PCB Design
Why a narrow neck in a PCB trace gets hot
A neck-down is a short stretch where a wide PCB trace narrows, to squeeze between two pads or past a mounting hole. Every bit of the current flows through it, and the narrow copper has more resistance, so by P = I² × R the neck takes a share of the heat far bigger than its length. Most burnt traces on damaged boards are burnt at a neck.
Same current, more ohms
The wide part and the neck are in series, so the same current flows through both. Heat goes where the resistance is:
Take a trace whose wide parts are 0.01 Ω each and whose neck is 0.1 Ω. At 2 A the neck makes 2² × 0.1 = 0.4 W, while each wide part makes just 0.04 W. A small patch of copper can shed only about a tenth of a watt into the board before the solder mask browns and the copper starts to lift. Widen the neck to 0.01 Ω and it makes 0.04 W: comfortable.
Count the squares
A trace's resistance depends on its shape, not its size: count how many squares fit along it, length ÷ width. Standard 1 oz copper is about 0.5 mΩ per square. A neck 10 mm long and 0.2 mm wide is 50 squares, about 25 mΩ. At 2 A that's 0.1 W in a strip a fifth of a millimetre wide: right at the edge.
Live experiment
Find the hot spot
The 5 V rail feeds a load through a wide trace that squeezes into a short neck between two pads. Pick the neck's resistance and the load current. At 2 A a 0.1 Ω neck makes 0.4 W, four times the 0.1 W it can shed, while the wide part stays cool. Twice the current is four times the heat.
“The narrow spot” is lesson 2 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the one before it first.
Current squared
Heat goes with the square of the current, so a neck that's fine at one current can cook at a little more. Push 3 A instead of 2 through the widened 0.01 Ω neck and the heat goes up 1.5², about 2.2 times, to 0.09 W: nearly its limit. Wide enough for 2 A is only just wide enough for 3.
Rules of thumb for outer-layer 1 oz copper make a 1 A trace roughly 0.3 mm wide, 2 A about 0.8 mm and 3 A about 1.3 mm. A neck is wherever a trace dips below its own rule.
Fixes
Keep power traces at full width all the way, and if a gap is tight, run two narrow traces in parallel or swap to a wider path on another layer with enough vias. Copper pours and thicker 2 oz copper help too. The check is always the same: current squared times the neck's resistance, against what that spot can shed.
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.
“The narrow spot” is lesson 2 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the one before it first.
Or start from the very first lesson, no account neededWork it out
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: Halve its resistance: twice the width. Heat is I² × R. The current is set by the load, so R is the only lever, and width is how you pull it.
- Why not “Add a capacitor”? A capacitor doesn't change the trace's resistance, and the load's steady current still flows through it.
- Why not “Halve its length and width together”? Halving both keeps the same number of squares, so the resistance, and the heat, stay the same.
- Why not “Double the current”? Twice the current makes four times the heat, the opposite of what's wanted.
Show the answer
Answer: In the neck: it has most of the resistance and carries the same current. The same current flows through every part, so heat follows resistance, I² × R, and the narrow neck has the most.
- Why not “In the wide part”? The wide part has fewer squares and less resistance, so less heat for the same current.
- Why not “Spread evenly”? Heat follows resistance, and the neck has two thirds of it, so it takes two thirds of the heat.
- Why not “Nowhere”? Any resistance carrying current makes heat: I² × R.
Show the answer
Answer: About 1.7 A, and the trace takes 1.5 W. 5 ÷ 2.9 is 1.7 A, and 1.7² × 0.5 is 1.5 W in a trace rated for half a watt.
- Why not “4 A”? More resistance can only lower the current. Without the trace it would be about 2 A, not 4.
- Why not “Nothing”? A thin trace still conducts. It just steals volts and turns them into heat.
- Why not “2 A”? The 0.5 Ω trace adds to the 2.4 Ω load, so the current falls to 5 ÷ 2.9, about 1.7 A.
“The narrow spot” is lesson 2 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the one before it first.
Questions people ask
Is a short narrow section in a power trace OK?
It can be, if its heat is small. A short neck has few squares, so little resistance, and the wide copper either side helps carry heat away. Work out I² × R for the neck alone and compare it with what that small patch of copper can shed; at high currents, widen it or add copper around it.
How do I work out a trace's resistance?
Count squares: length divided by width. Each square of standard 1 oz copper, 35 µm thick, is about 0.5 mΩ whatever its size. A 10 mm long, 0.2 mm wide neck is 50 squares, about 25 mΩ.
Can a trace neck be used as a fuse?
Some boards do use a deliberately thin section that burns through on a short circuit, before anything worse fails. It's cheap but imprecise: the current it blows at depends on the copper, the board and the air around it. A real fuse is far more predictable.
Related topics
- 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.
- How to calculate junction temperatureHeat flows like current: watts through °C/W give a temperature rise, so size the copper before it cooks.
- How to calculate voltage drop in a wireA wire is a resistor: how many volts a cable steals, the heat it makes, and why the grid runs at high voltage.
- What is a ground plane on a PCB?A whole layer of copper for ground: milliohms for steady current, about a nanohenry for fast current.