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Thermal Interface Materials (TIMs) & Dielectrics Thermal Interface Materials (TIMs) & Dielectrics

TIMs Fundamentals

As electronics become smaller and more powerful, the challenges associated with thermal management have become more intense and there is a need to explore options for cooling and thermal dissipation of the electronic. Thermal interface materials (TIMs) are a common material used in electronics to transfer the heat away from hot components to cooling hardware. TIMs are designed to fill in air gaps, which act as thermal insulation, to provide for improved heat transfer, lower thermal resistance, and overall better cooling of the electronic.

TIM illustration surface zoom

Representation of two surfaces (A, top; B bottom) in contact with a heat flow across the interface without (left) and with (right) thermal interface material applied.

Heat dissipation is also critical for electric vehicles (EVs), particularly for thermal management systems (TMSs) around electronics and batteries. Without proper thermal management, lithium-ion batteries can overheat during charging or discharging, and non-uniform temperature distribution within the battery packs may also happen, directly affecting optimal performance. So, the same principles can be applied here, as TIMs play an important role in the effectiveness of the TMSs, ensuring heat is conducted efficiently away from the key components by filling air gaps, and reducing interfacial thermal resistance between the heat sources and the dissipation components.

In summary, TIMs act as an intermediate to aid in heat transfer, usually between a heat-generating, temperature-sensitive component and a heat sink. They can be found in various formats, including pastes/greases, adhesives, potting compounds, phase change materials (PCMs), thermal gap pads, and more. Proper TIM selection will depend on a variety of factors, of which temperature conditions, material compatibility, and size/space availability can be viewed amongst the most important. In all cases, however, thermal conductivity plays a crucial role in the research and design of new thermal interface materials. Researchers seeking better performing TIMs create materials with high thermal conductivity values to improve heat transfer between layers. In this context, thermal conductivity can be used as a guiding metric to quantify the expected heat dissipation performance. It is therefore important to select an accurate method for thermal characterization for both performance improvement and thermal runaway avoidance.

Measuring Thermal Conductivity of TIMs

Different methods can be applied to measure the thermal conductivity of thermal interface materials, such as the C-Therm’s Modified Transient Plane Source (MTPS) and the Transient Plane Source (TPS). C-Therm’s Trident is the perfect solution in this context as it allows for having both methods available in one instrument.

MTPS

C-Therm’s MTPS method, which conforms to ASTM D7984, provides unique capabilities with respect to the measurement of TIMs being capable of testing not only solids but also liquids, powders, and pastes across a broad temperature range (-50ºC to 200ºC). This allows for the testing of all listed TIM formats using a single “plug-and-play” method.

Trident Thermal Conductivity Testing Heat Transfer Fluid with MTPS

Trident Thermal Conductivity Testing Heat Transfer Fluid with MTPS

TPS

The Flex TPS method employs a double-sided sensor to simultaneously determine thermal conductivity, thermal diffusivity, and specific heat capacity of materials from a single measurement, conforming to ISO standard 22007-2. This technique provides the user the greatest flexibility and control over experimental parameters and avoids the use of any contact agents, however, it is recommended for more experienced users.

Special Consideration – Compression

TIMs encompass a wide range of material types but are often compressible, introducing added complexity when it comes to properly characterizing their thermal conductivity. Many thermal characterization techniques rely on physical contact between the sample and the sensor which is accomplished by some form of applied force. The extent of the force applied can result in changes to the TIM structure, densifying under the applied load. Most TIMs are characterized under a controlled or specified range of compressions as this is well known to have an influence on thermal performance. It is therefore important that the method used can control for this to ensure data is collected in a way as to not bias the results.

Although methods such as the double-sided TPS can be used for TIM characterization, they might not be ideal in all situations. The ISO standard 22007-2 states “With soft materials, the clamping pressure shall not compress the specimen and thus change its thermal transport properties”, meaning the TIM can only be accurately tested in an “uncompressed” state which may not give truly meaningful data. On the other hand, the MTPS can be paired with C-Therm’s Compression Test Accessory (CTA), allowing for testing under a range of applied forces and compression states for truly representative results.

