Thermal Interface Material Applications Research Articles

Thermal conformance parameters for assessment of heat transfer between similar and dissimilar metal contacts

AbstractA novel approach to assess the thermal conformance between two metallic materials under transient conditions was proposed in the present investigation. Thermal conformance parameters (ƞ, ϴ, tg) were defined to quantify the contact condition between a metal–metal interface. To assess the effect of load and thermophysical properties of the sink and source materials on the degree of thermal conformance, a thermal conformance assessment parameter (TCAP) was proposed. Heat flux transients at the thermal interface was estimated by using an inverse heat conduction approach for various similar and dissimilar metallic surfaces in contact such as Cu─Cu, Al─Al, Al─Cu, and Cu─Al under both load and no load conditions. Commercially available silicone grease (SG) and thermal grease (CTG) were used as thermal interface materials (TIMs). The thermal conformance parameters increased with the increase in load for all the combinations of interfaces with and without TIMs. It was observed that, except for the copper–copper combination, thermal conformance parameters showed a linear relation with the TCAP. The enhancement in the heat transfer due to the application of load and TIM was validated by determining the maximum temperature difference (∆Tmax) across the interface. The experimental study revealed that the ∆Tmax decreases with the application of load and application of TIM leading to enhanced heat transfer. For the copper–copper combination, the thermal conformance depended solely on the load applied. Due to the lower thermal resistance offered by copper source/sink materials, the interfacial resistance between them becomes a dominant factor. The effect of TIM on heat absorbed by the sink was significant for the Cu/Cu interface.

Biomass- and Carbon Dioxide-Derived Polyurethane Networks for Thermal Interface Material Applications.

Recent environmental concerns have increased demand for renewable polymers and sustainable green resource usage, such as biomass-derived components and carbon dioxide (CO2). Herein, we present crosslinked polyurethanes (CPUs) fabricated from CO2- and biomass-derived monomers via a facile solvent-free ball milling process. Furan-containing bis(cyclic carbonate)s were synthesized through CO2 fixation and further transformed to tetraols, denoted FCTs, by aminolysis and utilized in CPU synthesis. Highly dispersed polyurethane-based hybrid composites (CPU-Ag) were also manufactured using a similar ball milling process. Due to the malleability of the CPU matrix, enabled by transcarbamoylation (dynamic covalent chemistry), CPU-based composites are expected to present very low interfacial thermal resistance between the heat sink and heat source. The characteristics of the dynamic covalent bond (i.e., urethane exchange reaction) were confirmed by the results of dynamic mechanical thermal analysis and stress relaxation analysis. Importantly, the high thermal conductivity of the CPU-based hybrid material was confirmed using laser flash analysis (up to 51.1 W/m·K). Our mechanochemical approach enables the facile preparation of sustainable polymers and hybrid composites for functional application.

Molecular Dynamics Simulation on the Heat Transfer in the Cross-Linked Poly(dimethylsiloxane).

In this work, the effect of cross-linking degree and stretching on the thermal conductivity of poly(dimethylsiloxane) (PDMS) is explored by performing a molecular dynamics simulation. Our results demonstrate that the thermal conductivity of PDMS exhibits a monotonous rise with an increase in the cross-linking degree. By decomposing the total heat flux into three microscopic heat transfer modes, the high cross-linking degree improves the contribution from bonding interactions to the heat transfer more than that from the nonbonding interactions. An analysis of the vibrational density of states shows a blue-shift of the vibrational modes at low frequencies, indicating a large phonon group velocity due to the strong interchain bonding interaction. From the spectral distribution of heat flux, the spectral contributions are shifted toward the higher frequencies with the increasing cross-linking degree, which reflects more contribution from the high-frequency modes to the heat transfer. Stretching can improve the thermal conductivity parallel to the tensile direction with the increase in strain. This is mainly due to the further improved contribution of bonding interactions or high-frequency modes to heat transfer. Interestingly, the anisotropy of the thermal conductivity first decreases and then increases with the increasing cross-linking degree. Our study conducts a detailed investigation of the thermal conductivity of cross-linked PDMS, providing guidance on the application of thermal interface materials.

