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What's the purpose of the ground plane?

I guess it's a pretty basic question and I tried to understand it by reading http://en.wikipedia.org/wiki/Ground_plane#Printed_circuit_boards but I still don't really get it.

The "allowing the designer to ground anything without having to run multiple tracks" part is clear but the others are not so much.

I observed that various PCBs have, but others don't have a ground plane. Also, how usual it is to have a ground plane on one side and a power plane on the other side? What are the benefits? Is it supposed to lessen electrical noise? In what applications is it preferred?

I guess it's a lot of questions but I'm really interested about the answers.

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A ground plane:

  • provides a low-impedance ground connection,

    This is important since "ground" is how different components relate to each other. When ground lifts because of impedance noise margin is reduced and errors can result.

  • acts as a EMI shield,

    If your product needs to undergo any sort of official certification then you can know that half of it is already shielded and you only need worry about the other half.

  • and can act as a heat sink.

    This is not that important for small-signal components, but any sort of power electronics, or even just a voltage regulator for the rest of the board, can benefit from this.

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  • \$\begingroup\$ what certification are you talking about? \$\endgroup\$
    – GoatZero
    Nov 30, 2014 at 23:54
  • \$\begingroup\$ also, if i design a circuit using a switching power supply would i need to design using a ground plane then? (due to the high freq) \$\endgroup\$
    – GoatZero
    Nov 30, 2014 at 23:56
  • \$\begingroup\$ FCC/CRTC/etc. (communications) certification. A ground plane is even more important with a switching power supply, but you may want to split it from the digital ground and have them meet at a single point. \$\endgroup\$ Nov 30, 2014 at 23:58

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