Measure thermal pastes and pads: ASTM D5470-17 and the Nanotest TIMA5 – Correctly determine bulk and effective thermal conductivity

1 - Introduction and efective thermal resistance 2 - Calculation of the interface resistance 3 - Thermal bulk resistance and bulk conductivity 4 - Equipment, datasheets and conclusion

Bulk resistance and bulk thermal conductivity

In order to determine the actual bulk thermal conductivity of a medium after the interface resistance _Rinterface has been determined, the measured thermal resistance can be corrected to exclude the influence of the interface resistance. As we remember from the previous page, it represents the resistance created by the thermal interface between two materials and must now be subtracted from the total measurement to calculate the pure thermal conductivity of the material itself.

First we have the already known total thermal resistance Rth_{, total}. This is made up of the interface resistance _Rinterface and the resistance of the bulk material _Rbulk:The bulk resistance _Rbulkdepends on the thermal conductivity _λbulk, the bondline thickness BLT and the cross-sectional area A of the medium:To determine the actual thermal conductivity of the bulk material _λbulk, you must first subtract the interface resistance from the total thermal resistance. From this you can now determine _Rbulk:Then simply rearrange the equation for Rbulk to calculate _λbulk:If you have measured thermal resistances for many different bondline thicknesses BLT and have already determined the interface resistance by extrapolating to BLT=0, you can use the corrected thermal resistance _Rbulk to determine the actual thermal conductivity of the material. This is particularly important in experiments or applications where accurate thermal properties of a material without the influence of interface resistance are required. Or for marketing, which is not viable without large numbers.

However, even this significantly higher value is still not in the spherical realms of the marketing cheaters that we are led to believe. Let’s take a look at the individual curve of the excellent DOWSIL TC-5888, whose thermal conductivity is stated by the manufacturer Dow Chemical as 5.2 W/m-K. And now let’s take a look at my measurement and see what was to be proven. It agrees down to the decimal place!

And now let’s compare this bulk value once again with the actual, effective values for the relevant layer thicknesses BLT from practice. This is still good, but slightly less and also increases with increasing BLT:

The disadvantages of ASTM Hotwire measurement (or how not to do it)

The ASTM Hotwire test for measuring the thermal conductivity of thermal pastes, which is actually quite inexpensive to implement, has serious disadvantages that can quickly affect the accuracy of the results. One of the main problems with this method is the potential misestimation of the actual thermal conductivity of the tested materials, which often leads to values that are far too high. One of the main disadvantages of the ASTM Hotwire test is the sensitivity of the test to the contact resistance between the thermal paste and the metallic plates.

These contact resistances are caused by unevenness on the surfaces of the plates and the imperfect distribution of the paste. Even small air bubbles or uneven layers can impair the heat flow and thus falsify the measurement results. As the test is based on the assumption that the paste conducts the heat flow evenly, such irregularities can quickly lead to systematic errors and usually do so for materials above 1 W/m-K.

In addition, insufficient consideration is given to the influence of temperature gradients along the metallic plates. If the temperature distribution is not uniform, this influences the temperature measurements and leads to an inaccurate calculation of the thermal conductivity. In addition, the hotwire test can be further distorted at high temperatures due to the thermal expansion of the materials, which can also lead to incorrect results.

The often too high measured values also result from the so-called “guarded hot plate” design of the test, in which the heating of the wire can lead to an overestimation of the thermal conductivity. In practice, this means that the test overestimates the thermal conductivity of the thermal paste because the method is sensitive to local overheating of the wire. The wire generates a localized heat source that is not necessarily representative of the uniform heat transfer in real applications. The shortcomings are therefore due to a combination of contact resistance, uneven temperature distribution, and the specific test set-up, which do not always correctly replicate the real-life operating conditions of the thermal pastes. As a result, these measurements are less reliable for more conductive pastes and can significantly overestimate the actual performance of the thermal conductive materials. But they are inexpensive, as such a system ultimately costs only a tenth of the price of a proper tester.

But let’s not get into that, we want to do it properly. That’s why I’ll introduce you to the equipment we used on the next page.