VCC & GND planes in a 2-Layer Board

Discussion in 'General Electronics Chat' started by miguelon, Jun 1, 2014.

  1. miguelon

    Thread Starter New Member

    May 20, 2014
    11
    1
    Hi all,

    this is my first post in this forum, but I've read a long time. Very helpful for newbies like me in the matter. So thanks to the people who make this possible.

    I'm designing a PCB with smd components on a 2-layers board. Nearly all components are in the top layer and some components are throu hole like the pin header to support a LCD, etc.

    The bottom layer has some lines that are imposible to route on the top layer so I use them to route this lines. The rest of the bottom layer is for the GND plane.

    The top layer have a lot of components that need a 3.3V supply so some need a very long line to get the VCC pin. So I thought that I could use a VCC plane to get a simpler design and minimice impedance in the VCC line. I use the poligon tool (in Cadsoft Eagle) to fill all the spaces left by the components in the top layer.

    I don't know if this way is erroneous or not. Can someone give me some tip/advice?. Thanks in advance.

    Sorry for the mistakes in english.

    Greetings from Spain
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    A flooded plane on the top layer between the components and traces is a common way to get a low impedance ground or power connection. You just have to make sure that all the segments of the top plane are connected together and not separated by a trace.

    On a switching regulator board I did I used the top plane for the ground so that the surface mount decoupling capacitors could go directly from the power pins on the top components to the ground plane. Thus there were no vias in series with the capacitors, which would otherwise add resistance and inductance, increasing the decoupling capacitor's impedance.
     
    miguelon likes this.
  3. miguelon

    Thread Starter New Member

    May 20, 2014
    11
    1
    Thanks for your fast answer. This is what i've done for the moment.

    PCB.gif

    I've split the top layer in 2 subplanes, one for 5v and other for 3.3v.

    So, is it valid to use a VCC filled for the top plane and a GND filled for the bottom?. I have always seen one (VCC or GND) or both planes of GND but never with top VCC and bottom GND what made me to doubt if I could do.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,439
    3,360
    Looks ok to me.

    Besides the composite view (both top and bottom) show us two more views with top layer only and bottom layer only.

    Show us layouts x2 or x4 enlargement.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    The PCB manufacturer actually likes planes on both sides since that means they have less copper to remove in the etching process.
     
  6. miguelon

    Thread Starter New Member

    May 20, 2014
    11
    1
    Top Layer.

    Top.gif

    Bottom Layer.

    Bottom.gif

    Thanks for your comments.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    Looks okay. My only suggestion is to place the decoupling capacitors closer to the IC pins (and before the vias connecting to the ground plane).
     
  8. miguelon

    Thread Starter New Member

    May 20, 2014
    11
    1
    I'll try put them closer as you say. Thanks you for your tips.
     
Loading...