Book Excerpt: The Printed Circuit Designer’s Guide to... Thermal Management with Insulated Metal Substrates, Vol. 2, Chapter 3

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Chapter 3: Thermal Resistance/Impedance and Measurement

Note that several measurement methods are in common use and typically produce different results. Obviously, this can cause confusion. The Rth value of a thermal resistance, or its thermal conductivity, is meaningless if the test method is not specified.

Following is a brief review of the different measurement methods:

ASTM E1461
The image on the left in Figure 3.1 shows the experimental setup and measurement principle for the test method ASTM E1461 (Standard Test Method for Thermal Diffusivity by the Flash Method). This is a contactless test method for high-temperature testing using a laser. One surface of the test sample is irradiated with a flash of the laser and the thermal transfer to the other side is measured using an infrared detector.

A test sample measured according to ASTM E1461 was shown to have thermal conductivity of 3.3 W/mK.

ISO 22007-2
The center diagram in Figure 3.1 describes the test setup and measurement principle for testing according to the ISO 22007-2 transient plane heat source (hot disc) method. This method allows fast testing and high accuracy, eliminating opportunities for measurement failures and yielding a value without transfer losses. The test sample needs to be at least 3 mm thick.

Using this setup to test the same sample as in the previous ASTM E1461 test showed the thermal conductivity to be 5 W/mK.

ASTM D5470
The diagram on the right in Figure 3.1 describes the test method ASTM D5470. A heat flow is set up through the sample, establishing a temperature gradient that is measured in the steady state. The thermal conductivity/resistance of the sample is then calculated using a method described in the standard.

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