Thermal management can be categorized as active and passive. Suitable selection of both types are applied in equipment and devices to obtain optimum energy efficiency in the operating system. Active thermal management is generally associated with a technology that generates energy from external devices, such as fan, liquid coolers, and thermoelectric coolers (TECs), to enhance heat transfer in a system. This type of management is mostly applied for large equipment but features limitations, such as noise pollution, complicated design, and external energy requirement for operation.
Passive thermal management
Among the different thermal management technologies, passive thermal management is the most widely adapted in the electronics industry. Passive thermal management includes small components in micro-scale devices and utilizes thermodynamics in the cooling system. This type of management is preferable, especially for microelectronic devices, due to the advantages of minimal components required in the design, low cost, and ease of operation for small devices. To date, modern electronic devices mostly operate at a scale of 10 nm. The implementation of passive thermal management is suitable for dimension operations that improve energy efficiencies. Passive thermal management can be divided into heat sinks, heat spreaders, heat pipes, and thermal interface materials (TIMS).
What are thermal interface materials (TIMs)?
The electronic devices sector has grown hugely in recent decades, defined by increased miniaturization and integration. As a result of the growing power requirements for these devices, the amount of heat generated in their operation has increased. For example, a modern smartphone can have several billion logic circuits in the main microprocessor. This circuit density creates a significant amount of heat that must be dissipated. If the heat is not adequately dissipated, the life expectancy and performance of the circuits are significantly reduced.
Thermal Interface Materials (TIMs) are a category of products used to aid thermal conduction between mechanically mated surfaces, such as a heat generating semiconductor device (e.g. microprocessor, photonic integrated circuits, etc.) and a heat dissipating device (e.g. heat sink). The addition of TIMs will also enhance material strength and surface adhesion, thus providing good material sustainability and reduce internal thermal resistance.
Microscopic surface roughness and non-planarity of the IC/heat spreader and heat sink surfaces result in asperities between the two mating surfaces. Standard machined surfaces are rough and wavy, leading to relatively few actual contact points between surfaces. The rest of the area is separated by an air-filled gap. These asperities prevent the two solid surfaces from forming a thermally perfect contact due to the poor thermal conductivity of the air that exists in the gaps between two mating surfaces.
The purpose of a thermal interface material is to fill gaps between mating surfaces with a substance that’s has better thermal conductivity than the air that would typically fill those gaps otherwise, in order to improve heat transfer between the two surfaces. The difference can be significant, as typical TIMs conduct heat roughly 100 times better than the air they displace. A variety of products adapted for different use cases are available. Proper thermal management minimizes any thermal fluctuations at these interfaces, which in turn reduces structural damage from overheating, leading to reliable, long-term performance of the electronic device over time. The excess heat generated from a component is transferred to TIMs, where surface contact between materials occur, through conduction. Then, heat is continuously transferred to the heat spreader and is finally released to the environment by convection. This process aims to reduce the temperature rise within the component in a short time and effectively avoid component damage.
The TIM is typically made of a material that is compliant and can fill the voids in between the two surfaces, thereby increasing the effective contact area. An ideal TIM would only fill the existing voids in the interface with a thermally conductive material.
Two types of TIMs are used to facilitate heat removal from the chip:
- TIM-1 is placed between the chip (or die) and the integrated heat spreader (IHS)
- TIM-2 is placed between the IHS and the heat sink.
Types of TIMs
There are different types of TIMs available both commercially as well as in the research and development phase. They can be broadly categorized as one of the following types:
- Polymer matrix composites: different types of carbon fibers combined with a variety of thermosetting and thermoplastic resins, including epoxy, cyanate ester, liquid crystal, nylon, polycarbonate, acrylonitrile butadiene styrene, polybutylene terephthalate, and polyphenylene sulfide.
- Metal matrix composites: silicon carbide particle-reinforced aluminium, beryilia particle-reinforced beryilium, carbon fiber-reinforced aluminium, copper-tungsten, copper molybdenum, aluminium silicon, and Invar silver.
- Carbon/carbon composites: carbon nanofibers, vapor-grown carbon fibers, nano-graphene platelets, pyrolitic graphite, and other carbon/ carbon mixes.
Advanced thermal interface materials can provide better thermal management characteristics as well as improvements in weight and strength and are being integrated into a range of thermal management solutions, including thermal greases, PCMs, thermal gels, thermal adhesives, thermal pads, thermal tapes, solder and carbon-based TIMs. Carbon-based TIMS have been extensively researched in recent years.
Carbon-based TIMs
Carbon materials, e.g., carbon black (graphite), graphite NPs, diamond, etc. possess high intrinsic thermal conductivities and have been extensively investigated and developed in the past decade for TIM applications. Some of these materials, e.g., carbon black, can be coated on TIM to enhance its specific contacting thermal conductance. Similarly, graphite and diamond particles can be reinforced into desired matrix to enhance the thermal conductance. Besides the filler applications in host matrix, Gr- and CNT-based structures can also be directly fabricated into TIMs.
