Lesson 6.2 · Module 6: From Schematic to Real PCB Hardware

PCB Trace Width, Current and Voltage Drop

Copper tracks on a circuit board are not perfect wires. Too thin, and they heat up and waste voltage.

The live, interactive simulation for this lesson opens here when JavaScript is on.

Key ideas

A PCB trace is a thin, flat strip of copper with a small resistance.
Thin traces carrying big currents get hot and drop voltage along their length.
Every trace also has a tiny inductance, which causes voltage spikes when the current changes very fast.

Try this in the simulation

Increase the trace current and watch the temperature rise.
Make the trace wider until it stays cool and the voltage drop is small.
Switch to the SMD Footprint view to see the real width.

How wide should a PCB trace be?

A copper track on a circuit board is a thin strip of metal with a small resistance. Carry too much current through a narrow trace and it heats up and drops voltage along its length. The IPC-2221 rule of thumb links the current, the allowed temperature rise and the copper thickness to the width you need. Every trace also has a little inductance, which matters when the current changes very fast.

What you’ll learn

Why a trace has resistance, and how its width, length and thickness set it.
How to find the width needed for a given current with the IPC-2221 rule.
The difference between 1 oz and 2 oz copper.
Why fast current changes make voltage spikes on long traces.

The formula, letter by letter

\begin{aligned}\htmlData{sym=I}{I} &= \htmlData{sym=k:ipc}{k}\,\htmlData{sym=Delta}{\Delta }\htmlData{sym=T}{T}^{0.44} \htmlData{sym=A:area}{A}^{0.725}\;\Rightarrow\; \htmlData{sym=Delta}{\Delta }\htmlData{sym=T}{T} = \htmlData{funit=°C,v=53.198793375783936}{\text{53.2 °C}}\;\;(\htmlData{sym=k:ipc}{k}=0.048)\\ \htmlClass{mv mv-v_drop}{\htmlStyle{font-size:1.00em}{\htmlData{sym=U_drop}{U_{\mathrm{drop}}}}} &= \htmlData{sym=I}{I} \htmlData{sym=R}{R} + \htmlData{sym=L}{L}\frac{\htmlData{sym=d}{\mathrm{d}}\htmlData{sym=i}{i}}{\htmlData{sym=d}{\mathrm{d}}\htmlData{sym=t}{t}} = \htmlData{funit=mV,v=0.18271474614146532}{\text{183 mV}} + 0\;\;(\htmlData{sym=w_min10°C}{w_{\mathrm{min,10°C}}} = \htmlData{funit=mm,v=0.0013788508548925142}{\text{1.38 mm}})\end{aligned}

Shown with this lesson’s starting values. In the simulation the numbers change as you move the sliders.

$I$: Current (A · ampere) : How much charge flows each second.
$k$: IPC-2221 constant k (no unit) : A number from the PCB standard: 0.048 for outer layers, 0.024 for inner ones.
$\Delta$: Change in Δ (delta) (no unit) : Δ in front of a letter means "the change in": ΔT is a temperature rise and ΔI_L is how much a coil current swings up and down. It has the unit of the letter after it.
$T$: Temperature (°C · degree Celsius) : How hot something is.
$A$: Cross-section area (mm²) : How much copper the current can flow through: width × thickness.
$U_{\mathrm{drop}}$: Voltage lost along the trace (V · volt)
$R$: Resistance (Ω · ohm) : How strongly something opposes current.
$L$: Inductance (H · henry) : How strongly a coil resists a change in current.
$\mathrm{d}$: A tiny change in (no unit) : d in front of a letter means a tiny change in it. dV/dt is how fast V changes per second; dI/dt is how fast current changes (A/s).
$i$: Current (while it changes) (A · ampere) : A lower-case i is a current that changes over time.
$t$: Time (s · second) : Time since something started.
$w_{\mathrm{min,10°C}}$: Narrowest trace for a 10 °C rise (mm)

Common questions

How do I calculate PCB trace width?

Use the IPC-2221 formula or a calculator. Give it the current, the copper thickness and the temperature rise you accept (often 10 °C), and it returns the minimum width.

What is 1 oz copper?

The standard copper thickness on a circuit board: one ounce of copper spread over one square foot, about 35 µm thick. 2 oz copper is twice as thick and carries more current in the same width.

Does trace length matter?

Yes. Resistance and inductance grow with length, so a long trace drops more voltage and picks up more spikes than a short one of the same width.

Keep going

Related components: Resistor, Inductor

Words used in this lesson

All lessons · How components work · Glossary