The continued miniaturization and proliferation of electronics is met with significant thermal management challenges. Decreased size, increased power densities, and diverse operating environments challenge the limitations of conventional thermal management schemes and materials. To enable the continuation of these trends thermal interface materials (TIMs) that are used to enhance heat conduction and provide stress relief between adjacent layers in a electronic package must be improved. Forests comprised of nominally vertically aligned carbon nanotubes (CNTs), having outstanding thermal and mechanical properties, are excellent candidates for next-generation thermal interface materials (TIMs). However, despite nearly a decade of research, TIMs based on vertically aligned CNT forests have yet to harness effectively the high thermal conductivity of individual CNTs. One of the key obstacles that has limited the performance of CNT TIMs is the presence of high thermal contact resistances between the CNT free ends and the surfaces comprising the interface. The aim of this research is to better understand the mechanisms by which the thermal contact resistance of CNT forest thermal interfaces can be reduced and to use this understanding towards the design of effective and to scalable processing methods. Contact area and weak bonding between the CNT tips and opposing surface are identified as factors that contribute significantly to the thermal contact resistance. Three strategies are explored that utilize these mechanisms as instruments for reducing the contact resistance; i) liquid softening, ii) bonding with surface modifiers, and iii) bonding with nanoscale polymer coatings. All three strategies are found to reduce the thermal contact resistance at the CNT forest tips to below 1 mm2-K/W, a value to where it is no longer the factor limiting heat conduction in CNT forest TIMs. These strategies are also relatively low-cost and amenable to scaling for production when compared to existing metal-based bonding strategies.

To allow electronic devices to operate within allowable temperatures, heat sinks and fans are employed to cool down computer chips. However, cooling performance is limited by air gaps between the computer chip and the heat sink, due to the fact that air is a poor heat conductor. To alleviate this problem, thermal interface material (TIM) is often applied between mating substrates to fill air gaps. Carbon nanotube (CNT) based TIM has been reported to have excellent thermal impedance; however, because it is non biodegradable, its potential impact on the environment is a concern. In this thesis research, two types of TIMs were designed, synthesized, and characterized. The first type, Designed TIM 1, consisted of anodic aluminum oxide (AAO) templates with nanochannels (pore size=80nm) embedded with copper nanowires by electrodeposition. This type of nanostructure was expected to have low thermal impedance because the forest-like structure of copper nanowires can bridge two mating surfaces and efficiently transport heat one dimensionally from one substrate to the other. The second type, Designed TIM 2, was fabricated by sandwiching Designed TIM 1 with commercially available thermal grease to further reduce thermal impedance. It was expected that the copper nanowire structures would secure the thermal grease in place, thus preventing grease pump-out under contact pressure, which is a common problem associated with the usage of thermal grease. The morphologies of the two designed TIMs were studied using scanning electron microscopy (SEM), and their thermal properties were determined using ASTM D5470-06, the standard method for testing thermal transmission properties of thermally conductive materials. Experiments were conducted to evaluate the proposed TIMs, as well as commercially available TIMs, under different temperature and pressure settings. Experimental results suggest that the thermal impedance of TIMs can be reduced by increasing contact pressure or reducing thickness. Designed TIM 2 yielded 0.255℃-cm2/W, which is lower than thermal grease and other available TIMs at the operating temperature of 50 to 60℃. Considering the application limitations and safety issues of thermal grease, phase change material, and CNT-based TIMs, our designed TIMs are safe and promising for future applications.

