LIBRARYGround and power planes on a real layout.

On a real board, ground is a filled sheet of copper, a plane that covers a whole layer, rather than a single thin trace. A plane gives every signal's return current a low-inductance path directly beneath it, which is what keeps a board quiet. Pour a ground plane, keep it unbroken, and stitch it with vias.

Why a plane beats a trace

Return current follows the path of least impedance, and at any real speed that path is the copper directly under the signal trace, not a far-off thin wire. A continuous plane offers that path everywhere, so the loop between a signal and its return stays tiny and radiates almost nothing. This is the return-path idea from the fundamentals, now poured in copper (All About Circuits).

Keep the plane whole

A plane only works while it is continuous. Route a trace across the ground layer and you cut a slot in it, and every return current that needed to cross that slot has to detour around the end, making a large loop that radiates and picks up noise. So keep signals off the ground layer, and where one must cross the plane, give its return a way across.

Stitch it with vias

On a two-layer board, ground exists on both sides, joined by stitching vias: plated holes that tie the two ground areas into one. Stitching keeps the whole ground at one potential and shortens the path a return takes to hop layers. Sprinkle stitching vias across the board, and especially wherever a signal changes layers.

Power pours and thermal relief

A power rail can be poured as a plane too, giving it a low-resistance spread to every part that taps it. Where a plane connects to a pad it uses a thermal relief, a few short spokes instead of solid copper, so the pad can still be soldered without the whole plane sinking the heat away from the iron.