What is W/mK?
Thermal conductivity ratings explained: what the number means, how it is measured, and when it changes real temperatures.
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.
On test method comparisons: Other methods, including hot-disk, guarded hot plate, and bulk specimen tests, measure thermal conductivity without an interface. Because they exclude contact resistance, they produce higher numbers for the same material. A product claiming 17 W/mK via a bulk test is not directly comparable to one claiming 15 W/mK via ASTM D5470. When evaluating products across brands, the test method matters as much as the number itself.
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.
Thermal conductivity reference
Amawi product values are ASTM D5470 rated. Reference material values are established material properties shown for context.
| Material / Product | W/mK | |
|---|---|---|
| Copper | 401 | |
| Aluminum | 237 | |
| Thermal Interface Materials — ASTM D5470 | ||
| Liquid Fusion Supreme Liquid Metal | 128.0 | |
| Fusion Supreme Paste Thermal Paste | 15.0 | |
| Fusion Supreme Pad Thermal Pad | 15.0 | |
| Fusion Ultra Paste Thermal Paste | 12.8 | |
| Fusion Ultra Pad Thermal Pad | 12.8 | |
| Frostbite Extreme Paste Thermal Paste | 8.5 | |
| Fusion Thermal Putty Thermal Putty | 8.0 | |
| Frostbite Extreme Pad Thermal Pad | 8.0 | |
| Frostbite Paste Thermal Paste | 6.0 | |
| Frostbite Pad Thermal Pad | 6.0 | |
| Frostbite Thermal Putty Thermal Putty | 6.0 | |
| Air gap (no TIM) | 0.025 | |
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.
GPU repaste
Modern discrete GPUs dissipate 300-600W across compact die areas, producing some of the highest heat flux densities in consumer hardware. Real-world repaste results on high-end graphics cards have documented GPU temperature reductions of 6-7°C and hotspot improvements of up to 14°C after replacing factory paste with premium compound. At GPU hotspot temperatures, each degree preserved directly extends sustained performance before the card throttles.
High-TDP desktop CPUs (150W and above)
Flagship desktop CPUs from Intel and AMD running sustained workloads above 150W sit at heat flux densities where paste quality produces measurable, repeatable temperature differences. Premium compound against standard alternatives consistently shows 5-10°C improvement in extended benchmarks at this TDP range, translating directly to more headroom for sustained boost.
Laptop and console repaste
Mobile CPUs and APUs are thermally constrained by design: small die, high sustained TDP, and a throttle margin of as little as 5-10°C. Premium paste reduces die temperature and pushes the throttle threshold further away. In many cases this determines whether a system holds full boost clock speeds or drops during extended gaming or rendering sessions.
Delidded CPUs (direct die)
Removing the integrated heat spreader places the paste in direct contact with silicon, eliminating the internal TIM layer. In this configuration the external paste becomes the primary variable in the thermal path, and performance differences between product tiers are most pronounced. A high-conductivity compound delivers its full rated benefit here.
Overclocking
Higher frequencies generate more heat. The margin between stable operation and thermal shutdown narrows as clocks increase. Premium paste extends this margin, enabling higher sustained frequencies without additional cooling hardware investment.
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.
Product datasheets and technical documentation
All Amawi Technologies datasheets include full ASTM D5470 specification data for every product in the range.
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