Resistance Of Thermal Interface Materials Research Articles

Thermal interface materials: From fundamental research to applications

AbstractThe miniaturization, integration, and high data throughput of electronic chips present challenging demands on thermal management, especially concerning heat dissipation at interfaces, which is a fundamental scientific question as well as an engineering problem—a heat death problem called in semiconductor industry. A comprehensive examination of interfacial thermal resistance has been given from physics perspective in 2022 in Review of Modern Physics. Here, we provide a detailed overview from a materials perspective, focusing on the optimization of structure and compositions of thermal interface materials (TIMs) and the interact/contact with heat source and heat sink. First, we discuss the impact of thermal conductivity, bond line thickness, and contact resistance on the thermal resistance of TIMs. Second, it is pointed out that there are two major routes to improve heat transfer through the interface. One is to reduce the TIM's thermal resistance (RTIM) of the TIMs through strategies like incorporating thermal conductive fillers, enhancing interfacial structure and treatment techniques. The other is to reduce the contact thermal resistance (Rc) by improving effective interface contact, strengthening bonding, and utilizing mass gradient TIMs to alleviate vibrational mismatch between TIM and heat source/sink. Finally, such challenges as the fundamental theories, potential developments in sustainable TIMs, and the application of AI in TIMs design are also explored.

Determination of Contact Resistance of Thermal Interface Materials Used in Thermal Monitoring Systems of Electric Vehicle Charging Inlets.

The rapid growth of the electric vehicle (EV) market is observed. This is challenging from a materials point of view when it comes to the thermal monitoring systems of charging inlets, for which requirements are very restrictive. Because the thermal conductivity of the thermal interface material is usually measured, there is a significant research gap on the contact thermal resistance of novel materials used in the electric vehicle industry. Moreover, researchers mainly focus on electrically conductive materials, while for thermal monitoring systems, the most important requirement is a high dielectric breakdown voltage. In this paper, the thermal contact resistance of materials for EV applications was thoroughly analyzed. This study consisted of experimental measurements with the Laser Flash Analysis (LFA) method, as well as a theoretical analysis of thermal contact resistance. The main focus was on the extraction of contact and material thermal resistance. The obtained results have great potential to be used as input data for further numerical modeling of solutions that meet strict thermal accuracy requirements. Additionally, the chemical composition and internal structure were analyzed using scanning electron microscopy, to better describe the material.

Effect of segment structure of monotrimethoxysilylethyl-terminated asymmetric polysiloxanes on the thermal management performance of AlN-filled silicone pastes

Three kinds of α-triethylsiloxyl-ω-trimethoxysilyethyl-terminated oligomers (TES-PDMS-TMOS, TES-PTFPMS-TMOS, and TES-PDES-TMOS) were synthesized through an anion catalyzed non-equilibrium ring-opening polymerization reaction of cyclotrisiloxanes and subsequent hydrosilylation reaction with vinyltrimethoxysilane. As the in situ surface modification agent for mixed-size AlN inorganic fillers (1, 20 and 50 μm blended in a mass ratio of 3:5:9), these oligomers have been mixed with Vi-PDMS in a mass ratio of 1:2. Relative to that of silicone pastes using Vi-PDMS as the polymer matrix, the thermal conductivity of silicone pastes made from AlN particles treated with these oligomers is enhanced, while their thermal resistances decreased. With the change of the repeating unit structure in the oligomers, the silicone pastes prepared from these oligomer-treated AlN fillers differ significantly in filler content, thermal conductivity, and thermal resistance. When TES-PTFPMS-TMOS is used and filled with 800 parts (70.3 vol%) of hybrid AlN powders, a silicone paste with a thermal conductivity of 4.56 W/(m. K) could be achieved, which was 37.8% higher than that silicone paste using pure Vi-PDMS as the matrix. When TES-PDES-TMOS is used and filled with 900 parts (72.7 vol%) of hybrid AlN powders, the thermal conductivity of the silicone paste reaches 4.13 W/(m. K). Despite its thermal conductivity being only 3.5% higher relative to the silicone paste using only Vi-PDMS as the polymer matrix, its thermal resistance decreased by 20.3%. When TES-PDMS-TMOS is used and filled with 1200 parts (78.0 vol%) of hybrid AlN powders, the thermal conductivity of the silicone paste could reach up to 4.59 W/(m. K), together with a thermal resistance 29.6% lower than that of silicone paste where 1000 parts (74.7 vol%) of hybrid AlN particles filled in the Vi-PDMS. The heat dissipation of the 30 W high-brightness LED chip also illustrated the excellent thermal conductivities of these silicone pastes. The effect of another terminal group on the monotrimethoxysilylethyl-terminated asymmetric polysiloxanes on the thermal resistance of thermal interface materials was also discussed in detail.

