ASTM D5470 is the standard steady-state method for measuring the thermal resistance of thin thermally conductive materials. If you are comparing TIM data sheets, understanding this one test will save you from most of the ways those numbers mislead.

How the test works

The sample is placed between two meter bars of known conductivity, usually copper. Heat flows from a heater through one bar, across the sample, into the second bar and out to a cooling block. Thermocouples along the bars give the temperature gradient, from which heat flux and the temperature drop across the sample are derived. The whole stack sits under a controlled, variable compressive load.

Steady state
Measurement taken after thermal equilibrium
Variable pressure
Resistance reported against applied load
Controlled thickness
Bond line set by shims or LVDT
°C·cm²/W or °C/W
Result normalised to area, or absolute

Four numbers to demand from any data sheet

  1. The pressure

    Thermal resistance falls as pressure rises. A figure quoted at 100 psi is not comparable to one at 10 psi. If pressure is not stated, the number is close to meaningless.

  2. The bond line thickness

    Resistance scales with thickness. A value quoted at 0.1 mm says nothing useful about performance at 0.8 mm.

  3. Whether contact resistance is included

    Some reports subtract it, some include it. In a real joint you always pay it.

  4. The aged value

    Day-one resistance is easy. Resistance after 1,000 hours at operating temperature, under load, is the number that predicts field performance.

The most common data sheet trick Quoting conductivity (W/m·K) without quoting the bond line and pressure at which the resistance was measured. A material can have an impressive k and still perform badly if it does not wet the surfaces — conductivity is a property of the material, thermal resistance is a property of the joint.

What we report

ItemLenecold reporting
MethodASTM D5470
Thermal resistance< 0.05 °C·cm²/W, at low mounting pressure
Curve dataResistance vs pressure, for 0.3 / 0.5 / 0.7 mm
Aging150 °C × 1500 h @ 50 psi, resistance drift ≤ 10%, compressibility drift ≤ 10%
SamplesCut to your footprint so you can measure it in your own fixture
Our recommendation Do not take our number on trust. Order a sample, run it in the fixture you already use, at the pressure your assembly actually delivers, on the surfaces you actually have. That measurement is worth more than any data sheet — including ours.
Not sure which way to go? Send your power level, gap range, mounting pressure and duty cycle and we will tell you plainly which material fits — including when a cheaper option is the right answer. Ask engineering →

Frequently asked questions

Is ASTM D5470 the only relevant test?
It is the standard for thin TIMs and the one most data sheets reference. Depending on your application you may also want power cycling, thermal shock, outgassing or flammability testing — tell us your qualification plan and we will confirm what we can support.
Why does your resistance number change with pressure?
Because contact resistance falls as the pad conforms more closely to the surface roughness of the mating parts. Every TIM behaves this way; the difference is how much pressure is needed to get most of the benefit. Ours reaches low resistance at modest pressure.
Can you test to our internal standard?
We can supply samples and material data to support your own qualification. Share the procedure and we will tell you what we can provide directly and what you would need to run in-house.

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