Thermal interface materials - a review of the state of the art

The past few decades have seen an escalation of power densities in electronic devices, and in particular in microprocessor chips. Together with the continuing trend of reduction in device dimensions this has led to dramatic increase in the thermal issues within electronic circuits. Thermal management is therefore becoming increasingly more critical and fundamental to ensuring that electronic devices operate within their specification. Although a thermal management system may make use of all modes of heat transfer to maintain temperatures within their appropriate limits and to ensure optimum performance and reliability, conductive heat transfer is typically used to spread the heat out from its point of generation and into the extended surface area of a heat sink. To minimise the contact resistance, thermal interface materials (TIMs) are introduced to the joint to fill the air gaps and are an essential part of an assembly when solid surfaces are attached together. This paper reviews the conventional interface materials and then goes on to present a comprehensive review of the emerging state-of-the-art research in the use of carbon nanotube based materials. The paper also outlines the advantages and disadvantages of each TIM category and the factors that need to be considered when selecting an interface materia

過度液相焼結法によるパワーモジュールと銅ヒートスプレッダの低温及び低圧力接合

早大学位記番号:新7685早稲田大

High power gallium nitride micro-electronics: thermal management using microfluidics

Last four decades have seen unprecedented development in communication, defence, electronics, and computing technologies. The increased power density plus the miniaturisation of the device present challenges in managing the high heat flux in the microchip level. Besides, the highly heterogeneous heat flux in electronic devices presents more challenges to thermal management (TM). This calls for the development of more efficient cooling technologies for these high-power microelectronic devices. This PhD study aims to address this challenge by developing the high performance of heat transfer fluids (HTFs) and compact cooling devices. Gallium Nitride (GaN) based transistors which acted as inhomogeneous high heat flux output were targeted in this work. The work involves formulating, characterisation and performance measurements of various heat transfer fluids (including base fluids and nanofluids), design, fabrication and assemble, and package and experiments of microfluidics including foam metal, micro-jet impingement. Both experimental work and modelling were performed and the following main conclusions were obtained. • Heat transfer fluids study Two types of nanofluids were formulated and investigated for the application in room temperature and the low temperature. The BN/DI water nanofluids used in the room temperature shows 5.2 % enhancement in the thermal conductivity compared to the base fluid for the 0.5 wt.%. The other material rGO/EG+DW nanofluids used for the temperature as low as -50 ℃ has 17 % thermal conductivity increase with the concentration of 2.0 wt.%. This suggests that the nanofluids can have a better thermal performance for the microfluidic channel than the base fluids. • Performance of the microfluidics With the experimental comparison of the copper-foam based microfluidic channel and the micro-jet channel, the micro-jet channel was chosen due to a higher heat transfer coefficient. Both base fluids and nanofluids were experimental tests and the numerical simulation was validated with the micro-jet channel. The test showed that the BN/DI water nanofluids with a concentration of 0.5 wt.% can increase the heat transfer coefficient 5 % compared to the DI water. Meanwhile, the 2.0 wt.% rGO/EG+DW nanofluids showed a similar trend with an 11% increase in the heat transfer coefficient compared to EG+DW base fluid. The direct measurement of the temperature with Raman thermography was used to measure the temperature in the finger of the die. The experiment test suggests that with the target power density of 5 W/mm in the finger (1×10⁷ W/mm² in the finger), the peak temperature in the devices was 120 °C far below 200 °C. The thermal resistance for the jetting channel was 19.76 °C/W. The device used in the experiment was GaN-on-SiC. For the GaN-on-Diamond, a higher power density can be obtained. Thus, for the thermal management of the GaN devices, the nanofluids, material selection for the devices thermal package and micro-jet channel play important roles once the specific GaN transistors are selected

Characterization of Multifunctional Nanomaterials for Electronics Thermal Management and Sintering Applications