Aligned carbon-doping to modulate thermal and electrical conductivity of boron carbon nitride grown from chemical vapor deposition

Hexagonal boron carbon nitride (hBCN) thin films with a wide range of carbon doping are synthesized in a chemical vapor deposition (CVD) process by varying 3 distinct precursors. Utilizing the optothermal method, the highest thermal conductivities of hBCN thin films is measured experimentally as 836 ± 312 W/m·K, for the hBCN with the highest carbon doping, surpassing the measured hexagonal boron nitride (hBN) conductivity of 462 ± 158 W/m·K. Moreover, we demonstrate that the electrical conductivity retains the insulative properties of hBN with lower carbon doping levels. Additionally, insights of how increasing amounts of carbon increase the conductivity by forming graphene like channels integrated into the hBN networks have been examined by molecular dynamics simulations. These simulations confirm a greater increase in high frequency phonon group velocities contribute to higher thermal conductivity, when aligning carbon rather than simply increasing the quantity of carbon. The aligned carbon channels are effective in overcoming the additional phonon scattering, that occurs especially in the LA mode, when adding carbon to the hBN network. By varying the carbon doping to maximize the thermal conductivity while minimizing the electrical conductivity, we have discovered an effective method of producing optimized hBCN thin films, which are ideal to be utilized in thermal interface material applications as a lower cost alternative to hBN.

A novel phase change composite with ultrahigh through-plane thermal conductivity and adjustable flexibility

With the rapid development of micro-nano electronics, thermally conductive materials with remarkable through-plane thermal conductivity (κ⊥) and great flexibility are greatly urgent to effective thermal management. The fillers with high thermal conductivity (TC) are commonly assembled to achieve this target. However, the imperfect contact between the joint fillers will lead to high thermal resistance (R), hindering thermal transport. Herein, we fabricated thermal interface materials (TIMs) with ultra-high κ⊥ (up to 168.4 W m−1 K−1) and adjustable flexibility by the combination of highly thermally conductive carbon fiber bundles (CF) and polymer encapsulated phase change materials (PCMs). The alignment of consecutive CF guarantees the continuity of thermal paths, greatly facilitating thermal transport. While the slantly slicing technique and softening effect of PCMs beyond phase transition temperature lead to an attenuated compression stress (1.6 MPa@10% strain with a CF loading of 50 wt%). What’s more, the latent heat absorbed by PCMs is also conducive to thermal management. Thus, during a CPU temperature-control experiment, the application of as-prepared TIMs manifests a temperature decline of 33.9 °C of CPU. This work opens a new perspective to fabricating TIMs with ultrahigh κ⊥ and low compression stress (σ) by the combination of thermally induced flexible matrix and the slantly slicing technique of aligned CFs.

Thermal percolation network in alumina based thermal conductive polymer

Polymers incorporated with high thermal conductivity fillers have numerous applications in thermal interface materials. Plenty of efforts have been made to improve the thermal conductivity of polymer composite. A possible method is to choose fillers with different morphologies, which can combine the advantages of various fillers. However, owing to the limitations of the effective medium theory as well as lack of researches of thermal percolation, there is still little understanding of the synergistic mechanism of fillers with different morphologies. In order to avoid the coupling effect of different materials, this work uses the same kind of alumina but with different morphologies to prepare different kinds of epoxy composites incorporated with spherical alumina, plate-like alumina and fillers mixed of 1:1 ratio. The thermal conductivity of each sample is measured by the steady state method. With the fitting of the thermal percolation theory, the synergistic effect of plate-like fillers and that of spherical fillers are verified to promote the formation of thermal percolation network. In addition, by observing the microscopic distribution of fillers, we try to explain the mechanism of this synergistic effect.

Thermal percolation network in Al<sub>2</sub>O<sub>3</sub> based thermal conductive polymer

Polymers incorporated with high thermal conductivity fillers have numerous applications in thermal interface materials. Plenty of efforts have been made to improve the thermal conductivity of polymer composite. A possible method is to choose fillers with different morphologies, which can combine the advantages of various fillers. However, owing to the limitations of the effective medium theory as well as lack of researches of thermal percolation, there is still little understanding of the synergistic mechanism of fillers with different morphologies. In order to avoid the coupling effect of different materials, this work uses the same kind of Al<sub>2</sub>O<sub>3</sub> but with different morphologies to prepare different kinds of epoxy composites incorporated with spherical Al<sub>2</sub>O<sub>3</sub>, plate-like Al<sub>2</sub>O<sub>3</sub> and fillers mixed of 1∶1 ratio. The thermal conductivity of each sample is measured by the steady state method. With the fitting of the thermal percolation theory, the synergistic effect of plate-like fillers and that of spherical fillers are verified to promote the formation of thermal percolation network. In addition, by observing the microscopic distribution of fillers, we try to explain the mechanism of this synergistic effect.

