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 (in-plane) | 1,200 | Spreading applications |
| 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
Through-plane for TIM, in-plane for spreaders and heat spreader plates.
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.