By John Clifford, Technical Specialist
Measuring the Thermal Conductivity of Thermal Interface Materials (TIMs)
Thermal conductivity is a critical performance indicator for Thermal Interface Materials (TIMs), which quantifies their ability to transfer heat. The TIM Tester (ASTM D5470) is a popular method used to measure the thermal impedance of TIMs and can be used to calculate an apparent thermal conductivity. However, relying solely on TIM Tester measurements may miss several specifics of the sample, leading to an inaccurate understanding of overall performance and increased risk of potential safety issues when applied to a thermal management system. Here are five added benefits for testing TIMs with the Modified Transient Plane Source (MTPS).
Most TIMs use nanoparticles to enhance their thermal conductivity. However, the filler distribution can substantially impact the performance of the TIM. For instance, particle settling can cause a higher thermal conductivity near the bottom of the sample and a lower one at the top. TIM Testers cannot detect side-to-side variations, providing instead an average thermal conductivity of the sample. Inaccurate data of this kind can lead to TIMs being applied to systems resulting in non-ideal performance and heightening potential for failure.
Anisotropic materials can improve heat dissipation in situations where directionality is important. TIM Testers cannot differentiate in-plane and through-plane of anisotropic materials. Unaccounted for this can have significant implications on material performance. Transient methods such as Transient Plane Source (TPS) or Modified Transient Plane Source (MTPS) have a long history of being applied for testing of anisotropic materials.
3. Representative Conditions
Measuring materials under representative conditions is essential for obtaining accurate thermal conductivity data, this is especially true for TIMs. The TIM Tester requires specific testing conditions as outlined in the ASTM standard which may not reflect the realistic application conditions of the TIM. This includes factors such as temperature and force applied during testing (sample compression). This can lead to inaccurate data and compromised product performance when applied to real world applications.
Precision and repeatability are crucial when measuring any property. However, ASTM D5470 for TIM Testers does not specify a precision or bias statement, making it challenging to acquire reliable data especially for product certification or technical specifications. The standard states that the thermal conductivity values for the same material measured in different laboratories are expected to be within 18% of the mean value, which does not meet the criteria required for many advanced industry applications.
The TIM Tester, like all steady-state methods, requires the system temperature to reach steady-state equilibrium before valid data is obtained, which can take upwards of an hour or longer depending on the material. This seriously limits the rate of data generation that is possible and can bottleneck groups dealing with high-throughput requirements. Transient methods provide an accelerated approach with measurement and analysis possible in only a few minutes or even seconds depending on the method employed.
In conclusion, while TIM Testers are useful for measuring thermal impedance and can provide a calculated apparent thermal conductivity value, relying solely on them may lead to an incomplete understanding of material performance. Transient methods can provide not only a faster, but more well-defined and representative measurements of TIMs. This can greatly assist in understanding product performance when applied to real world situations.
This blog post is a part of our Thermal Interface Materials (TIMs) & Dielectrics application page.
About the Author
John Clifford is a marketing intern at C-Therm. He is currently in his fourth year of his Chemical Engineering degree at the University of New Brunswick in Fredericton.