Multi-walled nanotubes (MWCNT)
The thermal conductivity of an isolated CNT at room temperature can reach 6600 W/mK by combining equilibrium and nonequilibrium molecular dynamics simulations with accurate carbon potentials. However, It is reported that the thermal conductivity of CNTs in transverse direction is much lower than that in the longitudinal axis. Nevertheless, the CNTs has been used as TIMs, which show good performance in dissipating heat for electronic devices. The thermal conductivity of CNTs is influenced by various factors, such as orientation, alignment, aspect ratio of the CNTs, and topological defect.
The conductivity of individual CNT is about 3000 W/mK for multi-walled CNTs (MWCNTs) at room temperature and about 2400 W/mK for single-walled CNTs (SWCNTs). In addition to the high thermal conductivity, low transverse elastic modulus of CNTs allows them to adapt to and conform to considerable amount of external loads without any permanent atomic alterations. Thus, they can absorb stresses induced by the mismatched coefficient of thermal expansion (CTE) in a device, resulting in lower interfacial thermal resistance. The thermal resistance of the CNT arrays is as low as 7 mm2 K/W, reduced significantly compared with the state-of-art commercial TIM. Furthermore, CNTs possess excellent properties, such as good chemical stability and low CTE.
Single-walled carbon nanotubes (SWCNTs)
SWCNTS are used as Conductive fillers in TIMs. SWCNTs have extremely high thermal conductivity, allowing more efficient transfer of heat between surfaces. Thermal conductivity over 3000 W/mK has been measured. Well-dispersed SWCNTs at loadings of 1-5% weight in TIMs can improve thermal conductivity by over 50% compared to traditional TIMs. SWCNT networks efficiently conduct phonons even across interfaces, reducing thermal interface resistance. TIMs using SWCNTs are mainly used in advanced electronics cooling such as CPUs, GPUs, insulated gate bipolar transistors (IGBTs).
Vertically aligned CNTs (VACNTs)
Vertically aligned CNTs (VACNTs) possess the intrinsic, extraordinary nanoscale properties (mechanical, electrical, and thermal) of individual CNTs, but present them in a hierarchical and anisotropic morphology. VACNTs are tubular nanostructures made of graphitic carbon (carbon nanotubes) grown on a substrate with their axis perpendicular to the substrate.
Vertically aligned CNTs (VACNTs) are ideal TIMs, as they offer both high thermal conductivity and good mechanical compliance. CNT arrays enhance the contact thermal conductivity and reduce the thermal interface resistance. The thermal interface resistance is reduced by making better contact at the interface or densified the CNTs.
CNTs grown in arrays with relatively high density can also further improve the thermal conductivity or reduce the contact thermal resistance. Furthermore, VACNT-TIMs show good stability when subjecting to thermal cycling and high-temperature baking, thus assuring long-term reliability in application.
The thermal conduction of the CNT-TIMs is enhanced if replacing the air in the CNTs with a thermally conductive material. Therefore, using CNTs as fillers filled with polymer as matrix, such as EP, polyimide (PI), PMMA, can combine the high thermal conductivity of CNT and the compliance of polymers to provide a better performance as TIMs. The thermal conductivity of these TIMs depends strongly on the matrix materials, the quality, loading percentages, dispersion, density and orientation of CNTs.
Graphene
Fillers: Graphene is utilized as thermal conductive fillers for polymer-based TIMs. Graphene as a filler material has been shown to increase the thermal conductivity of polymer by about 20–30 times. Such improvement is much higher than using CNTs at the same filler fraction, as well as easier to manufacture. The thermal conductivity of Gr-filled composites is higher than that of pure matrix, such as EP, PI, commercial grease and phase change materials. There are many factors that affects the thermal conductivity of the composites, such as the dispersion of Graphene in the matrix, orientation and arrangement, the number of Graphene layers, the size of the Graphene, functionalization, aspect ratio, thermal percolation threshold, and forming process of composites, etc.
Foam: Graphene Foam is a promising candidates for TIMs, due to high flexibility under compression, lightweight, capability of free standing, and isotropy. They offer high through-plane thermal conductivity and thinner BLT, which can lower the overall thermal resistance of TIMs.
Aerogels: Graphene aerogels display promise as TIMs due to the continuous network and the high thermal conductivity of Graphene. The pore size of graphene aerogels is smaller than graphene which results in m more thermal conductive paths. Although low density and high porosity of Graphene r aerogels lead to poor performance in thermal transport, the super-elastic property makes the Gr aerogels maintain its structure integrity under compression. Thus, denser Graphene aerogels with high thermal conductivity can be obtained through compression or increasing precursor density.
Other nanomaterials in TIMs
Metal nanoparticles – Highly conductive fillers like silver, gold or copper nanoparticles.
Ceramic nanoparticles – Alumina, boron nitride, aluminum nitride and silicon carbide nanoparticles used. Thermal conductivities over 200 W/mK.