The science of manipulating materials at their nanoscale level is nowadays allowing endless possibilities to disrupt the current limitations on the conventional production processes and products. In electronics, the need for more capable thermal management strategies led to the exploration of advanced approaches and focus on new materials and allowed to push further the thermal dissipation capabilities of each generation of products. In this thesis, we investigate different thermal management concepts and propose new solutions based on carbon and metallic nanomaterials, while we explore the possibility to combine the size effect with the composition effect of the nanoscale materials.Due to their high surface to volume ratio, nanoscale particles show different thermodynamics properties that led to their potential implementation in electronics fabrication processes. More specifically, silver nanoparticles (Ag NPs) have been under focus in recent years for applications to replace lead-free solder and contribute to energy saving. Due to a poor trade-off between the process parameters, the production costs, and the reliability of the silver related application, different strategies are being suggested to optimize its applications. In this present study, we investigate multiple sintering parameters of Ag NPs and use the nanoscale effect in a hybrid approach for the sintering of microscopic powder. The results of the sintering parameters are correlated to the density of the samples and their properties in terms of thermal and electrical conductivity. While the sintering of Ag NPs occurs at low temperatures and allows to obtain relatively high densities, the thermal and electrical properties are still limited and the increase in the temperature and fraction of the NPs higher than 400 degrees and 2wt.% has a much- pronounced effect to improve the physical properties of the samples.The sintering of Ag NPs was also explored in this thesis to propose a novel approach to use graphene foam as a heat sink. While graphene is known for its outstanding physical, chemical, and mechanical properties, its integration as a practical solution in electronics is still missing. The use of Ag NPs in this work allowed to successfully attach the 3D graphene foam on its substrate and further improve both its mechanical and thermal properties by coating the graphene with Ag NPs. Also, the integration of Ag NPs as a die-attach for the 3D porous structure allowed its further use as a container for Phase Change Materials (PCM). Different amounts of PCM were introduced in the lightweight foam and the junction temperature of the hot spot was correlated to the power and the presence of the PCM. We found that graphene foam presents a real advantage for its use in thermal dissipation strategies.2D graphene material is developed herein as a coating for micro-and nanoscale particles. Using Chemical Vapor Deposition (CVD) and Arc Discharge (AD) methods, we introduce the possibility to produce graphene coating on copper particles for application in thermal management. In addition, we explore the possibility to introduce a doping effect on the coated NPs to further study its effect on the thermal performances of NPs. The morphology and the composition of the coating were investigated and correlated with the bottom-up production process of CVD and AD. The thermal conductivity and chemical stability of the produced particles were studied for their use as fillers in thermally conductive pastes and additives water-based nanofluids. The thermal properties of the different systems were linked to the fraction of the additives and nanofillers. The graphene-coated particles were found to have a multifunctional effect. In both micro-and nanoscale particles, the graphene coating was found to act as a corrosion resistance that stabilizes the metallic core of the particles. The graphene coating also was found to act as a carbon source to reduce the microparticles in a bimodal powder at high temperatures. Finally, the encapsulation of the nanoscale powder allowed to observe a melting point depression related to the composition of the core of the nanoparticles and their nanoscale size.In an effort to combine the size effect of the nanoparticles and their compositions, different alloyed nanoparticles were produced using AC. The morphology, the composition, and their sintering properties were compared to highlight their composition effect. The produced nanopowders were also used as a sintering aid in the spark plasma sintering approach (SPS) and the results show a positive contribution of the nanopowders in the reduction of the sintering temperature and the densification of the samples. An additional effect is also reported and arises from the possibility to use those particles to fine-tune the chemical composition of the bimodal particles

Development of carbon nanotube-reinforced nickel matrix composites: processing, microstructure and physical properties

The present thesis is focused on the design of a processing route which would deliver a microstructurally tailored metal matrix composite. By controlling the final microstructure, the resulting physical properties can be predicted and therefore optimized for a certain application. Process parameters were optimized considering the CNT defect state after dispersion. Additionally, the microstructural evolution is analysed during sintering, considering the potential chemical interactions. It was observed that the CNTs act as microstructural controller due to boundary pinning, resulting in finer final microstructures for higher CNT concentrations up to 3.0 wt.% CNTs, concentration beyond which no further refinement is detected. This stagnation is a consequence of the CNT agglomeration due to the mass transport during sintering. The mechanical, thermomechanical, tribological and electrical characterization of the composites was performed with complementary techniques. The improvement of the mechanical properties is associated to a Hall-Petch effect. Thermomechanical behaviour shows a decrease in the coefficient of thermal expansion. The anchoring effect of the CNTs is the responsible for this reduction and three model mechanisms for the behaviour are proposed. Tribological behaviour showed reduced friction and wear loss in the composites. Finally, the influence of the CNT concentration and distribution are correlated to the improvement in the electrical conductivity.Die vorliegende Arbeit beschäftigt sich mit der Entwicklung einer Herstellungsroute, welche die Synthese mikrostrukturell maßgeschneiderter Metallmatrixkomposite erlaubt. Dies ermöglicht eine Vorhersage und Optimierung der sich einstellenden physikalischen Eigenschaften für bestimmte Anwendungsfälle. So wurden die Prozessparameter hinsichtlich des Defektzustandes von CNTs optimiert und die während des Sintervorganges potentiell möglichen chemischen Wechselwirkungen analysiert. CNTs fungieren durch Behinderung der Korngrenzbewegung als Steuerungselement für die Mikrostruktur. Mit steigender CNT-Konzentration (bis zu 3 Gew.-%) nimmt die Korngröße ab. Höhere CNT-Konzentrationen führen zu Agglomerationen aufgrund des Massentransportes während des Sintervorganges und tragen nicht mehr zum Kornfeinungseffekt bei. Eine mechanische, thermomechanische, tribologische und elektrische Charakterisierung der Komposite wurde mit komplementären Methoden durchgeführt. Die Verbesserung der mechanischen Eigenschaften kann mit dem Hall-Petch Effekt korreliert werden. Gleichzeitig wird eine Abnahme des Wärmeausdehnungskoeffizienten festgestellt, welche mit dem Verankerungseffekt der CNTs zusammenhängt und durch drei Modellansätze beschrieben wird. Die tribologischen Untersuchungen zeigen eine Verringerung des Reibkoeffizienten und des Verschleißes. Abschließend wird eine Verbesserung der elektrischen Leitfähigkeit auf den Einfluss von CNT-Konzentration und -Verteilung zurückgeführt

