\begingroup\$ @ThePhoton signal wires might no longer see the ground plane if the power plane is inserted in between. \$\endgroup\$
    – le_top
    Sep 11, 2018 at 14:53
  • \$\begingroup\$ @Puffafish I'ld want my high speed signals in the layer just next to the ground layer and it would not be the layer next to the power layer. Some manufacturers allow you some control about sheet and filler thicknesses. A filler of 0.1mm gives about 40pF/cm^2 with better frequency caracteristics than lump components (at 0.01mm it is about 400pF/cm^2). \$\endgroup\$
    – le_top
    Sep 11, 2018 at 14:53

2 Answers 2

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Its really a matter of capacitance and what your tying that capacitance to. Interplane capacitance beween signals and a power plane is usually in the pico farad to femto farad range. So if adding a ~1pf capacitor from the signal to the power plane makes a difference then go ahead and fill the plane, usually pF only matter with high speed signals after ~30-50Mhz and when transmission line effects start to take over.

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If you intend to route high speed control impedance traces above the power plane, then do so by pouring in ground in the empty regions. Remember, a power planes effectiveness as a true reference plane is limited by the bandwidth of your decoupling capacitors/network. Nothing beats a true broadband via stitched ground pour.

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