Effects of temperature and pressure on interfacial thermal resistance of thermal interface materials in coupled heat transfer process with vapor chamber

The vapor chamber (VC) is commonly acknowledged as an effective solution for the dissipation of high heat flux hotspots, as it dissipates the localized heat over the entire surface. There is a prevailing emphasis on enhancing the thermal performance of the VC and decreasing its thermal resistance. However, little attention has been paid to the interface thermal resistance (ITR) between the heat source and the VC. In actuality, the ITR of the thermal interface material (TIM) is much greater than the thermal resistant of the VC. Further research on coupling test of heat transfer performance between TIM and VC needs to be carried out. As a consequence, a coupling test method of TIM and VC is put forward in this paper. The thermal performance of dissimilar TIMs was experimentally explored, including thermal grease, thermal phase change materials (PCMs), low-melting-point alloy (LMPA) and thermal pad, and their performance was compared to the situation without using TIM. The results indicate that utilizing PCM leads to an 87.5% decrease in ITR, while thermal grease results in a 51% decrease and LMPA leads to a decrease of 81%. The proper TIM is PCM, LMPA, thermal grease and thermal pad in descending order of effectiveness. The heat transfer performance of TIM is influenced by both temperature and pressure. In the practical applications of TIM, ensuring consistency is also essential, meaning the ITR of TIM should not be easily affected by various manual operations, in addition to considering the heat transfer performance.

Ultralow Contact Resistance of Thermal Interface Materials Enabled by the Vitrimer Chemistry of a β-Hydroxy Phosphate Ester

Ultralow Contact Resistance of Thermal Interface Materials Enabled by the Vitrimer Chemistry of a β-Hydroxy Phosphate Ester

Investigations on heat flow management perspective-induced design criteria of thermal interface materials

Thermal interface materials (TIMs) are a critical component of thermal management systems, with efforts being made to improve their thermal conductivity (k) to enhance their thermal-management performance in electronic devices. However, the heat flow management ability should be the key factor for determining thermal-management performance, instead of the k of the TIMs alone as commonly assumed. Systematically looking for what factors will affect the heat flow management ability and understanding how they can influence the thermal-management performance is urgent. In this work, we combined both experiments and simulations to clarify this issue in a TIM-based thermal management system. The heat flow management ability of TIMs was classified into two categories based on the heat flow directions: the heat transport ability, responsible for transferring heat from the chip to heat sink in through-plane direction; and the heat flow dispersion ability, pertaining to the in-plane dispersion of heat flow within the chip. These two kinds of ability of TIMs were found to be affected by different properties beyond just k: the former was proportional to the thermal resistance of TIMs and the contact area between TIMs and chips, while the latter was influenced by the area and anisotropic k of TIMs. Through systematic analysis of heat flow, a clear understanding of the heat flow management ability was obtained, leading to the proposal of design criteria grounded in the perspective of heat flow management. These criteria aim to guide the development of efficient materials design and further enhance thermal-management performance.

