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Via Current & Resistance

PCB & Copper

Cross-section, resistance and current capacity of a via, and how many you need.

Inputs

Board profile

Fills Board thickness (barrel length), Operating temperature and Finished hole diameter from your board profile.

IPC-6012: 20 µm average for Class 2, 25 µm for Class 3
A
°C
°C

Results

1 passed
Vias required
4
Pass

Checked: 4 vias ≥ the 4 needed for a 10 °C rise, IPC-2221B §6.2, k = 0.024
Show working
  1. Barrel cross-section, an annulusA = π × (d + t_p) × t_p = π × 0.325 mm × 25 µm = 0.02553 mm² = 39.56 mil²
  2. Current per via, IPC-2221I = k × ΔT^0.44 × A^0.725 = 0.024 × 10^0.44 × 39.56^0.725 = 951.2 mA
  3. Vias requiredN = ⌈I / I_via⌉ = ⌈3 / 0.9512⌉ = 4
  4. Resistance of 4 in parallelR = (ρ × T / A) / N = 1.1 mΩ / 4 = 275.5 µΩ
Resistance per via
1.1mΩ
Current capacity per via

for 10 °C rise, k = 0.024
951.2mA
Voltage drop across transition
826.4µV
All results (4)
Barrel cross-section

39.6 mil², a tube, not a rod
25500µm²
4 vias in parallel
275µΩ
Power in the via field
2.479mW
Aspect ratio

standard fabrication
5.33:1
4 vias at 750 mA each, inside the 951.2 mA per-via limit. Minimum required: 4.

Spread the vias along the current path. A tight cluster shares heat and runs hotter than this calculation assumes.

Accuracy

Source
  • IPC-2221B §6.2 (2012)

IPC-2221B, Generic Standard on Printed Board Design, §6.2 (2012) fitted to measurements

Valid range
  • Barrel plating thickness 5 µm to 100 µm

Barrel plating thickness Barrel plating far outside the usual 15 to 35 µm is a different process from the one assumed here.

Precision
The IPC-2221 trace curve applied to the barrel cross-section.

It ignores the heat the barrel sheds into the planes it crosses, so the current it reports is conservative.

IPC-2221B §6.2

Verified against

2 independent cases. See the working.

This is a design aid. The engineer remains responsible for the design and for checking the standard itself.

Parameter sweep

Vary one input over a range and see the answer and verdict at each step, as a table and a curve.Pro

Worst-case corners

Put a tolerance on each input and get the worst-case band around the answer.Pro

Cross-section

The principle

A plated hole carries current in a thin copper tube, not the whole hole. A 0.3 mm via with 25 µm plating has about the cross-section of a 0.7 mm trace in 1 oz copper.

Barrel cross-sectional area
A=π(d+tp) tpA = \pi (d + t_p)\, t_p
Mean diameter d + tp. For 0.3 mm and 25 µm, A ≈ 0.0255 mm² (about 40 mil²).
Barrel resistance
Rvia=ρ lA=ρ tboardπ(d+tp)tpR_{via} = \frac{\rho\, l}{A} = \frac{\rho\, t_{board}}{\pi (d+t_p) t_p}
Typically 0.5–2 mΩ on a 1.6 mm board.
Current capacity (IPC-2221 applied to the barrel)
I=k ΔT0.44A[mil2]0.725I = k\,\Delta T^{0.44} A_{[\t{mil}^2]}^{0.725}
N vias in parallel
RN=RviaN,Iper=INR_N = \frac{R_{via}}{N}, \qquad I_{per} = \frac{I}{N}
Thermal resistance of one via
Rθ,via=tboardkCuAR_{θ,via} = \frac{t_{board}}{k_{Cu} A}
The same formula with copper’s thermal conductivity in place of its electrical conductivity. About 163 °C/W for the example above.
  • ddfinished (plated) hole diameter
  • tpt_pcopper plating thickness in the barrel, typically 20–30 µm
  • llbarrel length = board thickness
  • NNnumber of vias sharing the current
More detail

Treating the barrel as a trace of the same cross-section is conservative: it is bonded to copper at both ends and sheds heat better than a free trace.

Power vias. Spread them along the path rather than clustering, and add at least one more than the calculation needs. An open barrel is a field failure.

Fabrication limits

  • Aspect ratio (board thickness ÷ hole) above about 8:1 makes plating hard; 10:1 is the practical limit for standard processes.
  • Plating class. IPC-6012 Class 2 requires 20 µm average in the hole, Class 3 requires 25 µm. If your capacity assumes 25 µm, put it on the drawing.
  • Filled vias. Copper-filled vias conduct much better; epoxy-filled ones conduct no better than plain.
References: IPC-6012 (qualification and performance of rigid boards, plating thickness classes), IPC-2221B §6.2 for the current capacity relation.

Engine version ⁨1.18.3⁩