a power plane
A power plane is an entire copper layer of a multilayer board given over to carrying one supply voltage, instead of routing that supply as ordinary traces. Picture the difference between feeding water to a building through one fat shared reservoir on each floor, versus a tangle of thin individual pipes: the reservoir is the plane. Every chip that needs that voltage simply taps straight up (or down) to the plane with a short via, getting its power from a huge, low-resistance copper sheet rather than a long skinny trace.
The power plane almost always works as a pair with a ground plane on an adjacent layer. A standard four-layer stack is signal-ground-power-signal, and that ground-power sandwich does something subtle and valuable: the two big copper sheets, separated by a thin slice of FR-4, form a built-in capacitor spread across the whole board. This plane capacitance, combined with the plane's low inductance, holds the supply voltage steady when chips suddenly demand current — exactly the job decoupling capacitors do, but distributed everywhere at once.
Why it matters: a solid power plane gives every part a stiff, quiet supply with minimal voltage drop and minimal inductance, which is essential for fast digital chips that gulp current in nanosecond bursts. The honest caveat is that a power plane is not a free ground plane — it is referenced to a different voltage, so a signal trace running over a power plane (rather than a ground plane) does not get as clean a return path. And if the plane is split into several voltage regions, a trace crossing the split has its return current interrupted, which causes noise. Plan where signals cross, and pair power with ground.
On a four-layer board the inner ground and power planes sit a few tenths of a millimetre apart in FR-4, forming roughly a hundred picofarads of distributed capacitance that helps hold 3.3 V steady when the processor switches and grabs current.
A whole layer of supply — low drop, low inductance, and a free distributed capacitor with the ground plane.
A power plane complements decoupling capacitors, it does not replace them. The plane handles slower, board-wide supply demands; small local caps right at each chip still handle the fastest current spikes. And never route sensitive signals across a split in the plane.