Intermetallic growth and thermal impedance at the In32.5Bi16.5Sn/Cu interface

Present developing trends in the miniaturization and power escalation on electronic components have driven research interest in thermal interface materials (TIMs) to solve the fundamental issue of heat dissipation. Among the various TIMs, the In32.5Bi16.5Sn low melting alloy (LMA) has drawn much attention owing to its low melting point and great bondability, which could mitigate the warpage issue and provide very low thermal interface resistance. In this study, the Cu/In32.5Bi16.5Sn/Cu structure was used to simulate the practical conditions of the application of TIM in joining the substrate and heat sink. The thermal properties before and after an aging test at 80 °C were measured with a thermal tester, and the growth kinetics of the intermetallic compound (IMC) at the In32.5Bi16.5Sn/Cu interface were investigated. Without fast dissolution of Cu into the In32.5Bi16.5Sn alloy, the growth kinetics of the IMC at the interface were found to be diffusion-controlled at the temperature studied. It was found that the main change in the Cu/In32.5Bi16.5Sn/Cu system after the aging test was the thickening of the IMC, and the interface was free of cracks. As the aging time increased, the thermal impedance increased to twice that of the initial value, showing parabolic degradation, which is relevant to the growth kinetics of the IMC at the interface. Implications for the degradation of the thermal performance and further research are also discussed.

Core-shell Cu@Al2O3 fillers for enhancing thermal conductivity and retaining electrical insulation of epoxy composites

Heat accumulation has emerged as a critical issue that affects the performance and reliability of modern electronic devices in their evolving toward miniaturization, high power density and high integration. The application of thermal interface materials (TIMs) as a thermal management strategy is commonly employed to tackle this problem. Herein, we report the development and characterization of highly thermally conductive but electrically insulating Cu@Al2O3/epoxy composites. Stemming from the excellent thermal conductivity of Cu, the Cu@Al2O3/epoxy composites exhibit thermal conductivity of 1.32 W/m·K at 10 vol% filler content, which is nearly 7 times higher than that of the epoxy resin. Simultaneously, the Cu@Al2O3/epoxy composites still retain a high electrical resistivity of 2.3 × 1013 Ω·cm, which is 6 orders of magnitude greater than that of Cu/epoxy composites without Al2O3 barrier layers, because the dense nanoscale insulating Al2O3 shell effectively inhibits the electron transfer. In addition, the Cu@Al2O3/epoxy composites possess lower dielectric constant and dielectric loss, and better mechanical properties than those of Cu/epoxy composites.

Amino acid functionalized boron nitride nanosheets towards enhanced thermal and mechanical performance of epoxy composite

HypothesisThe practical applications of boron nitride nanosheet (BNNS) are dramatically limited by the harsh exfoliation and surface functionalization conditions due to the hydrophobic and chemically inert nature. This issue can be improved by selecting efficient modifiers with hydrophilic groups. ExperimentsA green and scalable amino acid-assisted ball milling method is presented to exfoliate and functionalize BNNS simultaneously. The different interactions between BNNS and four amino acids (tryptophan (Trp), phenylalanine (Phe), arginine (Arg), lysine (Lys)) are thoroughly investigated to rationalize the thermal and mechanical properties of their corresponding epoxy (EP) composites. FindingTrp and Phe display higher functionalization degree and dispersibility of BNNS than Arg and Lys thanks to the additional π-π interactions between the aromatic groups and BNNS. Moreover, both BNNS-Trp/EP and BNNS-Phe/EP exhibit higher cross-plane thermal conductivity of 2.1 and 1.96 W m−1 K−1 at 30 wt% filler loading. In addition, the mechanical strengths of all these amino acids functionalized BNNS filled epoxy composites are significantly enhanced due to stronger interfacial interactions between fillers and epoxy matrix. Thus, this work paves the way for the facile mass production of functionalized BNNS and expedites their applications in thermal interface materials of electronic components.

