PCB Design Basics — Trace Width, Current & Heat

A PCB trace is just a flat wire etched from copper. Like any wire, it has resistance, and that resistance generates heat when current flows through it. Make the trace too narrow and it overheats — potentially damaging the board, delaminating the copper, or starting a fire. Make it too wide and you waste board space. The IPC-2221 standard gives you the math to get it right.

The IPC-2221 formula

The industry-standard formula relates trace width to current capacity and acceptable temperature rise:

Width (mils) = (I / (k × ΔT^0.44))^(1/0.725) / (thickness × 1.378)

Where:

• I = current in amps
• ΔT = temperature rise above ambient in °C
• k = 0.048 for external layers, 0.024 for internal layers
• thickness = copper thickness in ounces (1 oz = 35 µm)

This is not a formula you want to solve by hand. The PCB trace width calculator computes it instantly — enter current, temperature rise, copper weight, and layer type.

Why internal traces need to be wider

Internal layers are sandwiched between FR4 substrate. They can't dissipate heat to the air as effectively as external layers. The k-constant for internal layers (0.024) is half that of external layers (0.048), meaning internal traces need to be roughly twice as wide for the same current.

Common scenarios

1A on external layer, 1 oz copper, 10°C rise

Width ≈ 10 mils (0.25 mm)

This is fine for signal traces and moderate power. Most PCB design tools default to 10 mil trace width.

3A on external layer, 1 oz copper, 10°C rise

Width ≈ 50 mils (1.27 mm)

Getting wider. This is typical for motor drivers, voltage regulators, and power input traces.

5A on external layer, 1 oz copper, 10°C rise

Width ≈ 110 mils (2.8 mm)

Now you need dedicated power traces that are clearly visible on the board. Consider using 2 oz copper to halve the width.

10A — use a copper pour

At 10A with 1 oz copper and 10°C rise, you need about 400 mils (10 mm). At this point, use a copper pour (filled polygon) rather than a trace. Many high-current designs route power through ground planes and copper fills.

Copper thickness matters

PCB manufacturers specify copper in ounces per square foot:

Copper weight	Thickness	Typical use
0.5 oz	17.5 µm	Low-cost, signal-only boards
1 oz	35 µm	Standard — most hobby and commercial PCBs
2 oz	70 µm	Power boards, motor controllers
3 oz	105 µm	High-current industrial boards

Doubling the copper thickness halves the required trace width for the same current. Going from 1 oz to 2 oz copper costs more but dramatically improves current capacity without wider traces.

Temperature rise: how much is acceptable?

The IPC-2221 formula uses temperature rise above ambient, not absolute temperature. Common targets:

ΔT	Use case
10°C	Conservative — suitable for most designs
20°C	Moderate — acceptable for power traces in ventilated enclosures
30°C	Aggressive — only for non-critical power paths with good airflow
40°C+	Not recommended unless you've done thermal analysis

For reference, FR4 substrate has a glass transition temperature (Tg) of 130–170°C. You never want traces approaching that.

Trace resistance and voltage drop

A trace has resistance: R = ρ × L / (W × T)

Where ρ is copper resistivity (1.7 × 10⁻⁸ Ω·m), L is length, W is width, and T is thickness.

For a 100mm long, 10 mil wide, 1 oz copper trace:

• R ≈ 0.49 Ω
• At 1A: voltage drop = 0.49V, power loss = 0.49W

That's significant for a 3.3V supply. For long power traces, wider is better. Use the power calculator to check power dissipation.

Via current capacity

Current also flows through vias (plated holes connecting layers). A standard via (0.3mm drill, 25µm plating) can handle about 0.5A with a 10°C rise. For higher currents, use multiple vias in parallel or larger via sizes.

Rule of thumb: Use 3–4 vias per amp for standard 0.3mm vias.

Design rules summary

Parameter	Signal traces	Power traces (1–3A)	Power traces (3–10A)
Width	6–10 mil	20–50 mil	50–200+ mil
Copper	1 oz	1–2 oz	2–3 oz
ΔT target	N/A	10°C	10–20°C
Via current	N/A	2–3 vias per amp	Copper pour

Comparing traces to wire

Sometimes you need to connect a PCB to external wiring. The wire gauge reference shows AWG wire sizes and their current ratings, so you can match your PCB trace capacity to the appropriate wire gauge.

AWG	Diameter	Current (chassis)
30	0.25mm	0.52A
26	0.40mm	1.3A
22	0.64mm	3A
18	1.02mm	7A

Tools for PCB design

1. Trace width calculator — IPC-2221 calculation for external and internal layers
2. Wire gauge reference — AWG table for off-board connections
3. Power calculator — verify power dissipation in traces

Most PCB EDA tools (KiCad, EasyEDA, Altium) have built-in trace width calculators, but having a quick web tool open while you're designing the schematic helps you plan before you route.