Post History

<blockquote>For power polygon that primarily carry DC, how critical is it that it is located above a ground plane?</blockquote>

Practically not at all.

Power planes are overrated.  Think about what problem you are really trying to solve.  If you have high power currents, then you need to use wide and/or thick traces.  In some cases, you might even add wires to carry some of the current.  I've seen boards where the solder mask was deliberately left off a long power trace so that a wire could be soldered over it in production.  I don't recommend that unless it's either a one-off, or you have access to very cheap labor.

In some exceptionally high frequency cases, having a power plane can be useful to get a high-quality distributed capacitance with the ground plane.  In such cases, you put the power and ground planes on the two most closely-spaced layers.

In most ordinary cases (no exceptionally high currents or high frequencies), power planes are silly.  A good ground plane with small and few interruptions is much more important.  If you properly bypass the power at each point of use with a 100 nF to 1 &micro;F SMD ceramic cap, then the power leads can have some impedance at high frequencies.  The high frequency impedance is then made low at each point of use by the bypass caps.

Usually, <b>with a good ground plane and proper power bypassing, you don't need power planes.</b>