What is W/mK?

What W/mK measures

Thermal conductivity is the rate at which a material transfers heat. The unit W/mK (watts per metre-kelvin) expresses how many watts of heat flow through one metre of material for every degree Celsius of temperature difference across it. For a thermal interface material, a higher W/mK value means heat moves through the compound faster, reducing the temperature gradient between a hot processor die and its heatsink.

No heatsink base and no processor die is truly flat at a microscopic level. Thermal compound fills the voids between these surfaces, replacing trapped air that would otherwise act as an insulating layer at the contact point. The compound's conductivity determines how quickly heat crosses that interface.

How W/mK is measured: ASTM D5470

The figure on a datasheet is meaningful only if you know which test produced it.

ASTM D5470 is the international standard for measuring thermal transmission properties of compressible interface materials. It tests actual interface resistance: the compound is sandwiched between two precision-machined metal plates under defined clamping pressure, and thermal resistance is measured at multiple bondline thicknesses. This captures both the material's bulk conductivity and the contact resistance at each surface, producing a result that reflects real interface conditions rather than an idealized sample measured in isolation.

All Amawi Technologies thermal interface materials are tested to ASTM D5470. Full technical datasheets are available on the downloads page.

Heat flux: why modern systems demand more

Thermal conductivity becomes more critical as heat flux increases. Heat flux is the amount of power dissipated per unit of die area, measured in W/cm².

A 65W desktop CPU spread across 200mm² produces around 33 W/cm². A 200W+ CPU running through a 140mm² die produces over 140 W/cm². Modern high-performance processors have moved toward smaller die areas while TDP targets have continued to increase, producing heat flux densities that amplify every unit of thermal resistance in the path from silicon to heatsink.

At moderate heat flux, the temperature difference between a 6 W/mK and a 15 W/mK compound at a real-world bondline is 2-4°C. At the heat flux densities produced by a 200W+ CPU under sustained workload, or a high-end GPU under gaming conditions, that same conductivity difference can account for 5-10°C at the die. Each degree of reduction is additional headroom before thermal throttling engages.

From W/mK to actual degrees: a worked example

The thermal resistance of a compound layer follows a simple relationship: resistance equals layer thickness divided by conductivity times contact area. This makes the effect of W/mK on real temperatures directly calculable.

Take a realistic case: a CPU with a 150mm² die dissipating 150W through a 0.05mm (50 micrometre) compound layer. At 6 W/mK, the layer alone adds about 0.056 K/W of thermal resistance, which at 150W means roughly 8.3°C of temperature rise across the compound. At 15 W/mK, the same layer adds 0.022 K/W, or about 3.3°C. The conductivity difference alone is worth 5°C at the die in this scenario.

The same relationship explains why application technique matters as much as the rating: halving the layer thickness halves the resistance, exactly like doubling the conductivity would. A thin layer of a mid-tier compound can match a thick layer of a premium one.

Thermal conductivity reference

Amawi product values are ASTM D5470 rated. Reference material values are established material properties shown for context.

Thermal pads: why thickness beats the rating

For thermal pads, W/mK is only half the story. Pads bridge gaps of 0.5mm to 3mm, ten to sixty times thicker than a paste bondline, and thermal resistance scales with thickness divided by conductivity. The result is counterintuitive: a 0.5mm pad rated at 6 W/mK has less than half the thermal resistance of a 1.5mm pad rated at 8 W/mK, despite the lower number on the label.

The practical rule when selecting thermal pads: measure the actual gap and choose the thinnest pad that still makes full contact under mounting pressure. Only after thickness is fixed does the W/mK rating decide the remaining performance, and at 1mm and above it matters far more per point than it does for paste. This is why VRM and memory cooling, where gaps are large and fixed, benefits most from high-conductivity pads.

Where high W/mK makes the biggest difference

High thermal conductivity delivers its greatest benefit where heat flux is highest and thermal headroom is narrowest.

Reading a datasheet: W/mK red flags

Because W/mK is the headline number, it is also the most manipulated one. Three patterns are worth checking before you trust a rating:

• 1.

  No test method named. A conductivity claim without a named standard is unverifiable. If neither the listing nor the datasheet says how the value was measured, treat it as a marketing figure, not a specification.

• 2.

  Bulk values presented as interface performance. Hot-disk and bulk specimen tests exclude contact resistance and systematically produce higher numbers than ASTM D5470 for the same material. A bulk 17 W/mK and an interface-tested 15 W/mK are not the same class of claim.

• 3.

  Ratings out of proportion to price and formulation. High-conductivity fillers are expensive. A paste sold for the price of a coffee claiming the conductivity of premium silver- or aluminium-filled formulations warrants skepticism; independent tests of such products regularly measure a fraction of the printed value.

Every Amawi Technologies product states its ASTM D5470 result and publishes a full datasheet on the downloads page, so the number you compare is the number the material actually delivers as an interface.

Application: reaching the rated spec

Thermal conductivity is a property of the material. Reaching the rated value requires correct application: a thin, even layer across the full die surface, with no excess that unnecessarily increases bondline thickness. Every additional micrometre of bondline adds thermal resistance regardless of the compound's W/mK rating.

Clean surfaces are equally important. Oils, residues, and traces of old compound at the interface add contact resistance directly on top of the new material. Both die and heatsink surfaces should be cleaned with 99.9% isopropyl alcohol before applying fresh compound.

Longevity: premium compound over time

Thermal compounds are not permanent. Every thermal cycle stresses the interface between compound and surface. Lower-quality formulations, including some that claim high W/mK ratings via non-interface test methods, are prone to pump-out, dry-out, and phase separation after 50-100 thermal cycles. The result is steadily increasing thermal resistance over 18-24 months as the compound degrades in place.

Premium compounds use stable polymer matrices and fine particle distributions engineered to maintain their thermal properties through years of cycling. A correctly applied, quality installation lasts 3-5 years before replacement becomes worthwhile.

Frequently asked questions