A major challenge in the formulation of thermal interface materials (TIM), used in the microelectronics industry to facilitate heat transfer from an electronic package to a heat sink, is to ensure that the material is electrically insulating while achieving a high thermal conductivity. Several parameters influence thermal conductivity, but it will be shown that proper selection of the polymers serving as binders and additives in the formulation is important. The incorporation of electrically insulated metallic particles as fillers can also help to increase the thermal conductivity of a TIM. This Dissertation is concerned with exploring different strategies for the preparation of thermally conducting, but electrically insulating TIM compositions. Among these, the synthesis and the application of functionalized (telechelic) poly(ethylene oxide), PEO, in the preparation of TIM will be explored in more details. To achieve this goal, we conducted the synthesis of telechelic oligomers containing a primary amine functional group at one end, and investigated their influence on the properties of metallic surfaces such as copper. The results obtained indicate that the PEO-NH2 oligomers can bind to copper metal and/or the oxide layer at its surface, leading to much lowered electrical conductivity for the particles. The composition of TIM formulations was optimized in a systematic fashion using these materials and effective thermal conductivities reaching up to 9.4 W/mK were attained, much higher than for two commercial TIM used as benchmarks (1.5 W/mK for Arctic Silver 5, and 3.5 W/mK for ShinEtsu X23-7783D). Moreover, thinner layers (down to 0.004 mm) were achieved for the materials developed as compared to commercial TIM (0.07 mm). Additionally, we used a computational approach based on the method of random resistor networks to predict the effective thermal conductivity of the TIM. We studied the effects of mono- and polydispersed filler particle size distributions, as well as geometry (spherical particles and flakes). The influence of the PEO-NH2 oligomers acting as surfactants on the effective thermal conductivity was also investigated by defining surfactant concentration-dependent thermal conductivities. We finally compared the results of the numerical simulations with our experimental data and obtained excellent agreement for most compositions.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Graham, Samuel; Committee Member: Das, Suman; Committee Member: Joshi, Yogendra. Part of the SMARTech Electronic Thesis and Dissertation Collection.

abstract: Thermal interface materials (TIMs) are extensively used in thermal management applications especially in the microelectronics industry. With the advancement in microprocessors design and speed, the thermal management is becoming more complex. With these advancements in microelectronics, there have been parallel advancements in thermal interface materials. Given the vast number of available TIM types, selection of the material for each specific application is crucial. Most of the metrologies currently available on the market are designed to qualify TIMs between two perfectly flat surfaces, mimicking an ideal scenario. However, in realistic applications parallel surfaces may not be the case. In this study, a unique characterization method is proposed to address the need for TIMs characterization between non-parallel surfaces. Two different metrologies are custom-designed and built to measure the impact of tilt angle on the performance of TIMs. The first metrology, Angular TIM Tester, is based on the ASTM D5470 standard with flexibility to perform characterization of the sample under induced tilt angle of the rods. The second metrology, Bare Die Tilting Metrology, is designed to validate the performance of TIM under induced tilt angle between the bare die and the cooling solution in an "in-situ" package testing format. Several types of off-the-shelf thermal interface materials were tested and the results are outlined in the study. Data were collected using both metrologies for all selected materials. It was found that small tilt angles, up to 0.6°, have an impact on thermal resistance of all materials especially for in-situ testing. In addition, resistance change between 0° and the selected tilt angle was found to be in close agreement between the two metrologies for paste-based materials and phase-change material. However, a clear difference in the thermal performance of the tested materials was observed between the two metrologies for the gap filler materials. / Dissertation/Thesis / M.S. Mechanical Engineering 2011

It has been known that a significant part of the thermal budget of an electronic package is occupied by the thermal interface material which is used to join different materials. Research in reducing this resistance through the use of vertically aligned multiwall carbon nanotube based thermal interface materials is presented. Transferred arrays anchored to substrates using thermal conductive adhesive and solder was analyzed through a steady-state infrared measurement technique. The thermal performance of the arrays as characterized through the measurement system is shown to be comparable and better than currently available interface material alternatives. Furthermore, a developed parametric model of the thermal conductive adhesive anchoring scheme demonstrates even greater potential for improved thermal resistances. Additionally, a developed transient infrared measurement system based on single point high speed temperature measurements and full temperature mappings is shown to give increased information into the thermophysical properties of a multilayer sample than other steady-state techniques.