Nanocomposites for future electronics device Packaging: A fundamental study of interfacial connecting mechanisms and optimal conditions of silane coupling agents for Polydopamine-Graphene fillers in epoxy polymers

With the ultra-fast development of high-performance semiconductor devices through the increase of power and on-chip integration density, it is essential to maximize heat transfer efficiency and explore new nanomaterials for applications in thermal interface materials (TIMs). Graphene-based epoxy nanocomposites are becoming one of the most promising candidates for the next generation of highly efficient, lightweight TIMs due to the unique properties of graphene – extraordinarily high thermal conductivities and large surface areas. However, they can have poor interfacial interactions with polymers owing to the lack of functional groups on the surface of graphene, leading to phonon scattering within the contact interfaces. In this work, graphene nanosheets are fabricated by a large-scale and low-cost liquid exfoliation method. Through fundamental understandings of interfacial connecting mechanisms and intelligent modifications of graphene with polydopamine (PDA) and silane coupling agents, optimized reaction conditions between silane coupling agents and PDA-graphene serve as the main contributor to the high thermal conductivity of the epoxy nanocomposites with low viscosity and effective filler dispersion. The graphene epoxy nanocomposites thus obtained have an out-of-plane thermal conductivity of 0.73 W/mK at 5 wt% loading, corresponding to a 400% improvement in thermal conductivity as compared to that of neat epoxy. In addition, the nanocomposites also have a lower coefficient of thermal expansion (CTE), higher thermal stability, lower moisture adsorption, and effective electrical insulation. From performance tests on the TIMs, they present better cooling capabilities and heat dissipation as corroborated through both experimentation and simulation, supporting the idea that our findings have potential to be applied in the next generation of TIMs for high-power and high-density integrated circuits (ICs) as well as in advanced packaging.

Specifics of Thermal Transport in Graphene Composites: Effect of Lateral Dimensions of Graphene Fillers.

We report on the investigation of thermal transport in noncured silicone composites with graphene fillers of different lateral dimensions. Graphene fillers are comprised of few-layer graphene flakes with lateral sizes in the range from 400 to 1200 nm and the number of atomic planes from 1 to ∼100. The distribution of the lateral dimensions and thicknesses of graphene fillers has been determined via atomic force microscopy statistics. It was found that in the examined range of the lateral dimensions, the thermal conductivity of the composites increases with increasing size of the graphene fillers. The observed difference in thermal properties can be related to the average gray phonon mean free path in graphene, which has been estimated to be around ∼800 nm at room temperature. The thermal contact resistance of composites with graphene fillers of 1200 nm lateral dimensions was also smaller than that of composites with graphene fillers of 400 nm lateral dimensions. The effects of the filler loading fraction and the filler size on the thermal conductivity of the composites were rationalized within the Kanari model. The obtained results are important for the optimization of graphene fillers for applications in thermal interface materials for heat removal from high-power-density electronics.

Thermal interface materials for cooling microelectronic systems: present status and future challenges

Thermal management has become a challenging aspect particularly in the field of microelectronics due to rapid miniaturization and massive scale integration. This has resulted in the generation of enormous amounts of heat that needs to be efficiently transferred from microelectronic devices to ensure longer life cycles. The efficient transfer of heat offers advantages such as achieving higher operating temperatures and prevents component failure. A device engineer has to, therefore, identify methods that would facilitate the efficient transfer of heat from the systems. The existence of an interface between the heat source and the heat sink impedes the efficient transfer of heat. Over the years, researchers have identified techniques that could be employed to reduce the interface impediments. Among these techniques, the application of thermal interface materials (TIMs) at the interface is the most promising and has become an integral part of applications where an efficient transfer of heat across interfaces is desirable. In the present paper, the assessment of contact resistance, properties of interface materials and thermal management of microelectronic devices using TIMs are discussed. The present status of TIMs is critically reviewed and the future challenges are highlighted.

