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Technology Review: Thermal Interface Materials

Posted on February 19, 2019 by | 4 comments

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

Qpedia continues its review of technologies developed for electronics cooling applications. We are presenting selected patents that were awarded to developers around the world to address cooling challenges. After reading the series, you will be more aware of both the historic developments and the latest breakthroughs in both product design and applications.

Thermal Interface Materials
This Technology Review will focus on recent developments in Thermal Interface Materials. (Wiklmedia Commons)

We are specifically focusing on patented technologies to show the breadth of development in thermal management product sectors. Please note that there are many patents within these areas. Limited by article space, we are presenting a small number to offer a representation of the entire field. You are encouraged to do your own patent investigation.

Further, if you have been awarded a patent and would like to have it included in these reviews, please send us your patent number or patent application.

In this issue our spotlight is on thermal interface materials.

There are many U.S. patents in this area of technology, and those presented here are some recent. These patents show some of the salient features that are the focus of different inventors.

Thermal Interface Material with Carbon Nanotubes
US 7253442 B2 – Hua Huang, Chang-Hong Liu and Shou-Shan Fan


A thermal interface material includes a macromolecular material, and a plurality of carbon nanotubes embedded in the macromolecular material uniformly. The thermal interface material includes a first surface and an opposite second surface. Each carbon nanotube is open at both ends thereof, and extends from the first surface to the second surface of the thermal interface material. A method for manufacturing the thermal interface material includes the steps of: (a) forming an array of carbon nanotubes on a substrate; (b) submerging the carbon nanotubes in a liquid macromolecular material; (c) solidifying the liquid macromolecular material; and (d) cutting the solidified liquid macromolecular material to obtain the thermal interface material with the carbon nanotubes secured therein.

An object of the present invention is to provide a thermal interface material having a reduced thickness, small thermal interface resistance, good flexibility and excellent heat conduction. To achieve the above-mentioned object, the present invention provides a thermal interface material comprising macromolecular material and a plurality of carbon nanotubes embedded in the macromolecular material uniformly. The thermal interface material also comprises a first surface and an opposite second surface. Each carbon nanotube is open at two ends thereof, and extends from the first surface to the second surface of the thermal interface material.

Unlike in a conventional thermal interface material, the carbon nanotubes of the thermal interface material of the present invention are disposed in the macromolecular material uniformly and directionally. Thus, each carbon nanotube of the thermal interface material can provide a heat conduction path in a direction perpendicular to a main heat absorbing surface of the thermal interface material. This ensures that the thermal interface material has a high heat conduction coefficient. Furthermore, the thickness of the thermal interface material of the present invention can be controlled by cutting the macromolecular material. This further enhances the heat conducting efficiency of the thermal interface material and reduces the volume and weight of the thermal interface material.

Moreover, each carbon nanotube is open at two ends thereof, and extends from the first surface to the second surface of the thermal interface material. This ensures the carbon nanotubes can contact an electronic device and a heat sink directly. Thus, the thermal interface resistance between the carbon nanotubes and the electronic device is reduced, and the thermal interface resistance between the carbon nanotubes and the heat sink is reduced. Therefore, the heat conducting efficiency of the thermal interface material is further enhanced.

Transferrable Compliant Fibrous Thermal Interface
US 6676796 – Michael Pinter, Nancy Dean, William Willet, Amy Gettings and Charles Smith

In one aspect of the invention there is provided a fibrous interface material sandwiched between two layers of a removable paper or release liner. The interface comprises flocked, e.g. electroflocked, mechanically flocked, pneumatically flocked, etc., thermally conductive fibers embedded in an adhesive or tacky substance in substantially vertical orientation with portions of the fibers extending out of the adhesive. An encapsulant is disposed to fill spaces between portions of the fibers that extend out of the adhesive, leaving a free fiber structure at the fiber tips.

Another aspect of the invention is a method of making a fibrous interface. In the method, thermally conductive fibers of desired length are provided and, if necessary, cleaned. A release liner is coated with an adhesive or tacky substance, and the fibers are flocked to that release liner so as to embed the fibers into the adhesive or tacky substance with a portion of the fibers extending out of the adhesive.