Effective application of thermal interface materials (TIM) sandwiched between silicon and a heat spreader in a microelectronic package for improved heat dissipation is studied through thermal and mechanical characterization of high thermally conductive carbon nanotubes (CNTs) integrated into eutectic gallium indium liquid metal (LM) wetting matrix. Thermal conductivity data from Infrared microscopy tool reveals the dependence of experimental factors such as matrix types, TIM contacting interfaces, orientation of CNTs and wetting of CNTs in the matrix on the thermal behavior of TIM composite. Observed generalized trend on LM-CNT TIM shows progressive decrease in effective thermal conductivity with increasing CNT volume fractions. Further thermal characterizations LM-CNT TIM however show over 2x increase in effective thermal conductivity over conventional polymer TIMs (i.e. TIM from silicone oil matrix) but fails to meet 10x improvement expected. Poor wetting of CNT with LM matrix is hypothesized to hinder thermal improvement of LM-CNT TIM composite. Thus, wetting enhancement technique through electro-wetting and liquid crystal (LC) based matrix proposed to enhance CNT-CNT contact in LM-CNT TIM results in thermal conductivity improvement of 40 to 50% with introduction of voltage gradient of 2 to 24 volts on LM-CNT TIM sample with 0.1 to 1 percent CNT volume fractions over non voltage LM-CNT TIM test samples. Key findings through this study show that voltage tests on LC- CNT TIM can cause increased CNT-CNT networks resulting in 5x increase in thermal conductivity over non voltage LC-CNT TIM and over 2x improvement over silicone-CNT TIMs. Validation of LM wetting of CNT hypothesis further shows that wetting and interface adhesion mechanisms are not the only factors required to improve thermal performance of LM-CNT TIM. Anisotropic characteristic of thermal conductivity of randomly dispersed CNTs is a major factor causing lower thermal performance of LM-CNTs TIM composite. Other factors resulting in LM-CNT TIM decreasing thermal conductivity with increasing CNT loading are (i) Lack of CNT-CNT network due to large difference in surface tension and mass density between CNTs and LM in TIM composite (ii) Structural stability of MWCNT and small MFP of phonons in ~5um MWCNTs compared to the system resulted in phonon scattering with reduced heat flow (iii) CNT percolation threshold limit not reached owing to thermal shielding due to CNT tube interfacial thermal resistance. While mixture analytical models employed are able to predict thermal behaviors consistent with CNT-CNT network and CNT- polymer matrix contact phenomenon, these models are not equipped to predict thermo-chemical attributes of CNTs in LM-CNT TIM. Extent of LM-CNT wetting and LM-solid surface interfacial contact impacts on interfacial thermal resistance are investigated through LM contact angle, XPS/AES and SEM-EDX analyses on Au/Ni and Ni coated copper surfaces. Contact angle measurements in the range of 120o at both 55oC and 125oC show non wetting of LM on CNT, Au and Ni surfaces. Interface reactive wetting elemental composition of 21 days aged LM on Au/Ni and Ni surfaces reveals Ga dissolution in Au and Ni diffusion of ~0.32um in Au which are not present for similar analysis of 1 day LM on Au/Ni surface. Formation of Au-Ni-Ga IMC and IMC-oxide iono-covalency occurrence at the interface causes reduction in surface tension and reduction in interfacial contact resistance.

Polymer nanocomposites are used in a variety of applications due to their good mechanical properties. Specifically, better performance of lithium ion batteries and thermal interface material can be obtained by using conductive materials and polymer composites. In the case of lithium ion batteries, electrochemical properties of batteries can be improved by adding conductive additives and conducting polymer into the cathode. Several samples, to which different conductive additives and conducting polymer were added, were prepared and their electrical resistance and discharge capacity measured. In the thermal interface material case, also, thermal properties can be enhanced by polymer nanocomposites. In order to confirm the thermal conductivity enhancement, samples were synthesized using different filler, polymer and methods, and their thermal conductivity measured. The influence of polymer nanocomposites and results are discussed and future plan are presented. In addition, reasons of thermal conductivity changing in each case are discussed.

This thesis presents an experimental apparatus and methodology for measuring the interface conductance of thermal interface materials (TIMs) as a function of clamping pressure. The experimental apparatus is a steady state characterization device based on the basic premise presented in ASTM D5470 – 06. The setup is designed to develop an approximately one dimensional heat transfer through a TIM sample which is held between two meter bars. The temperature is measured along the meter bars using resistance temperature detectors (RTDs) and the temperature drop across the interface is extrapolated from these measurements and then used to calculate the conductance of the interface. This setup and methodology was used to characterize six commercial TIMs at pressures ranging from 0.17-2.76 MPa (25-400 psi). These TIMs included: Tgrease 880, Tflex 720, Tmate 2905c, Tpcm HP105, Cho-Therm 1671, and Cho-Therm T500. The measured conductance values for the various tests ranged from 0.19 to 5.7 W/cm2K. A three dimensional FEA model of the experimental setup was created in COMSOL Multiphysics 4.2a. This model was compared to the experimental data for a single data point and showed good correlation with the measured temperatures and conductance value.

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 109-111).