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About decoupling capacitors on the other side

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  • About decoupling capacitors on the other side

    Hi everyone,

    Usually decoupling capacitors are on the same board side than the component for which they are used.
    Sometimes for space issues, we want to place them on the other side of the board.

    Since the capacitor and the component are not on the same side of the board, we obviously need one via between both layers.
    We have 2 options as shown on the attached screenshots.
    I think that both configurations are not good.
    They are not because, if we assume having a power plane in internal layer, the current flows directly from the via to the component.
    It acts as if the capacitor were not present.

    When we have the capacitor on the same side than the component, the capacitor is located between the via and the component.
    In that case, the component is supplied with a current that first flows into the capacitor.

    Would it mean that placing decoupling capacitors on the other side is always worse than having them on the same side ?

    Thanks for ideas !

  • #2
    What is the stack-up?
    Often, power, gnd or both come from another layer. Also, 3V3 is on one end and gnd on the other end of the component.
    Both traces and via's have inductance (as do planes, but they are a lot smaller due to their width.)
    Alternatively, if the power plane is close to the gnd plane, you do not need the decoupling capacitors close to the component. (<=0.2mm or 8mil)

    But why not place the capacitor directly underneath the component like the attached picture?
    Click image for larger version

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    • #3
      The stack-up is:

      Signal
      GND Plane
      Signal
      Power Plane
      Power Plane
      Signal
      GND Plane
      Signal

      In your picture, the link to the power supply directly comes the power plane and not from the capacitor.
      I think there is no other solution.

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      • #4
        Once the cap is on the other side, from the point you are asking about, I don't think it is really important where exactly the via is. We just would like to keep both tracks as short as possible. At least that is how I would do it.

        However, when connecting positive and negative pads of decoupling caps, often we may want to have these two vias placed close to each other.

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        • #5
          "...power supply directly comes the power plane and not from the capacitor" - it all depends on the impedance.
          Do you need two power planes in the middle?
          Decoupling...
          https://www.youtube.com/watch?v=5Ca0Eah7eKI
          https://www.youtube.com/watch?v=XumNc480qYo
          And even the latest is good: https://www.youtube.com/watch?v=WdlN8bHw-w0

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