The adhesive is cured and the space between fibers if filled with a curable encapsulant. A second piece of release liner is placed over the fiber ends. Then the fibers in the adhesive with the release liner over the fibers in the adhesive with the encapsulant in the spaces between the fibers is compressed to a height less than the normal fibers’ length and clamped at the compressed height.

Thereafter the encapsulant is cured while under compression to yield a free fiber tip structure with the fiber tips extending out of the encapsulant.

Liquid Metal Thermal Interface for an Integrated Circuit Device
US 7348665 B2 – Ioan Sauciuc and Gregory Chrysler


One possible solution to meet the heat dissipation needs of microprocessors and other processing devices is to employ an active cooling system—e.g., a liquid based cooling system that relies, at least in part, on convective heat transfer initiated by the movement of a working fluid—rather than (or in combination with) heat sinks and other passive heat removal components. Disclosed herein are embodiments of a cooling system for an integrated circuit (IC) device—as well as embodiments of a method of cooling an IC device—wherein the cooling system includes a liquid metal thermal interface that is disposed between a die and a heat transfer element, such as a heat spreader or a heat sink. Embodiments of a method of making a liquid metal thermal interface are also disclosed.

This patent is for a liquid metal thermal interface for an integrated circuit die. The liquid metal thermal interface may be disposed between the die and another heat transfer element, such as a heat spreader or heat sink. The liquid metal thermal interface includes a liquid metal in fluid communication with a surface of the die, and liquid metal moving over the die surface transfers heat from the die to the heat transfer element. A surface of the heat transfer element may also be in fluid communication with the liquid metal.

Per Figure 2, the cooling system 200 is coupled with an IC die 10. During operation of the IC die 10, the die may generate heat, and the cooling system 200 is capable of dissipating at least some of this heat, such as may be accomplished by transferring heat away from the IC die 10 and to the ambient environment. The IC die 10 may comprise any type of integrated circuit device, such as a microprocessor, network processor, application specific integrated circuit (ASIC), or other processing device.

Heterogeneous Thermal Interface for Cooling
US 7219713 B2 – Jeffrey Gelorme, Supratik Guha, Nancy LaBianca, Yves Martin and Theodore Van Kessel

The present invention is a thermal interface for coupling a heat source to a heat sink. One embodiment of the invention comprises a mesh and a liquid, e.g., a thermally conductive liquid, disposed in the mesh. The mesh and the thermally conductive liquid are adapted to contact both the heat source and the heat sink when disposed there between. In one embodiment, the mesh may comprise a metal or organic material compatible with the liquid. In one embodiment, the liquid may comprise liquid metal. For example, the liquid may comprise a gallium indium tin alloy. A gasket may optionally be used to seal the mesh and the liquid between the heat source and the heat sink. In one embodiment, the heat source is an integrated circuit chip.

In another aspect of the invention, a method for cooling a heat source with a heat sink is provided. In one embodiment, the method includes providing a thermal interface having a mesh and a liquid disposed in the mesh. The thermal interface is interposed between the heat source and the heat sink, such that the mesh and the liquid are in contact with the heat source on a first side of the thermal interface and in contact with the heat sink on a second side of the thermal interface.

In another aspect of the invention, a method of fabricating a thermal interface for assisting the thermal transfer of heat from a heat source to a heat sink is provided. In one embodiment, the method includes providing a mesh. A liquid is disposed in the mesh in sufficient quantity to substantially fill the mesh. The liquid comprises liquid metal. Optionally, the liquid metal may subsequently be frozen in place.

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. To register for Qpedia and to get access to its archives, visit https://www.qats.com/Qpedia-Thermal-eMagazine.

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Industry Developments: Advances in Thermal Interface Materials for Electronics Cooling

Posted on January 24, 2018 by | 2 comments

By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)

For decades, thermal interface materials (TIMs) have been used as pathways allowing heat to flow from one location to another. TIMs are often part of cooling systems that remove heat from component dies by dissipating it into heat spreaders, such as heat sinks, and ultimately out of the dies’ surrounding enclosures.