Innocuous, Highly Conductive, and Affordable Thermal Interface Material with Copper-Based Multi-Dimensional Filler Design.

Thermal interface materials (TIMs), typically composed of a polymer matrix with good wetting properties and thermally conductive fillers, are applied to the interfaces of mating components to reduce the interfacial thermal resistance. As a filler material, silver has been extensively studied because of its high intrinsic thermal conductivity. However, the high cost of silver and its toxicity has hindered the wide application of silver-based TIMs. Copper is an earth-abundant element and essential micronutrient for humans. In this paper, we present a copper-based multi-dimensional filler composed of three-dimensional microscale copper flakes, one-dimensional multi-walled carbon nanotubes (MWCNTs), and zero-dimensional copper nanoparticles (Cu NPs) to create a safe and low-cost TIM with a high thermal conductivity. Cu NPs synthesized by microwave irradiation of a precursor solution were bound to MWCNTs and mixed with copper flakes and polyimide matrix to obtain a TIM paste, which was stable even in a high-temperature environment. The cross-plane thermal conductivity of the copper-based TIM was 36 W/m/K. Owing to its high thermal conductivity and low cost, the copper-based TIM could be an industrially useful heat-dissipating material in the future.

Enhanced thermal conductivity of natural rubber based thermal interfacial materials by constructing covalent bonds and three-dimensional networks

With the rapid development of electronic integration technology, the timely and efficient heat dissipation capacity has become increasingly important to ensure reliability and lifetime of electronics. Thermal interface materials (TIMs) were considered as the best candidate to tackle this issue. In this work, the three-dimensional structures with covalent bond connections of boron nitride/natural rubber (BN/NR) thermal interface composites were successfully prepared and its possessed an ameliorated through-plane thermal conductivity of 0.79 W m−1 K−1 when BN content were 25 wt%. The results demonstrated that three-dimensional network structures and chemical interactions between BN and NR had a significant contribution in reducing interfacial thermal resistance. Meanwhile, the sensitive thermal response, satisfactory electrical insulation and good mechanical properties made its own high probability for the TIMs applications. Importantly, this tactics created a new means to construct elastomer-based thermal management materials and applied for the electronic packing materials.

Pressure-Activated Thermal Transport via Oxide Shell Rupture in Liquid Metal Capsule Beds.

Liquid metal (LM)-based thermal interface materials (TIMs) have the potential to dissipate high heat loads in modern electronics and often consist of LM microcapsules embedded in a polymer matrix. The shells of these microcapsules consist of a thin LM oxide that forms spontaneously. Unfortunately, these oxide shells degrade heat transfer between LM capsules. Thus, rupturing these oxide shells to release their LM and effectively bridge the microcapsules is critical for achieving the full potential of LM-based TIMs. While this process has been studied from an electrical perspective, such results do not fully translate to thermal applications because electrical transport requires only a single percolation path. In this work, we introduce a novel method to study the rupture mechanics of beds composed solely of LM capsules. Specifically, by measuring the electrical and thermal resistances of capsule beds during compression, we can distinguish between the pressure at which capsule rupture initiates and the pressure at which widespread capsule rupture occurs. These pressures significantly differ, and we find that the pressure for widespread rupture corresponds to a peak in thermal conductivity during compression; hence, this pressure is more relevant to LM thermal applications. Next, we quantify the rupture pressure dependence on LM capsule age, size distribution, and oxide shell chemical treatment. Our results show that large freshly prepared capsules yield higher thermal conductivities and rupture more easily. We also show that chemically treating the oxide shell further facilitates rupture and increases thermal conductivity. We achieve a thermal conductivity of 16 W m-1 K-1 at a pressure below 0.2 MPa for capsules treated with dodecanethiol and hydrochloric acid. Importantly, this pressure is within the acceptable range for TIM applications.

