Planes!

Community PCB Design
1

Hello,

I know using ground planes is a good idea, but a few questions:

Is it a good idea to have a ground plane on my bottom layer and a V+ plane on my top? It’s advantageous to my design, but I’m not sure if it’s an electrically sound idea.

Is it wrong to engulf my entire board (top and bottom) with a signal-less plane, so that any space that isn’t filled with a trace still has copper?

Thanks!

2

Unconnected copper areas are a bad idea from an EMC point of view. They should be grounded.

3

What if they can’t be connected to ground? I have areas that are surrounded by traces so that a ground connection can’t be made.

4

Remove them. The Pulsonix software I use won’t create them in the first place unless I tell it to.

5

I agree with Leon - remove any copper that can’t be connected to ground. In Protel, the default option when pouring a plane is to remove unconnected copper.

6

What about the V+ plane?

7

Geek 2.0:
What about the V+ plane?

OK, I meant remove any copper that can’t be connected to the approriate net (wheether that’s ground or Vcc or whatever)

8

Okay, cool.

Is the capacitance between the V+ plane on the front and the ground plane on the back something I need to take into account? Seems to me as if it would be like an extra decoupling cap, if anything.

9

Geek 2.0:
Is the capacitance between the V+ plane on the front and the ground plane on the back something I need to take into account? Seems to me as if it would be like an extra decoupling cap, if anything.

For a normal double-sided PCB, this capacitance will be tiny compared to the decoupling capacitors. For very high speed multi-layer boards, it can be beneficial to use a very thin dielectric layer between the ground and power planes, thus increasing the capacitance. This has an advantage at high speeds, as much of the the inductance of a normal capacitor is avoided.
10

Sometimes copper pours can help a board to remain dimensionally stable during processing. “Copper imbalance” can (e.g. large areas where the copper is etched away versus areas dense with copper) may warp and bend during processing because of the uneven expansion and contraction of the copper remaining versus areas where the copper is voided. If the board is large it may actually be beneficial to leave disjoint copper in certain areas to help with balance.

I agree with the concern the others voiced about such copper being an EMC risk and if you can wing it, connecting it to common or earth ground (depending on your design) would be preferred. This is something you’d have to assess for your own design. What signals pass nearby or under/over the area in question? Clocks? High-speed? Analog? Sensitive? Accessible? SIP/SOPs? You might be able to reduce coupling by increasing spacing of such copper away from high-speed or sensitive circuits while still gaining the benefit of mechanical stability.

Check with your board fab regarding thieving patterns…