W/m·K appears on every thermal datasheet and is one of the most misused numbers in electronics thermal design. Understanding what it measures — and what it deliberately omits — is the difference between a working thermal design and an expensive surprise.
What W/m·K actually measures
Thermal conductivity, k, quantifies how readily heat flows through a material. Formally, it is the heat flow in watts through a one-metre cube when there is a one-kelvin temperature difference across it.
Rearranged into the form engineers actually use:
Reference values
| Material | k (W/m·K) | Context |
|---|---|---|
| Still air | 0.026 | Why trapped air in an interface is so damaging |
| Silicone grease | 3–8 | Typical commercial compounds |
| Filled thermal pad | 1–6 | Standard gap filler |
| Stainless steel | ~16 | — |
| Aluminium | ~205 | Common heat sink material |
| Copper | ~400 | Best practical bulk conductor |
| Graphene (in-plane) | ~1,500–5,000 | Why graphene is interesting at all |
| Lenecold graphene pad (through-plane) | 180 | Directionally engineered |
| Lenecold graphene plate (through-plane) | 1,200 | High-power stacks |
| Diamond | ~2,000 | Benchmark, not practical |
The directional issue: through-plane vs in-plane
This is where most confusion lives. Heat in an electronics assembly usually travels through the thickness of the interface — from die upward into the heat sink. That direction is called through-plane (or z-axis).
Graphene is unusual because it is highly anisotropic: within the plane of the sheet it conducts extremely well, but across the stack of sheets it conducts far less. A randomly-oriented graphene composite therefore shows modest through-plane conductivity despite the impressive raw material number.
Why a big number can still perform badly
Three common reasons:
- Thickness dominates. A 3 W/m·K grease at 50 μm beats a 6 W/m·K pad at 500 μm. Conductivity alone tells you nothing about the joint.
- Contact resistance is excluded. The W/m·K figure describes the bulk material. The interfaces between pad and die, and pad and sink, add resistance that can exceed the bulk term at low mounting pressure.
- Measurement conditions vary. Values depend on temperature, pressure and test method. Always check the standard — ASTM D5470 for resistance under pressure, laser flash for bulk diffusivity.
How to use the number correctly
Confirm the direction
Always match the quoted axis to the direction heat travels in your stack — for a thermal pad and for our plate, that is through-plane.
Convert to resistance at your thickness
R = L / (k × A). This is the number that matters.
Add contact resistance
Or better, get measured R-versus-pressure data from the supplier.
Check the test standard and pressure
A value measured at 100 psi is not valid for your 10 psi assembly.
Verify at your operating temperature
Conductivity of polymer-based materials changes with temperature.