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.
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).
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.  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.”
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 firstname.lastname@example.org.