Thermal Vias
Theory: conducting heat through copper tubes
Overview
There are lots of small holes under power IC thermal pads. How much do they help?
Those are thermal vias, and they help enormously. FR-4 board material has a through-plane conductivity of only about 0.3 W/mK — practically an insulator. A via's copper plating is a thin tube about 25 µm thick, but copper conducts about 1,300 times better, so even a thin tube is a major heat path. Let's calculate how count, plating thickness and filling change the thermal resistance under a 5 mm square pad.
Parallel thermal resistances
$t_b$ is board thickness, $r_o$ hole radius, $t_p$ plating thickness. Via copper, fill material and remaining FR-4 act in parallel. This 1D estimate excludes in-plane spreading in copper layers.
When boards became heat sinks
Component heat was once removed mainly through a heat sink on top of the package. In the 2000s, small packages like QFNs with exposed bottom thermal pads spread, and routing most heat into the board became standard. The board itself is then the heat sink, with thermal vias under the pad and internal copper planes as the heat path. LED lighting and automotive electronics went further, adopting aluminium-core boards and copper-filled vias.
Worked example: vias under a 5 mm pad
Board 1.6 mm, FR-4 through-plane 0.3 W/mK, vias 0.3 mm drill with 25 µm copper plating (390 W/mK). One via's resistance, 190 K/W, is about the same as the entire 5 mm square of FR-4 (213 K/W).
Number of vias
| Vias | Resistance | Effective conductivity | ΔT at 1 W |
|---|---|---|---|
| 0 | 213 K/W | 0.3 W/mK | 213 K |
| 4 | 38.9 K/W | 1.6 W/mK | 38.9 K |
| 9 | 19.2 K/W | 3.3 W/mK | 19.2 K |
| 16 | 11.3 K/W | 5.7 W/mK | 11.3 K |
| 25 | 7.4 K/W | 8.7 W/mK | 7.4 K |
| 36 | 5.2 K/W | 12.4 W/mK | 5.2 K |
| 64 | 2.9 K/W | 21.8 W/mK | 2.9 K |
Filling and plating (25 vias)
| Condition | Resistance |
|---|---|
| Unfilled | 7.35 K/W |
| Resin-filled (0.6 W/mK) | 7.33 K/W |
| Solder-filled (50 W/mK) | 5.74 K/W |
| Copper-filled | 2.30 K/W |
| 0.5 mm drill, 35 µm plating | 3.17 K/W |
| 18 µm plating (thin) | 9.85 K/W |
One thin hole conducts as much heat as all the FR-4 under the pad.
The plating's copper cross-section is only 0.022 mm², but at 1,300 times FR-4's conductivity it conducts as much as 25 mm² of FR-4. So resistance falls nearly in inverse proportion to via count — about a thirtieth with 25 vias. Thinning plating to 18 µm worsens resistance by over 30%, so plating thickness matters. Resin filling serves manufacturing purposes such as stopping solder wicking, with almost no thermal benefit; filling for heat requires copper-filled vias. In practice it also matters whether heat leaving the vias can spread sideways in back-side or internal copper planes — with small planes the vias' benefit is lost.
Design tips
- Place as many vias as practical under the pad (about 1 mm pitch is common).
- Specify plating thickness on drawings; thin plating loses much benefit.
- Provide ample copper planes on the back and inner layers for lateral spreading.
- To prevent solder wicking, consider smaller holes, tenting or filling.
- In board-level analysis, model via regions as anisotropic material with effective conductivity.
“Vias, but still hot”
On a power board with 36 thermal vias under the pad, the IC still ran over 20 K hotter than expected. The back side under the vias had only a small copper pad, because the area was needed for routing, so the heat had nowhere to spread. Adding a large copper plane on the back, stitched to other layers with vias, brought temperatures down as expected. Vias are only a path; without a destination they don't work.
Common mistakes
Mistakes and fixes
| Mistake | Effect | Fix |
|---|---|---|
| Treating FR-4 as isotropic | Through-plane overrated | Separate in/through-plane |
| Omitting vias | Temperature overestimated | Use effective conductivity |
| Unchecked plating | 30%+ error | Specify it |
| Expecting resin fill to help | No benefit | Use copper fill |
| Too little back copper | No spreading | Enlarge planes |
I'd like to learn related topics.
Try the PCB thermal tool. Related pages include board-level thermal analysis, junction temperature, thermal resistance and thermal interface materials.
Related Topics
Experience the theory firsthand with the interactive simulator for this field
All Simulators