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.

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).

Shrinking volume, coupled with higher performance, microprocessors and integrated circuits have led to serious heat dissipation issues. In an effort to mitigate the excessive amounts of waste heat and ensure electronic survivability, heat sinks and spreaders are incorporated into heat generating device structures. This inevitability creates a thermal pathway through an interface. Thermal interfaces can possess serious thermal resistances for heat conduction. The introduction of a thermal interface material (TIM) can drastically increase the thermal performance of the component. Exceptional thermal properties of multiwall carbon nanotubes (MWCNTs) have spurred interest in their use as TIMs. MWCNTs inherently grow in vertically-oriented, high aspect ratio arrays, which is ideal in thermal interface applications because CNTs posses their superior thermal performance along their axis. In this paper, laser flash thermal characterization of sandwich‐bonded and cap‐screw‐bonded aluminum discs for both adhesive-infiltrated and “dry”, 100% MWCNT arrays, respectively. Thermal contact resistances as low as 18.1 mm2K/W were observed for adhesive‐infiltrated arrays and, even lower values, down to 10.583 mm2K/W were measured for “dry” MWCNT arrays. The improved thermal performance of the arrays compared to thermal adhesives and greases currently used in the electronics and aerospace industries, characterize MWCNT arrays as a novel, lighter‐weight, non‐corrosive replacement.

High performance/small package electronics create difficult thermal issues for integrated circuits. Challenges exist at material interfaces due to interfacial contact resistances. Multiwall carbon nanotube (MWCNT) arrays are considered to be excellent candidates for use as thermal interface materials (TIMs) due to outstanding thermal/mechanical properties. In this work, MWCNT array TIMs are analyzed in aluminum and carbon fiber composites via flash diffusivity analysis. The effect of TIM thickness, areal/bulk density, surface cleanliness, and volumetric packing fraction; along with the effect of substrate finish and interfacial contact pressure on thermal performance are analyzed. Trends show the best TIMs possess low thickness, high bulk density and packing fraction, and clean surfaces. Pressure dramatically increases thermal performance after establishing contact, with diminishing returns from additional pressure. Diffusivities approaching 40 mm2/s and 0.65 mm2/s are recorded for aluminum and composite systems. Oxygen plasma etching and high temperature annealing (“Graphitizing”) are investigated as methods to remove amorphous carbon from array surfaces. Graphitized TIMs report diffusivity improvements up to 53.8%. Three methods of incorporating MWCNTs into composites are attempted for thermal/mechanical property enhancement. Conductance calculations show increasing diffusivity without increasing thickness enhances thermal performance in composites. MWCNTs for mechanical property enhancement produce no change, or detrimental effects.

abstract: Soft polymer composites with improved thermal conductivity are needed for the thermal management of electronics. Interfacial thermal boundary resistance, however, prevents the efficient use of many high thermal conductivity fill materials. Magnetic alignment of ferrous fill material enforces percolation of the high thermal conductivity fill, thereby shifting the governing boundary resistance to the particle- particle interfaces and increasing the directional thermal conductivity of the polymer composite. Magnetic alignment maximizes the thermal conductivity while minimizing composite stiffening at a fill fraction of half the maximum packing factor. The directional thermal conductivity of the composite is improved by more than 2-fold. Particle-particle contact engineering is then introduced to decrease the particle- particle boundary resistance and further improve the thermal conductivity of the composite. The interface between rigid fill particles is a point contact with very little interfacial area connecting them. Silver and gallium-based liquid metal (LM) coatings provide soft interfaces that, under pressure, increase the interfacial area between particles and decrease the particle-particle boundary resistance. These engineered contacts are investigated both in and out of the polymer matrix and with and without magnetic alignment of the fill. Magnetically aligned in the polymer matrix, 350nm- thick silver coatings on nickel particles produce a 1.8-fold increase in composite thermal conductivity over the aligned bare-nickel composites. The LM coatings provide similar enhancements, but require higher volumes of LM to do so. This is due to the rapid formation of gallium oxide, which introduces additional thermal boundaries and decreases the benefit of the LM coatings. The oxide shell of LM droplets (LMDs) can be ruptured using pressure. The pressure needed to rupture LMDs matches closely to thin-walled pressure vessel theory. Furthermore, the addition of tungsten particles stabilizes the mixture for use at higher pressures. Finally, thiols and hydrochloric acid weaken the oxide shell and boost the thermal performance of the beds of LMDs by 50% at pressures much lower than 1 megapascal (MPa) to make them more suitable for use in TIMs. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2019

This thesis describes the synthesis of a low modulus, thermally conductive thermal interface materials (TIM) using metal decorated nanotubes as fillers. TIMs are very important in electronics because they act as a thermally-conductive medium for thermal transfer between the interface of a heat sink and an electronic package. The performance of an electronic package decreases with increasing operating temperature, hence, there exists a need to create a TIM which has high thermal conduction to reduce the operating temperature. The TIM in this study is made from metal decorated multi-walled carbon nanotubes (MWCNT) and Vinnapas®BP 600 polymer. The sample was functionalized using mild oxidative treatment with nitric acid (HNO3) or, with N-Methly-2-Pyrrolidone (NMP). The metals used for this experiment were copper (Cu), tin (Sn), and nickel (Ni). The metal nanoparticles were seeded using functionalized MWCNTs as templates. Once seeded, the nanotubes and polymer composites were made with or without sodium dodecylbenzene sulfonate (SDBS), as a surfactant. Thermal conductivity (k) measurement was carried out using ASTM D-5470 method at room temperature. This setup best models the working conditions of a TIM. The TIM samples made for this study showed promise in their ability to have significant increase in thermal conduction while retaining the polymer’s mechanical properties. The highest k value that was obtained was 0.72 W/m-K for a well dispersed aligned 5 wt percent Ni@MWCNT sample. The Cu samples underperformed both Ni and Sn samples for the same synthesis conditions. This is because Cu nanoparticles were significantly larger than those of Ni and Sn. They were large enough to cause alloy scattering and too large to attach to the nanotubes. Addition of thermally-conductive fillers, such as exfoliated graphite, did not yield better k results as it sunk to the bottom during drying. The use of SDBS greatly increased the k values of the sample by reducing agglomeration. Increasing the amount of metal@MWCNT wt percent in the sample had negative or no effect to the k values. Shear testing on the sample shows it adheres well to the surface when pressure is applied, yet it can be removed with ease.