Wettability and thermal contact resistance of thermal interface material composited by gallium-based liquid metal on copper foam

As a promising thermal interface material (TIM), gallium-based liquid metal (GBLM) could significantly reduce the thermal contact resistance (TCR) at the solid-solid interfaces, while the leakage of GBLM and the wettability between liquid metal and solid surface restrict further application. In this study, the GBLM was wrapped with a copper foam (CF) skeleton, which prevents leakage and improves the thermal conductivity of the GBLM. Gallium-based liquid metal-copper foam (GBLM-CF) TIM was prepared by the self-wetting of the CF surface through the galvanic corrosion of GBLM. As the concentration of HCl increases from 0 to 5 M, the interfacial tension (IFT) in the HCl solution decreases from 536.80±1.00 mN/m to 481.52±0.49 mN/m, demonstrating that the IFT of the GBLM is inversely proportional to the concentration of HCl. Meanwhile, the decrease of contact angle (CA) by 8° indicates that the presence of CuGa2 on the surface promotes the wettability of GBLM. With the increase of external pressure and the improvement of wettability, the TCR between the GBLM and the copper sheet decreased by 58.28%. The TCR of the sample using GBLM-CF as TIM is 83% lower than that of thermal grease. Moreover, the thermal conductivity of GBLM-CF TIM (51.58±0.01 W/(m·K)) is 212.6% higher than that of GBLM (16.50±0.02 W/(mK)). The GBLM-CF TIM presented in this study can be used for the efficient thermal management of electronic equipment.

Thermally conductive and stretchable thermal interface materials prepared via vertical orientation of flake graphite

With the increase of power density of electronic devices, there is a compromise between thermal conductivity and stretchability of thermal interface materials to reduce thermal contact resistance, enhance interfacial heat transfer, and relieve the warpage failure caused by stress concentration. Here, we report on the styrene-ethylene/butylene-styrene block copolymer (SEBS)/flake graphite composite thermal interface materials, fabricated via the vertical orientation of flake graphite. When the mass ratio of flake graphite to SEBS is 1:1, the thermal interface material exhibits a high out-of-plane thermal conductivity of 10.08 W/(m K) and maintains a considerable stretchability (elongation at break of 63%). The balance of thermal conductivity and stretchability keeps the thermal contact resistance of thermal interface material at a low value of 0.51×10-4 K ·m2/W. The thermal interface material consisted of SEBS/flake graphite builds a new way to address the challenge of thermal management in modern electronic products.

Synergistic effect of carbon fiber and graphite on reducing thermal resistance of thermal interface materials

Thermal interface materials (TIM) have become a leading solution to achieve lower operating temperature in electronic devices. To improve thermal conductivity and to simultaneously reduce thermal resistance, this work investigates the thermal properties for interface materials with carbon fiber and graphite. When the ratio between carbon fiber to graphite is 1:1, the thermal resistance can be reduced to 1.8 × 10−4 K m2 W−1 at 30 psi, although the bulk thermal conductivity is lowered from 34 to 19 W m−1 K−1. The synergistic effect between carbon fiber and graphite is investigated. Surface roughness of TIMs was measured and two different types of thermal conductivity measurement were applied to demonstrate that graphite makes a contribution to bulk thermal conductivity where a CF played an important role for smoothing the surface, thereby lowering the thermal resistance. In real applications, the hybrid filler is shown to perform better in enhancing the heat conduction for electrical packaging.

A Hertzian contact based model to estimate thermal resistance of thermal interface material for high-performance microprocessors

Thermal interface material (TIM) is applied at the interface between the high-performance microprocessors and heat sink to create a robust heat transfer path. Thermally speaking, TIM is typically characterized by a TIM tester and results are reported in terms of thermal impedance with bond line thickness (BLT) and/or averaged contact pressure. However, the characterization data collected from a TIM tester often cannot represent TIM’s behavior under usage conditions for high-performance microprocessors. The primary reason being the discrepancy in the contact conditions between TIM tester and microprocessor application, mainly including warpage of the microprocessor package and heat sink. In this paper, a Hertzian contact model is proposed to bridge this gap by incorporating in-situ warpage of the microprocessor package and heat sink in usage into the consideration. By applying the model, the thermal resistance of TIM under usage conditions for high performance microprocessors can be estimated from laboratory characterization data. A validation experiment is also presented, and good correlation has been obtained between the prediction and measurement.

