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

Measuring the Thermal Conductivity of TIMs

Trident Thermal Conductivity Instrument

C-Therm’s Trident Thermal Conductivity Instrument employs three methods of thermal conductivity measurement, allowing for greater versatility depending on the TIM sample type

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, which conforms to ASTM D7984 provides many unique capabilities in regards to TIMs. The MTPS can test solids, liquids, powders, and pastes, across a broad temperature range (-50°C to 200°C), making it versatile for many different TIMs. It can also be outfitted with the Compression Test Accessory (CTA) to test under representative loads.

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 – Additives

Another aspect to consider when testing TIMs are that many have filler materials added into the base material to improve thermal performance. 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. 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. Furthermore, due to the short test times the MTPS can be used for in-line or on-line process monitoring applications. By taking rapid thermal conductivity and effusivity values, the MTPS can be used for quality control purposes on the dispensing of TIMs.

  • Thin film dielectric materials are becoming increasingly common in the electronic industry, necessitating accurate thermal characterization

    Thin film dielectric materials are becoming increasingly common in the electronic industry, necessitating accurate thermal characterization

  • Many TIMs employ filler materials to improve their heat transfer performance; however, an uneven distribution of these particles can drastically change thermal behavior. The thermal mapping of the MTPS shows the property impact from poor distribution.

    Many TIMs employ filler materials to improve their heat transfer performance; however, an uneven distribution of these particles can drastically change thermal behavior. The thermal mapping of the MTPS shows the property impact from poor distribution.

  • TIMs have long been used on small scale electronics, from CPUs to phone batteries. However, as EV batteries demand greater volumes, particle settling is becoming a large problem.

    TIMs have long been used on small scale electronics, from CPUs to phone batteries. However, as EV batteries demand greater volumes, particle settling is becoming a large problem.

  • Large volumes of filled TIMs will eventually experience sedimentation, where the filled particles gather near the bottom, causing drastically different thermal performance.

    Large volumes of filled TIMs will eventually experience sedimentation, where the filled particles gather near the bottom, causing drastically different thermal performance.

  • The MTPS can be used to monitor the degree of sedimentation for quality control purposes; over time the thermal conductivity drastically increases as the filler settles towards the sensor.

    The MTPS can be used to monitor the degree of sedimentation for quality control purposes; over time the thermal conductivity drastically increases as the filler settles towards the sensor.

TIM 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.

  • Crosslink Technology Ltd.

    Our C-Therm MTPS combines speed, precision and accuracy to provide our development, quality and production teams trustworthy thermal conductivity measurements. It has enabled us to more rapidly customize solutions for our customers as well as provide quick and timely customer support.

    J.K.,
    Lab Manager

    More Testimonials

  • Eaton Cooper Power Systems

    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)

    More Testimonials

  • Huntsman Advanced Materials

    It’s user friendly, easy to use, easy to clean … means very productive and time saving. The MTPS technique supports us on knowing the level of thermal conductivity for the resin systems, a critical value to support thermal conductivity projects.

    Yen Nguyen,
    Manager

    More Testimonials

Case Highlights

Thermal Impedance and Apparent Thermal Conductivity

A TIM Tester diagram from ASTM D5470, a steady state method with the sample placed between the hot and cold plate to measure thermal impedance and apparent thermal conductivity

Thermal impedance is a combination of thermal resistance and thermal contact resistance. This is often measured using a TIM Tester (ASTM D5470), pictured above. With this data, apparent thermal conductivity can be calculated; however, while thermal impedance can be a useful property, apparent thermal conductivity can be a misleading value. TIM Testers require very specific pressure requirements, temperature differentials, and sample thickness measurement, which makes it difficult to achieve data at representative conditions. Furthermore, as a TIM Tester is a steady state method, it reports apparent thermal conductivity for the sample on average, meaning it is unable to detect uneven properties. This is especially important for heterogeneous TIMs, as particle settling can cause noticeable differences in thermal conductivity from the top and bottom of the sample. These uneven properties can cause performance and safety issues if they are not detected. Learn more about thermal impedance here.

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.

Particle Sedimentation in Large Volumes

As EVs become more and more common in the marketplace, the volume requirement for TIMs drastically increases, particularly in the form of two component (2k) adhesives. These materials are needed for the thermal management of the battery pack, and due to its size, these are often dispensed out of large drums. However, over time the conductive fillers in the TIM will begin settling towards the bottom of the drum due to a combination of gravity and intermolecular forces. When this happens, the properties of the 2k adhesive will be noticeably different, depending on if the top or bottom of the drum is sampled. This can be detrimental to the performance of an EV, as if a TIM is dispensed with a much lower thermal conductivity than the specifications call for, performance and safety issues arise. Therefore, it is crucial to be able to monitor the degree of sedimentation of TIMs – this can be done using thermal conductivity and effusivity values, using the short test times of the MTPS sensor.

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