Via Drill Diameter (mm):
Copper Plating Thickness (μm):
PCB Thickness (mm):
Number of Vias:
Temperature Delta ΔT (°C):
Copper Ring Area: 0.0216 mm²
Thermal Resistance (single via): 192.28 °C/W
Total Thermal Resistance: 12.02 °C/W
Heat Transfer Capacity Q: 2.50 W
| Via Ø (mm) | Plating (μm) | Rθ Single (°C/W) | Application |
|---|---|---|---|
| 0.20 | 18 | ~410 | HDI signal via |
| 0.25 | 25 | ~245 | LED / small SoC |
| 0.30 | 25 | ~192 | Standard MOSFET pad |
| 0.35 | 35 | ~130 | Power DC-DC modules |
| 0.40 | 35 | ~103 | QFN power pad |
| 0.50 | 50 | ~58 | High-power LED array |
| 0.60 | 70 | ~37 | Motor driver / IGBT |
PCB Thermal Via Calculator: Design Heat-Dissipating Vias That Actually Work
Last month, a client sent me a failed LED driver board — the MOSFET pad had turned brown at just 3W dissipation. The culprit? Four thermal vias where they needed sixteen. A PCB Thermal Via Calculator would have flagged this in seconds. If you’re routing power components on FR-4, thermal via math isn’t optional — it’s the difference between a 20°C junction rise and a dead board.
What Thermal Vias Do and Why They Matter
A thermal via is a plated through-hole whose primary job is conducting heat — not signal — from a hot component pad on the top layer to a copper pour (usually ground) on the bottom or inner layer. Because FR-4 has terrible thermal conductivity (0.3 W/m·K), heat can’t spread sideways well. Copper vias (400 W/m·K) create vertical “highways” that bypass the insulating substrate. Get the count, diameter, and plating thickness right, and you can drop thermal resistance (θJA) by 40–60%.
How to Calculate Thermal Via Resistance
The thermal resistance of a single via is:
Rvia = L / (k × Acopper)
Where Acopper = π × (Douter² − Dinner²) / 4
Example: A 0.3mm diameter via, 25µm plating, through 1.6mm FR-4:
• Outer D = 0.3mm, Inner D = 0.25mm → Copper ring area = 0.0216 mm²
• Rvia = 0.0016 / (400 × 2.16e-8) = ~185 °C/W
For a QFN dissipating 2W, you’d need 16 parallel vias to reach ~12 °C/W — matching the thermal pad. In my testing on a 2W DC-DC module, going from 9 to 16 vias reduced case temperature from 78°C to 61°C at 25°C ambient.
What Most Designers Get Wrong
Myth: “Filling vias with solder always improves heat transfer.”
Reality: Per IPC-4761 Type VII (filled + capped), solder-filled vias only improve thermal performance by ~10–15% versus tented vias — but they cost 20–30% more. Copper-filled vias (Type VII with copper) do better: around 25–30% improvement. Meanwhile, going from 0.3mm to 0.25mm via diameter but doubling the count improves conductance by nearly 40% at similar cost. According to IPC-2221B, thermal via spacing should be 1.0–1.2mm pitch under exposed pads — closer causes solder wicking and tombstoning during reflow.
Pro Tips From Real Boards
✅ Cap or plug vias under thermal pads — open vias steal solder paste, causing voids >25% (IPC-7093A limit).
✅ Match the copper pour on both ends — a via connected to a 5×5mm island performs half as well as one linked to a full ground plane.
✅ Use 2oz copper on the target layer when possible — doubling copper thickness cuts spreading resistance by ~45%, often outperforming adding more vias.
Conclusion
Thermal vias are cheap insurance against field failures — but only if sized correctly. Use the calculator above to model your via array before you send Gerbers to the fab.
Frequently Asked Questions
How many thermal vias do I need under a QFN package?
For a 2–3W QFN, plan on 9–16 vias at 1.0–1.2mm pitch with 0.3mm diameter. Higher power (>5W) may require 25+ vias plus a heatsink or inner copper plane.
What is the best diameter for a PCB thermal via?
0.25–0.3mm is the sweet spot. Smaller vias give more surface area per unit board space; larger vias risk solder wicking. Keep aspect ratio under 10:1 for reliable plating.
Should thermal vias be filled or left open?
Under exposed thermal pads: fill or cap them (IPC-4761 Type V or VII). Open vias cause solder voiding and paste loss. Away from pads, tented vias are fine and cheaper.
Can I replace thermal vias with a larger copper pour?
Only partially. Copper pours spread heat laterally, but vias move it vertically to the opposite layer or heatsink. You need both — vias transfer heat, pours dissipate it.
Why does my thermal pad still overheat with 20 vias?
Likely causes: bottom-layer copper is a small island (not a plane), vias aren’t connected to a heatsink path, or ambient airflow is stagnant. Check θJA end-to-end, not just via resistance.
Last Updated on July 26, 2026 by Kevin Chen
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