ASTM C518 Steady-State Thermal Transmission Properties

Scope:

ASTM C518 is a quick way to determine Steady-State Thermal Transmission properties that guide the design and prototyping of electronic products. 

Steady-State Thermal Transmission properties of a material change with temperature change. It is important for designing electronics that dissipate fixed amounts of heat in a given time. This ensures that the device stays at the desired temperature and doesn’t overheat under operation. It is also important to model the worst-case power dissipation situations.  

Laptops and phones now can store way more energy than they could. Electronics designers are therefore under more pressure than ever to design cost-effective products great at dissipating heat. ASTM C518 helps achieve that. 

Test procedure:

ASTM C518 involves an extensive calibration procedure. After proper calibrations, a sample of known thickness Δx is placed between a heated and a cooling plate. The temperature difference ΔT between the plates is fixed.

A temperature gradient is applied to the sample and the resulting heat flowing through the sample is measured. A heat flow meter is used to measure the rate of heat flow q through the test sample. The data from these tests can be used to determine thermal transmittance (U-value) and thermal conductivity.

Sample size:

The sample should be between 150 mm and 300 mm square in size and have parallel surfaces. 

Result:

From the measured heat flow q, the thermal transmittance u (U-value) can be calculated by

U = q/(A × ∆ T )

Where A denotes the area of the sample and ΔT the applied temperature gradient.

With the U-value and the thickness d of the sample, the thermal conductivity K can be determined

K = qD/A∆T

Conclusion

ASTM C518 test method may frequently be used instead of ASTM C177, which uses a separate apparatus called a Guarded Hot Plate Apparatus (GHP) to test the same Properties.