PCB Design
What is a ground plane on a PCB?
A ground plane is a whole layer, or a large poured area, of a circuit board's copper connected to ground. On a schematic every ground symbol is the same point. On a board they're joined by copper, and a ground plane makes that copper so wide and so close to everything that ground really does stay at nearly one voltage: milliohms for steady current, and about a nanohenry for fast current.
A ground trace has resistance
A thin ground trace snaking back to the supply might be 0.1 Ω. A motor's 1 A through it lifts the ground at a chip by 100 mV, and the chip measures everything from there, so its readings are 100 mV off. A plane, as wide as the board, is a few milliohms.
A ground trace has inductance
The same trace has about 50 nH. To a fast current that's a reactance that grows with frequency:
A chip driving a 10 Ω load at 10 MHz through that ground shakes its own ground by about 30% of its swing. That's ground bounce: noise on every signal the chip produces or reads.
Live experiment
Pour the plane
A clock drives a 10 Ω chip, and its current comes home through the ground: a thin trace of 50 nH or a plane of 1 nH. Pick the return and the clock's frequency. At 10 MHz the thin trace shakes the ground by 30% of the clock; the plane by 0.6%.
“Pour the plane” is lesson 3 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.
Return current follows the trace
At high frequencies, a return current doesn't take the shortest path back. It takes the path of least inductance, which is the smallest loop: in a plane, the strip directly under its signal trace. That loop is tiny, about 1 nH, so the ground barely moves, and the trace and its return radiate far less.
That's why many boards past the simplest use four layers, often signals, ground, power and signals, with the ground layer right under the parts.
Making a good plane
- Connect it at both ends. A plane joined only at the chip carries nothing; the current needs it at both ends to use it.
- Stitch with vias. On two-layer boards, vias tie top-side ground pours to the bottom plane so returns can switch layers.
- Decouple into it. A decoupling capacitor's short path to the plane is what lets it feed a chip's fast current.
- Use thermal reliefs on pads in the plane, so they can still be soldered.
When a board has noisy and quiet sections, split grounds are sometimes used, joined at exactly one bridge, under the point where analog and digital meet.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Pour the plane” is lesson 3 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: It's milliohms for steady current and about a nanohenry for fast current. Wide copper keeps the resistance tiny, and a return path right under each trace keeps the inductance tiny.
- Why not “It carries no current”? A ground plane carries all the return currents. It just does so with tiny resistance and inductance.
- Why not “It's further from the parts”? A plane sits right under the parts, not further away. Being close is what keeps return loops small.
- Why not “It's thicker copper”? A plane uses the same copper foil as the traces. Its advantage is its width and its closeness to every trace.
Show the answer
Answer: About 0.3 mΩ: nothing. 2π × 1000 × 50 nH ≈ 0.31 mΩ. Inductance only matters when things change fast.
- Why not “About 0.3 Ω”? Reactance is 2π × f × L. At 1 kHz that's 2π × 1000 × 50 nH, only about 0.3 mΩ.
- Why not “About 3 Ω”? About 3 Ω is 50 nH at 10 MHz. At 1 kHz the frequency is ten thousand times lower, and so is the reactance.
- Why not “About 30 Ω”? An inductor's reactance falls at lower frequencies. 30 Ω would need a far higher frequency, not a lower one.
Show the answer
Answer: Each ground is joined by copper with resistance and inductance, so they differ. The schematic hides the copper. Current through it makes ground at one chip a little different from ground at another.
- Why not “They are the same point, exactly”? On the schematic they're one point. On the board they're joined by copper, which has resistance and inductance.
- Why not “Only the supply's ground matters”? Every ground connection matters. A chip measures its signals from its own ground pin, wherever that sits.
- Why not “They differ only at DC”? They differ at DC because of resistance, and even more at high frequency because of inductance.
“Pour the plane” is lesson 3 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.
Questions people ask
What is a ground plane?
A whole layer, or a large area, of a circuit board's copper connected to ground. It gives every part a very low-resistance, low-inductance path back to the supply, so ground stays at nearly the same voltage everywhere on the board.
Why is a ground plane better than a ground trace?
A thin ground trace might be 0.1 Ω and 50 nH, so currents lift and shake it: 1 A lifts it 100 mV, and at 10 MHz it's about 3 Ω. A plane is a few milliohms, and a fast return current flows right under its signal trace in a loop of about a nanohenry.
Should I put a slot or split in the ground plane?
Avoid running a fast signal across one. Its return current wants to flow directly under the trace; a slot forces it to detour around, enlarging the loop, adding inductance and radiating noise. If you must split grounds, join them at one bridge and route signals over it.
How many layers do I need for a ground plane?
A two-layer board can pour most of its bottom layer as ground. Many boards past the simplest use four layers, often signals, ground, power, signals, so every fast signal has an unbroken ground layer right beneath it.
Related topics
- Star grounding explainedGive each load its own return to one point, so a big current never flows through a sensitive circuit's ground.
- What is a decoupling capacitor?Why every chip has a capacitor beside its power pins, how big, and why it must be close.
- PCB trace width for currentCount the squares: how a trace's width sets its ohms, its voltage drop and its heat, and how wide to draw it.
- Capacitive and inductive reactance explainedWhy a capacitor passes treble and a coil passes bass: X_C, X_L and Ohm's law for AC.