PCB Design
How many vias do you need for a high current?
A via is a drilled, copper-plated hole that carries a trace from one layer of a PCB to another. Its plating is thin, so it has a small resistance and only a little copper to shed heat, and a big current through one via can overheat and crack it. The fix is several vias in parallel: they share the current, and since heat goes as current squared, each one runs far cooler.
One via's budget
Treat each via as a small resistor with a heat limit. Here each is 20 mΩ and can shed about 0.1 W before its barrel is at risk. The most current one can carry is where I² × R reaches that:
Real vias vary a lot with drill, plating and board thickness, so take your own via's numbers from a calculator; the method is the same.
Share the current
Four amps through one via makes 4² × 0.02 = 0.32 W, three times its budget. Add a second via joining the same two traces and each carries 2 A:
Splitting the current two ways cut each via's heat four times. With n vias, each makes 1 ÷ n² of the heat, and the total falls n times.
Live experiment
Give each load enough vias
The current drops from the top layer to the bottom through one to four vias, each 20 mΩ and able to shed 0.1 W. Pick the 4 A or the 6 A load and add vias. One via at 4 A makes 0.32 W and its barrel cracks; two share it at 0.08 W each. 6 A needs three.
“Through the board” is lesson 6 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 5 before it first.
How many?
Divide the current by one via's budget and round up, then add one for luck:
Three vias carry 2 A each at 0.08 W. Two would carry 3 A each at 0.18 W, nearly double the budget.
Placing them
Group power vias close together, right where the current enters and leaves each layer, so the paths through them are equal and they share evenly. Under hot chips, grids of thermal vias carry heat rather than current to copper on the far side, and rows of stitching vias tie ground layers together around a board's edge.
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.
“Through the board” is lesson 6 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 5 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: Heat goes as current squared, so splitting the current n ways cuts each via's heat n² times. Each via's heat drops by n², and there are n of them, so the total drops n times and no single via gets hot.
- Why not “Vias are cheap”? Cost isn't the reason. The heat per via drops much faster than the number of vias rises.
- Why not “It isn't”? It is: halving each via's current quarters its heat, because heat goes as current squared.
- Why not “They are shorter”? Each via is the same length as before. It's the current through each one that changes.
Show the answer
Answer: 0.18 W each: too much for 0.1 W vias. 9 × 0.02 is 0.18 W in each, nearly double the budget. Three vias bring it to 2 A and 0.08 W.
- Why not “0.08 W each”? 0.08 W is what each makes at 2 A, with three vias. At 3 A each it's 3² × 0.02 = 0.18 W.
- Why not “0.6 W each”? Each via carries 3 A, not the full 6 A, so it makes 0.18 W.
- Why not “0.02 W each”? 0.02 W is what 1 A would make. Each via here carries 3 A.
Show the answer
Answer: Add their resistance and carry the full current each: worse. In series the whole 6 A goes through every via and the resistances add. Parallel is the only way to share.
- Why not “Share the current”? Only parallel vias share. In series every via carries the full 6 A.
- Why not “Halve the heat”? In series each via still carries all the current, so each makes the same heat, and there are more of them.
- Why not “Make no difference”? It makes things worse: the resistances add, and every via still carries the whole current.
“Through the board” is lesson 6 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 5 before it first.
Questions people ask
How much current can a single via carry?
It depends on the drill size, the plating thickness, the board thickness and how much temperature rise you allow. A small 0.3 mm via in standard copper is often treated as good for around an amp; for anything near the limit, use a calculator based on the IPC-2152 standard, and add spares.
Do parallel vias share current equally?
Only if their paths are equal. Vias in a tight cluster right at the pad share well. A via further along the trace has more copper in its path, so it carries less, which is why power vias are grouped close to where the current enters and leaves.
What are thermal vias?
Vias placed under a hot part's pad to carry heat, not current, down to copper on the other side of the board, where it can spread out. A grid of them under a regulator or power chip can drop its temperature noticeably.
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.
- Why a narrow neck in a PCB trace gets hotEvery amp goes through the narrow bit, so that's where the ohms are and where the heat goes.
- How to calculate equivalent resistanceShrink any series-parallel network to one resistor, one step at a time, and spot what's really in parallel.
- How to calculate junction temperatureHeat flows like current: watts through °C/W give a temperature rise, so size the copper before it cooks.