As a product line, TIMs have continuously evolved, driven by market needs for higher thermal conductivity, lower thermal impedance, new applications and lower costs.

Thermal Interface Materials

Figure 1. Thermal Interface Materials are Used Both Inside and Outside Chip Packages. (Indium) [1]

From a broad view, most TIMs fall into three material categories. Some are made from elastomers or other polymers with a thermally-conductive filler added. Other TIMs are solder-based. Like elastomers, these solder-based TIMs are soft and conformable to mating surfaces, filling air gaps that compromise thermal transfer. Finally, newer carbon-based TIMs have emerged that demonstrate superior performance, though many of these are not yet commercialized.

Here is a brief look at recent developments within the major TIM categories:

A New TIM Filler

A new generation of polymer-based TIMs uses boron nitride nanosheet (BNNS) fillers to enhance thermal conductivity. BNNS is a two-dimensional crystalline form of hexagonal boron nitride (h-BN), also known as white graphene. BNNS ranges in thickness from just one to a few atomic layers. It has a similar geometry to its all-carbon analog graphene, but some very different properties. For example, graphene is highly electrically conductive while BN nanosheets are electrical insulators.

Figure 2. Edges of boron nitride nanosheets are atoms of all boron, all nitrogen, or alternating elements. (Wikimedia Commons)

Hexagonal boron nitride (h-BN) has other desirable properties, including a large surface area, high-thermal transport, and chemical inertness. The thermal conductivity of bulk h-BN can reach 400 W/mK at room temperature. [3]

A recent study from Rice University, which continues to expand on its original simulations of graphene’s effect on nanoscale heat transfer, demonstrated that an h-BN thin film composed layer-by-layer of laminated h-BN nanosheets can enhance lateral heat dissipation on a substrate, in this case glass. Thermal performance improved with the BN coating due to its anisotropic thermal conductivity. It had a high in-plane thermal conductivity of 140 W/mK for spreading and a low cross-plane thermal conductivity of 4 W/mK to avoid a hot spot beneath the tested device. [4]

Researchers have also created simulations showing that 3-D structures of h-BN planes connected by boron nitride nanotubes could transfer heat (move phonons) in all directions, whether in-plane or across planes. The number and length of the nanotubes connecting the h-BN layers have an effect on heat flow: more and/or shorter pillars slow conduction, while longer pillars speed heat transfer along.

Figure 3. 3-D structure of highly thermally conductive h-BN sheets connected by BN nanotubes. (Shahsavari Group/Rice University)

Solder-Based TIMs

With ever-increasing power and heat dissipation needs across the electronics industry, solder-based TIMs may be better suited to take the heat away from dies than thermal grease where electrical insulation isn’t required.

Issues with thermal grease include:

  • Grease has a low bulk thermal conductivity of 3-12 W/mK. Some solder-TIMs provide a high bulk thermal conductivity of 87 W/mK.
  • Over time, thermal grease tends to pump-out and migrate away from the center of the power die. It gets hotter and can fail prematurely. There is no pump-out with a solder-TIM.
  • Over time, grease tends to bake-out and dry (becomes powdery), thereby increasing thermal resistance and reducing heat-dissipation effectiveness. With solder-TIMs, there is no bake-out. [5]

Figure 4. SMA-TIMs conform to surface disparities over time to increasingly reduce thermal resistance. (Indium Corp.) [6]

Recent solder-based TIMs developed by Indium Corporation include a new SMA-TIM (soft metal alloy). This is made from an indium solder base and offers uniform thermal resistance at lower applied stresses in compressed interfaces. It is provided as a compressible metal foil that can be used as a TIM between a heat source and a heat sink, heat spreader, or heat pipe.

The malleability of the indium minimizes surface resistance and increases heat flow (conductance). Over time, the malleability of the solder helps fill the interface gaps even better. Thus, thermal interface resistance decreases over time as opposed to thermal grease where the thermal interface resistance increases over time. [7]

Another newer indium-containing material has been developed for use in the TIM 1 position, between the die top and its case. The material is part of a system, developed by Indium Corporation, called mdTIM. It provides a thermal conductivity of 87 W/mK.

