PCB via current capacity: the barrel formula, plating classes, parallel vias and a worked example
A via is a hole plated with a thin tube of copper, and that tube is all the conductor there is. Its current rating comes from the plating's cross-section run through IPC-2221's trace formula, so a common 0.3 mm via with 25 µm plating carries about 1.7 A for a 10 °C rise, and a bigger drill helps less than you would expect. This calculator gives the current per via from the drill, the plating and the rise, the barrel's resistance, drop, loss and thermal resistance through the board, how many vias a load needs and how they share it, and draws the via in cross-section beside the array you would place.
How to use the via current calculator
- Type the drill size (the hole before plating) and the temperature rise you can accept, 10 °C unless you know better.
- Pick the plating thickness in the menu row: 25 µm for IPC class 3, 20 µm for class 2, or what your fab certifies.
- Add the current the load needs and the calculator counts the vias and shows how they share it; the board thickness and the ambient set the barrel's resistance and heat path. The Via sizes tab compares the common drills.
The barrel formula
A = π t (D − t) I = 0.048 · ΔT0.44 · A0.725
The plated barrel is a ring of thickness t inside a hole of diameter D, so its cross-section is π t (D − t). That area, in square mils, goes into the same IPC-2221 relation used for traces with the outer-layer constant 0.048, which is how the standard via calculators rate a via and is conservative for a barrel buried in laminate and connected to planes. A square mil is 0.000645 mm². The finished hole, D − 2t, is what the fab measures.
Plating thickness
Everything rides on the plating. IPC-6012 asks for 20 µm average in class 2 (most commercial boards) and 25 µm in class 3; many fabs quote 18–25 µm and some plate more on request. The plating is thinnest in the middle of the barrel on thick boards with small holes: above an aspect ratio of about 8:1 (board thickness over drill) the fab may not reach the nominal figure. Ask what they guarantee, and size for that.
Vias in parallel
For more current, use more vias rather than a bigger one: the barrel area grows with the circumference, so doubling the drill gives only about 1.65 times the current and takes four times the board area. Several 0.3 mm vias in a tight group share the current by their conductance; since alike barrels have alike resistance they share evenly as long as the copper feeding them is symmetrical. Round up and add one for the plating tolerance. The current then splits and each via's rise falls below the allowed figure.
Thermal vias
The same tube of copper is a heat path: its thermal resistance along the board is L / (k A), with copper at 385 W/m·K, around 190 K/W for a 0.3 mm via through 1.6 mm. That sounds poor, but a grid of twenty-five of them under a thermal pad is under 8 K/W, far better than the laminate. Fill or tent the vias under a pad so that reflow solder does not drain down them.
Your via, step by step
- Barrel: π × 0.025 mm × (0.30 mm − 0.025 mm) = 0.0216 mm² = 33.5 mil².
- Current: 0.048 × 100.44 × 33.50.725 = 1.69 A, like a 0.62 mm 1 oz trace.
- Barrel at 35 °C: 1.35 mΩ through 1.60 mm, dropping 2.27 mV; 192 K/W thermally.
- For 5 A: 3 vias, 1.67 A each at 9.7 °C.
Worked example: a 0.3 mm via with 25 µm plating
The barrel's area is π × 0.025 × (0.3 − 0.025) = 0.0216 mm², or 33.5 mil². For a 10 °C rise, 0.048 × 100.44 × 33.50.725 = 1.69 A, the same as a 0.62 mm outer trace in 1 oz copper. Through a 1.6 mm board the barrel is 1.35 mΩ at 35 °C and drops 2.27 mV at that current, with a thermal resistance of 192 K/W. A 5 A supply rail therefore needs 3 vias, 1.67 A each, which rise only 9.7 °C; a single 0.6 mm via would carry just 2.88 A.
Questions
Does a filled or tented via carry more?
Conductive fill adds a little copper, but ratings are given for the plating alone; non-conductive fill and tenting change nothing electrically. Both help heat and assembly.
Is the IPC-2221 trace formula right for vias?
It is the accepted approximation and errs on the safe side. IPC-2152 notes that vias with the same cross-section as the trace feeding them do not add temperature rise, which is a good rule in itself: match the barrel area to the trace.
What about microvias?
Laser microvias are 0.1–0.2 mm and shallow, with plating of 12–20 µm; their rating is small and they are stacked or staggered for power. Use the fab's figures for the plating.
Why is the resistance so low but the via still matters?
A milliohm barrel drops almost nothing, but it is also tiny and surrounded by laminate, so the same milliohm at a few amps warms it noticeably. The rise, not the drop, sets the limit.