A Review of the Performance and Characterization of Conventional and Promising Thermal Interface Materials for Electronic Package Applications

Thermal interface materials (TIMs) play a key role in reducing thermal resistance between jointed solid surfaces in order to increase thermal transfer efficiency. A TIM is a thermally conductive material which is applied between the interfaces of two components (such as circuit board and heat sinks) to enhance the thermal conductance between them. The present paper provides a detailed review in order to characterize conventional and advanced TIMs in terms of thermal performance. The paper also discusses the measurement of TIM performance using different techniques. It was found that greases are the most widely used TIMs and offer thermal resistance in the range of 0.1–1 cm2°C/W. However, greases are messy and difficult to apply and remove due to their high viscosity. Moreover, they have reliability issues such as pump-out, phase separation and dry-out, which limits their use as an efficient TIM. The thermal resistance of TIMs which contain carbon nanotubes (CNT) fall in the range of 0.01–0.19 cm2°C/W. However, the use of CNT as a TIM at high temperatures does not allow for uniform distribution of heat on cooling of electronic packaging systems. Although TIMs with the addition of nano--metal particles are considered promising, it is necessary to carry out extensive research on CNT as a TIM.

Evaluation of Contact Thermal Resistance of Ag Paste under High Heat Flux Conditions

This study describes the evaluation of thermal contact resistance under high heat flux. Next-generation semiconductor devices have advantage, which are high voltage device, and can be used under very high temperature and high heat flux. The contact thermal resistance of TIM (Thermal Interface Material) used at high temperature, thus, should be evaluated. Ag paste is expected as TIM used at high temperature. In this study, the contact thermal resistance is experimentally evaluated by steady state method. The experimental apparatus has heat transfer surface of φ 20.0 mm, and applied contact pressure to the heat transfer surface is 0.09 MPa. The results show that the temperature gap between the heat transfer surfaces show complete different tendency before / after solidification of the Ag paste.

Optimized Characterization of Thermoelectric Generators for Automotive Application

New developments in the field of thermoelectric materials bring the prospect of consumer devices for recovery of some of the waste heat from internal combustion engines closer to reality. Efficiency improvements are expected due to the development of high-temperature thermoelectric generators (TEG). In contrast to already established radioisotope thermoelectric generators, the temperature difference in automotive systems is not constant, and this imposes a set of specific requirements on the TEG system components. In particular, the behavior of the TEGs and interface materials used to link the heat flow from the heat source through the TEG to the heat sink must be examined. Due to the usage patterns of automobiles, the TEG will be subject to cyclic thermal loads, which leads to module degradation. Additionally, the automotive TEG will be exposed to an inhomogeneous temperature distribution, leading to inhomogeneous mechanical loads and reduced system efficiency. Therefore, a characterization rig is required to allow determination of the electrical, thermal, and mechanical properties of such high-temperature TEG systems. This paper describes a measurement setup using controlled adjustment of cold-side and warm-side temperatures as well as controlled feed-in of electrical power for evaluation of TEGs for application in vehicles with combustion engines. The temperature profile in the setup can be varied to simulate any vehicle usage pattern, such as the European standard driving cycle, allowing the power yield of the TEGs to be evaluated for the chosen cycle. The spatially resolved temperature distribution of a TEG system can be examined by thermal imaging. Hotspots or cracks on thermocouples of the TEGs and the thermal resistance of thermal interface materials can also be examined using this technology. The construction of the setup is briefly explained, followed by detailed discussion of the experimental results.

Considerations in the Use of the Laser Flash Method for Thermal Measurements of Thermal Interface Materials

With increasing thermal fluxes, the performance of thermal interface materials (TIMs) that are used to reduce the thermal resistance between contacting surfaces in electronic devices, such as at the die-to-heat sink or heat spreader-to-heat sink interfaces, is becoming critical. However, measuring the thermal resistances of TIMs in a manner representative of actual applications is difficult. The laser flash method is a technique that may be used to determine the thermal resistance of TIMs and their degradation under environmental exposure. This paper examines three issues associated with using the laser flash method that could limit its effectiveness in calculating thermal resistance: sample holder heating, clamping, and error in the Lee algorithm outputs due to coupon-TIM thermal diffusivity differences. As a case study, the thermal performance of polymer TIMs in pad form, as well as an adhesive and a gel, were examined. Finite element simulations indicated that, without proper consideration, sample holder heating can lead to significant error in the calculated TIM thermal conductivity values.