Tag Archives: thermal impedance

What is the Thermal Impact of Imperfections in Phase-Change Material

Advanced Thermal Solutions, Inc. (ATS) engineers have received several questions from customers about the phase-change material that comes standard on the base of all ATS heat sinks. Engineers have asked whether imperfections on the surface of the grey foil that protects the phase-change material, such as dents or wrinkles, have a significant impact on the thermal interface material’s thermal performance. Do these imperfections have any impact at all? Should the liner be removed?

ATS uses Parker Chomerics Thermflow™ T766 thermal interface material (TIM), which comes with a thin, protective layer of metal foil that should not be removed when placing the heat sink on the device it is intended to cool.

Phase-Change Materials

ATS heat sinks come with Chomerics T766 phase-change material standard. (Advanced Thermal Solutions, Inc.)

When pressure is applied, the phase-change material (and the metal foil) conform to both surfaces, completely removing air gaps or voids to maximize heat sink performance. The phase-change material will “attain minimum bond-line thickness” and “maximum surface wetting,” according to information from Chomerics, to limit the thermal resistance path and ensure almost no thermal contact resistance between the device being cooled and the heat sink. For the T766, the phase-change temperature is listed as 55°C. The liner should remain in place when placing heat sink on the device it is intended to cool (see the video below).[1]

Should engineers be concerned about the appearance of the metal foil lining? Do the dents or wrinkles in the lining impact the performance of the phase-change material and potentially impact the efficiency of the heat sink?

To reassure engineers that the appearance of the metal foil would have a negligible impact on the thermal performance of the TIM, the Chomerics Research and Development Department released the results of tests that the company performed on the T766 conformable metal foil. [2] Chomerics studied the impact on thermal impedance when the foil was wrinkled, dented, and even folded.

Researchers tested materials that were not wrinkled, lightly wrinkled, moderately wrinkled, and severely wrinkled under different pressures (20 psi, 50 psi, and 100 psi). The results (shown below) demonstrated that even when wrinkled “to a far greater extent than would be expected in actual handling” thermal impedance never increased more than 0.02°C-in22/W. The report explained, “For 50 W of power, through one square inch of material, that’s only 1.0°C change!”

The dent test was created using a wooden tongue depressor and included a sample with five dents per square inch and a second with 15 per square inch. As was demonstrated in the wrinkle study, the dents smoothed out during the testing process and showed a minimal impact on thermal impedance. “Once again, the thermal impedance did not increase by more than 0.01°C-in2/W. For 50 W of power, through one square inch of material, that’s only 0.5°C change! The metal foil carrier is so conformable that the dents were almost unidentifiable after testing with 100 psi of pressure.”

The final test was performed on T766 that was folded. One sample was folded under on one edge and the second was folded to overlap down the center. The results indicated that small folds of up to 5% of the pad’s area does not significantly impact thermal impedance. A large fold, which tripled the thickness of the foil in the center of the sample, had a significant impact on the thermal impedance of the material.

The report concluded, “T766 will perform extremely well even when the pad is wrinkled or folded, or the foil is scratched or dented. The high conformability of the metal foil carrier will allow it to smooth out and erase almost any imperfection.”

1. https://www.parker.com/literature/Chomerics/Parker%20Chomerics%20

2. http://www.parker.com/parkerimages/Parker.com/Divisions-2011/Chomerics%20Division/SupportAssets/Parker%20Chomerics%20THERMFLOW

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

Testing Thermal Interface Materials

Illustration: Parker Chomerics

Thermal interface materials, TIMs, provide the thermal pathway for transferring heat from components to heat sinks. At one time, most TIMs were simple, homogenous pads filled with thermally conductive fillers. But increasing power levels of processors and other components present a continuous need for improved thermal material performance. Today, a much wider range of TIMs is available, including phase change materials, compounds, and gap fillers.

When choosing a TIM, its essential to understand the testing methods to accurately determine the materials bulk thermal properties and in its performance.

The most common test is ASTM D5470: Linear Rod Method. This is the standard for measuring the thermal impedance of a TIM. Heat flow is carefully controlled through a test sample of a TIM. Typically, a heater is attached to an aluminum cylinder that has thermocouples arranged in series.

The thermocouples not only report temperature, but also the heat transfer through the known aluminum cylinder. Next, the interface material is compressed between the raised cylinder and an identical lower unit. Finally, a cold plate is attached to the bottom of the assembly to ensure the direction of heat transfer. The assembly can accommodate various material thicknesses and apply a range of pressure to the sample.

Another TIM test is laser flash diffusivity. Here, a small sample of interface material is subjected to a short pulse of laser energy. The temperature rise of the material is then recorded at a very high sample rate. Diffusivity is calculated using the equation shown below.

k = D/ρCp


k= thermal conductivity;

D = thermal diffusivity,

ρ = density of sample,

and Cp = specific heat.

The halftime of the sample is defined as the time between the start of the laser pulse to when the temperature of the back side of the sample has risen to half of its maximum value. The other variable in equation 1 is L, the thickness of the sample, which may be directly measured. Once diffusivity is known, it can be used in equation 2 to calculate thermal conductivity.

This laser flash method is very accurate as long as the density and specific heat are well known. However, it only measures thermal conductivity, as opposed to the ASTM standard which also measures thermal impedance. Thus, a key drawback to laser flash testing is that it doesn’t provide the contact resistance.

In comparisons of interface materials must be carried out by the user to provide meaningful results. Interface material testing procedures are different than heat sink testing methods. When testing several heat sinks it is possible to affix a thermocouple to the component’s case surface or to the heat sink itself and draw direct comparisons of performance. However, this approach will not work if the interface material is changed. To accurately compare interface materials, die-level temperature measurements must be taken, while the same heat sink is used in identical PCB and flow conditions.