Enhanced through-plane thermal conductivity and high electrical insulation of flexible composite films with aligned boron nitride for thermal interface material

Thermal interface materials (TIMs) with high thermal conduction and electrical insulation properties in through-plane direction are highly desirable in high-power and dense electronic devices. Thus a generalizable step-by-step orientation method without chemical modification or special treatment is adopted to prepare the composite films (boron nitride/poly (vinylidene fluoride) (i.e. x-BN/PVDF)). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results indicate that quite part of the BN platelets (BNs) are aligned well along the through-plane direction. A thermal conductivity of 3.50 W m−1 K−1 is obtained at a BN loading of 30 wt%, which has been improved by 1448% compared with the pure PVDF (0.226 W m−1 K−1). The volume resistivity and alternating current (AC) resistivity of 30-BN/PVDF sample are 6.51 × 1013 Ω·cm and 4.56 × 107 Ω·m (1 kHz), respectively, indicating much potential as high-frequency insulation material. In addition, the strong thermal conduction capability of the composite film is demonstrated by comparing temperature changes, which shows the importance of higher through-plane thermal conductivity in TIM applications.

Thermal Properties of the Binary‐Filler Hybrid Composites with Graphene and Copper Nanoparticles

AbstractThe thermal properties of epoxy‐based binary composites comprised of graphene and copper nanoparticles are reported. It is found that the “synergistic” filler effect, revealed as a strong enhancement of the thermal conductivity of composites with the size‐dissimilar fillers, has a well‐defined filler loading threshold. The thermal conductivity of composites with a moderate graphene concentration of fg = 15 wt% exhibits an abrupt increase as the loading of copper nanoparticles approaches fCu ≈ 40 wt%, followed by saturation. The effect is attributed to intercalation of spherical copper nanoparticles between the large graphene flakes, resulting in formation of the highly thermally conductive percolation network. In contrast, in composites with a high graphene concentration, fg = 40 wt%, the thermal conductivity increases linearly with addition of copper nanoparticles. A thermal conductivity of 13.5 ± 1.6 Wm−1K−1 is achieved in composites with binary fillers of fg = 40 wt% and fCu = 35 wt%. It has also been demonstrated that the thermal percolation can occur prior to electrical percolation even in composites with electrically conductive fillers. The obtained results shed light on the interaction between graphene fillers and copper nanoparticles in the composites and demonstrate potential of such hybrid epoxy composites for practical applications in thermal interface materials and adhesives.

Spherical core-shell Al@Al2O3 filled epoxy resin composites as high-performance thermal interface materials

With the miniaturization, high power density, and high integration of modern electronics, heat accumulation has emerged as a critical issue that affects their performance and reliability. The application of thermal interface materials is a common thermal management strategy. Herein, we report thermally conductive and insulating thermal interface materials composed of core-shell structured Al@Al2O3 and epoxy resin. The composites exhibit thermal conductivity of 0.92 W m−1 K−1 at 60 wt% filler contents, which is about 4.2 times higher than that of the epoxy resin. The oxidation of Al particles results in the formation of dense nanoscale insulating Al2O3 shell, which effectively restricts the electron transfer, thus leading to a very high electrical resistivity and puncture voltage of the composites. The polymer composites filled with Al@Al2O3 particles have excellent insulation property, high thermal conductivity and outstanding thermos-mechanical performance, which could be a potential thermal interface material in advanced electronic packaging techniques.

Thermal Interface Materials Based on Vertically Aligned Carbon Nanotube Arrays: A Review

As the feature size of integrated circuit devices is shrinking to sub-7 nm node, the chip power dissipation significantly increases and mainly converted to the heat. Vertically Aligned Carbon Nanotube arrays (VACNTs) have a large number of outstanding properties, such as high axial thermal conductivity, low expansion coefficient, light-weight, anti-aging, and anti-oxidation. With a dramatic increment of chip temperature, VACNTs and their composites will be the promising materials as Thermal Interface Materials (TIMs), especially due to their high thermal conductivity. In this review, the synthesis, transfer and potential applications of VACNTs have been mentioned. Thermal Chemical Vapor Deposition (TCVD) has been selected for the synthesis of millimeter-scale VACNTs. After that, they are generally transferred to the target substrate for the application of TIMs in the electronics industry, using the solder transfer method. Besides, the preparation and potential applications of VACNTs-based composites are also summarized. The gaps of VACNTs are filled by the metals or polymers to replace the low thermal conductivity in the air and make them free-standing composites films. Compared with VACNTs- metal composites, VACNTs-polymer composites will be more suitable for the next generation TIMs, due to their lightweight, low density and good mechanical properties.