While pure indium metal has a superb thermal transfer rate, air or gas pockets (voids) can degrade the performance of the material. These voids are created by entrapped air or gasses produced by flux component evaporation that fail to escape during reflow.

Indium’s mdTIM uses a patented system of materials and reflow technology does not use flux so there are no outgassing issues.

Carbon-Based TIMs

The very high thermal conductivity of pure carbon has long made it attractive for use in TIMs. Today’s carbon-based TIM fillers include diamond, carbon nanotubes (CNT), graphite and graphene. Often these fillers are dispersed in a spreadable (grease-like) polymer matrix.

In some cases, different forms of carbon fillers are being combined. For example, highly thermally conductive CNT have been mixed with less expensive carbon substrates like graphite and graphene to reduce costs but still deliver very high thermal conductivity.

Recent research has been made with graphite nanoplatelets (GNP) in thin thermal interface layers. These studies concerned the through-plane and in-plane alignment of GNP in a spreadable matrix. When dispersed, the GNP fillers take a naturally in-plane alignment, meaning the great majority of heat flow is in parallel to an interface. However, at the same time, the desired through-plane heat transfer from one surface to the other is much less. [8]

Figure 5. The top SEM images are graphite nanoplatelets with in-plane alignment. Bottom images show hybrid mix of GNP with a 45% volume of Al2O3 spheres. [8]

A solution was found by adding spherical microparticles. Spherical Al2O3 and Al filler particles were tested. The hybrid filler formulations resulted in enhanced through-plane thermal conductivity by disrupting the natural in-plane alignment of the GNP. This led to the disruption of the GNP in-plane alignment and the improvement of the through-plane thermal conductivity of the tested thermal greases.

Costs and other factors pose development challenges to TIMs with carbon-based heat transfer schemes. But given the high thermal conductivity and various configurations available from carbon-based materials, these will likely be at the heart of many upcoming performance advancements in TIMs.

References
1. Indium Corporation, http://www.indium.com/thermal-management/tim/
2. http://news.rice.edu/2015/07/15/white-graphene-structures-can-take-the-heat/
3. http://pubs.acs.org/doi/10.1021/acsami.5b03967
4. Nanoscale, http://pubs.rsc.org/en/content/articlelanding/2017/nr/c7nr07058f#!divAbstract
5. IEEE Xplore, http://ieeexplore.ieee.org/document/6142407/%5D
6. Engineering 360, http://www.globalspec.com/FeaturedProducts/Detail/Indium/Thermal_Interface_MaterialHeatSpring/256323/1
7. Indium Corporation, http://www.indium.com/thermal-interface-materials/heat-spring/
8. Nature, https://www.nature.com/articles/srep13108.pdf

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

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Thermally Conductive Adhesive Tape

Posted on August 24, 2015 by | Leave a comment

Top 3 questions customers ask when dealing with Thermally conductive adhesive tape for heat sinks.

The three most frequently asked questions about thermally conductive adhesive tape specifically made for heat sinks. This video covers how to apply the adhesive step-by-step for easy installation and removal.

Next, are the dimensions we carry for precise matching with your heat sinks. Finally, we go over the types and brands we carry. We only the best on the market from the companies who manufacture thermally conductive adhesive tape for heat sinks.

For more information on thermally conductive adhesive tape for heat sink please visit www.qats.com

Please don’t forget to watch our installation and removal video here: How to Remove Thermal Interface Material

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Testing Thermal Interface Materials

Posted on June 19, 2012 by | 1 comment

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

Where:

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.

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How To Video shows Best Ways to Remove Thermal Tape from Heat Sinks

Posted on December 23, 2011 by | Leave a comment

In this Advanced Thermal Solutions “how to” video, we teach you how to remove three kinds of thermal interface material from a heat sink. Thermal Tape, Phase Change Material and Thermal Grease.

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