Thermal Insulating Pads

Their standard test method is ASTM D5470.

ASTM D5470 measures thermal impedance (resistance) of a flat disk-shaped specimen or controlled volume of a liquid TIM between two flat polished calorimeter surfaces under controlled load.

What is apparent thermal conductivity?

Apparent thermal conductivity is a calculated value that uses the thermal impedance (resistance) measured from ASTM D5470 and the sample thickness to calculate a thermal conductivity value. This value is influenced by how effectively the sample contacts (or 'wets out' if a dispensable) the calorimeter surfaces. The thermal resistance at the interface between the sample and the probes is called contact resistance. Contact resistance adds to the overall thermal impedance and may produce a lower measurement than bulk thermal conductivity.

Is there a recommended surface roughness?

As a general rule, increased surface roughness will create a larger surface area available for wetting. For vertical applications, increased surface roughness will provide an greater resistance to slide. Generally, increasing the shot size, contact area and surface roughness will aid in slide resistance of the material. Surface roughness of N8 or rougher is recommended.

What materials do you recommend where applications require thin bondline thickness from a thermally conductive material?

This type of application would require a smooth/flat surface roughness. For this we recommend Ablefilm, ​Thermattach tapes, Cho-Therm, thermal greases or pastes/gels.​

Why is apparent thermal conductivity useful?

A material can have a very high intrinsic bulk thermal conductivity but be outperformed by a material of lower bulk conductivity that is softer and conformable. Measuring apparent thermal conductivity can help better identify real world performance of a thermal interface materials in many cases.

What should be considered when comparing reported thermal conductivity values for materials from varying sources on supplier datasheets?

It is always difficult to compare values since it is unlikely that reported values from varying sources were generated using the same test method and parameters. There are many test instruments and methods used in the marketplace.

What thermal data do Parker Chomerics report and what is the frequency that these measurements are made for their products?

Parker Chomerics report bulk thermal conductivity for most TIM products on technical datasheets. Thin bond line product (such as phase change materials and thermal greases) datasheets will show thermal impedance at fixed pressure instead of bulk thermal conductivity as this is more practical and useful to the designer. For lot-to-lot conformance testing, Parker Chomerics measures and retains apparent thermal conductivity and thermal impedance for every manufacturing lot of product.