Isotropically conductive adhesive filled with silver metalised polymer spheres

Isotropic conductive adhesives (ICAs) have a growing range of applications in electronics packaging and have recently emerged as an important material in photo-voltaic module interconnections, particularly for thin-film and other non-silicon technologies where soldering processes are often unsuitable due to the nature of the metallisation or the limited maximum temperature the assembly can be exposed to. ICAs typically comprise of a high volume fraction of solid metallic flakes, usually silver, in an adhesive matrix because of its highly conductive oxide however, this thesis will focus on adhesives containing a large volume fraction of silver coated/metalised mono-sized polymer spheres (Ag-MPS). Incorporating silver coated mono-sized polymer spheres is anticipated to deliver specific advantages such as a significant reduction in the required silver content, improvement of the overall mechanical properties and flexibility to tune the properties of the filler according to the application compared with conventional flake filled adhesives. In this research advancements in the understanding of Ag-MPS filled ICAs, both through theory and experiments, have been made. Analytical models to predict an individual Ag-MPS resistance and Ag-MPS filled ICA resistance have been developed. The experiments based on the flat punch nanoindentation technique have been conducted to determine individual Ag-MPS resistances. The theoretical and experimental studies establish Ag-MPS diameter, coating resistivity, coating thickness, contact radius, and contact geometry as the main contributors towards the resistance of an Ag-MPS filled ICAs. These studies showed that Ag-MPS resistance decreases with increasing coating thickness and contact radius but increases with increasing coating resistivity. The experiments have also been conducted to investigate the effect of Ag-MPS volume fraction, diameter, coating thickness, curing conditions and shrinkage (affecting contact radius) on ICA conductivity and comparisons are made with flake filled and commercial ICAs. The results showed that ICA conductivity increases with increasing volume fraction and coating thickness but decreases with diameter. More importantly the results showed that conductivities similar to those of flake filled ICAs, including those commercially available, can be obtained using 70% less silver. The results show that, Ag content can be reduced further to just 7% with use of larger 30μm Ag-MPS but with a lower resulting conductivity. Thus for applications where very high conductivity is not required larger Ag-MPS may offer even greater potential cost benefits, which is something flake filled ICAs cannot offer. This is a significant achievement which can allow tuning of ICA formulations according to the demands of the application, which is not possible with the use of silver flakes as there is only a limited range of silver flake volume fractions that will yield useful levels of conductivity

Synthesis, Production and Characterization of Next Generation Thermal Interface Materials for Electronic Applications

The inefficient dissipation of heat is a crucial problem that limits the reliability and performance of all electronic systems. As electronic devices get smaller and more powerful, and moving components of machinery operate at higher speeds, the need for better thermal management strategies is becoming increasingly important. Heat removal during the operation of electronic, electrochemical, and mechanical devices is facilitated by high-performance thermal interface materials (TIMs), which are utilized to couple devices to heat sinks. Herein, we report a new class of TIMs involving the chemical integration of boron nitride nanosheets (BNNS), soft organic linkers, and a metal matrix - which are prepared by chemisorption coupled electrodeposition approach. Thermal and mechanical characterization of the copper-based hybrid nanocomposites involving thiosemicarbazide demonstrates bulk thermal conductivities ranging from 211 to 277 W/(m.K), which are very high considering their relatively low elastic modulus values on the order of 15 to 30 GPa. The synergistic combination of these properties leads to the lowest measured total thermal resistivity to date for a TIM with a typical bondline thickness of 30-50 µm: 0.38 to 0.56 mm^2.K/W. Moreover, its coefficient of thermal expansion (CTE) is 11 ppm/K, forming a mediation zone with a low thermally-induced axial stress due to its close proximity to the CTE of most coupling surfaces needing thermal management. Furthermore, preliminary electrochemical tests revealed that the presence of organic ligands and BNNS in the hybrid nanocomposite TIMs improves the corrosion protection behavior of the TIMs by nearly 72%. Further analysis of the hybrid nanocomposite TIMs included the replacement of thiosemicarbazide with various organic ligands and the replacement of copper matrix with silver. Compared to all the ligands that were used in copper-based hybrid nanocomposites, the most promising thermal and mechanical test results were obtained from thiosemicarbazide. On the other hand, the best silver-based nanocomposite TIM was determined to be the one involving the ligand 2-mercapto-5-benzimidazolecarboxylic acid, in which the thermal conductivity was near 360 W/m.K, and elastic modulus and hardness were about 35 GPa and 0.25 GPa, respectively. The promising results indicate that metal-inorganic-organic nanocomposite TIMs can be great alternatives to currently used TIMs in the market

導電材料やダイアタッチ材料としての低温焼結と高接合特性を有する銅ペースト

Tohoku University村松淳司課