Compression Test Accessory

Compression Test Accessory

Special Consideration – Additives

Another aspect to consider when testing TIMs are that many have filler materials added into the base material to improve thermal performance. These additives can take a material from < 1W/mK into the 10+ W/mK range. Filler loading and dispersion are important factors to optimize to ensure the end-product does not result in “hot spots” and can evenly dissipate the generated heat.

Electric motor with conductive potting compound

Electric motor with conductive potting compound

Due to the relatively small active area of the MTPS (<18 mm diameter) it can be used to thermally map materials to ensure even dispersion. Also, important to note is that the MTPS works based on a pre-selected calibration. This reduces operator bias and allows for fast and easy testing. Due to the significant influence of additives, the MTPS calibrations may be pushed outside their preset limits for some TIMs. To overcome this, the MTPS calibrations can be refined by manually inputting material density (p) and heat capacity (Cp), which allows for truly accurate measurement of the filled TIM. This input requirement is similar in nature to how traditional Laser Flash methods operate following ASTM E1461. While not required in all cases, the manual input can be beneficial for certain test scenarios, especially further downline such as product certification.

A cured potting compound was tested using both MTPS and TPS methods under minimal applied load for comparability. The data is provided below.

Thermal conductivity measurements using both MTPS and TPS methods.

Thermal conductivity measurements using both MTPS and TPS methods.

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    My C-Therm TCi is working well and it has been used to generate valuable data. The instrument is a great addition to our lab’s testing capabilities.”

    Richard Baumann,
    Eaton Cooper Power Systems (Sector: Dielectric)

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Case Highlights

Thermally Conductive and Highly Electrically Resistive Grease

The basis behind thermal interface materials is the use of a polymer base such as silicone oil or epoxy resin and the addition of high thermally conductive fillers such as ceramics or metals. This particular study focuses on using a gallium-alloy based filler due to its high thermal conductivity value (39 W/mK) pure & 13.6 W/mK oxidized)

Four different mixtures were tested with liquid metal fractions of 60%, 71.4%, 77.8% and 81.8%. The graph below demonstrates the experimentally obtained thermal conductivity values of the liquid metal poly greases, using the C-Therm’s MTPS method.

TCi thermal grease measurement

Figure 1. Experimentally measured (using C-Therm’s MTPS method) and theoretically calculated thermal conductivities of the LMP thermal greases [1]

These values show that the addition of liquid metal significantly improves the thermal conductivity of the thermal interface greases.

[1] Mei, S., Gao, Y., Deng, Z., and Liu, J. (January 24, 2014). “Thermally Conductive and Highly Electrically Resistive Grease Through Homogeneously Dispersing Liquid Metal Droplets Inside Methyl Silicone Oil.” ASME. J. Electron. Packag. March 2014; 136(1): 011009.

Thermal Paste

Thermal pastes/greases are a longstanding, traditional TIM used in many electronic devices. As with any TIM, they act as an intermediate to aid in heat transfer, usually between a heat-generating, temperature-sensitive component, and a heat sink. By removing the void space (air), the TIM provides a better path for heat flow, thus reducing the overall thermal load on the system.

Thermal conductivity of these non-solid materials can be tedious to measure with more traditional measurement techniques. The following data demonstrates the simplicity and accuracy of the Modified Transient Plane Source (MTPS) technique for the measurement of a standard thermal grease compound.

TIMs MTPS 2

Five total measurements were taken with the MTPS, and the average value is reported. All testing was done under ambient conditions. Compared to the reference value (0.735 W/mK), results from the MTPS are within 6%, demonstrating good agreement.

SIMPLIFYING THERMAL CONDUCTIVITY

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