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Thermal interface materials based on the carbonallotropes, including diamond, graphite, amorphous carbon,the buckminster fullerene and the carbon nanotube, haverecently attracted massive interest within the researchcommunity. The obvious rationale behind this is the highthermal conductivity offered by the carbon allotropes and incertain cases the cost versus performance ratio. A few goodpapers about the carbon nano structures and their thermalproperties are provided in references [7-9]. Other kinds offillers with high thermal conductivity, such as siliconcarbide, boron nitride, aluminum nitride, aluminum oxide,silver and other metals, have also been extensively studied.The distribution and alignment of the thermally conductivefillers are also important factors. For example, thermalconductivity enhancement with aligned CNTs is claimed tobe at least 3 times of that with randomly distributed CNTsaccording to the calculation [10]. Another example is thatbimodal distribution of conductive particles is found toproduce better thermal properties [11, 12]. Table 1 gives ancomprehensive overview of the recent related researchworks, listing the materials structures, fabrication methods,test methods, results, and so on. The table is divided into twoparts: TIMs with carbon-based fillers and those filled withother materials.IV. STATE-OF-THE-ART RESEARCH: OTHER ASPECTSBesides the massive work on developing novel thermalinterface materials, research work is also performed in otheraspects of TIM technology.Solder-based TIMs and phase change materials are studiedin some companies, such as Intel [68], IBM [69], andHoneywell [70, 71]. Theoretical research of TIMs based onanalytical, statistical and numerical methods is alsoperformed intensively. Some examples can be found from,41, 72-78]. The modeling and simulation of TIMs usingmolecular dynamics or finite element method is alsoexecuted [79-82]. Difficulties still exist in direct linking oftheoretical calculation, modeling and simulation to theexperimental work. However these studies are veryimportant as valuable guidance and assistance to theexperimental research. It’s also worthwhile to mention theresearch on small scale structures within TIMs, such as voidsand contacts [83-87].Some research has also been done on modifying theinterface surface other than the TIM itself to improve theheat transfer. An unique technique called hierarchical nestedsurface channels (HNC) developed at IBM has been shownto reduce interface resistance due to thinner bondlines andshown to reduce the required squeeze loads [93]. The surfacechannels ability to easily evacuate material from theinterface with low assembly loads help maintain thinbondlines at the center of the interface for large area and thinsubstrate applications that are mounted with only peripheralloads applied (i.e. bolted). Because the surface channels alterthe flow pattern of viscous material during assembly theyalso prevent the non-uniformities that typically developduring bondline assembly such as particle stackingformations between chip corners. The individual HNC cellsactually create periodic arrays of particle stacks typically24-26 September 2008, Rome, Italywith a thinner bondline due to the delay in particleinteractions until later in the squeeze process. The smallscale stacking pattern within each HNC cell is formed due tothe bifurcation of the flow to each edge of the cell. Becausethe orientation and concentration of suspended particles ishighly dependent on localized shear flow, the ability of thechannels to redirect flows as desired may allow theexploitation of anisotropic properties in materials with highaspect ratio micro and nanoparticles. Oriented or optimizedparticle stacks could produce arrays of micro thermal andelectrical vias that significantly improve performance. Theconcept and particle stacking results of this HNC technologyis shown in Fig. 1.Fig. 1 Particle stacking associated with flat interfaces (left)and HNC (middle) with a standard nested channel layoutshown on right.V. DISCUSSIONSThermal interface materials have become a popularresearch topic in recent years, drawing attention from notonly material scientists, but also chemists, physicists, andmany researchers in other fields. There are a few reasonsbehind the popularity of TIM research. One driving force isthe urgent necessity of novel high performance materialsfrom the industry. The discovery and great achievements ofhigh thermally conductive nano- structures, especiallycarbon nanotubes and nanofibers, form another obviousreason.It can be clearly seen from the discussion in previoussections and summarily table that the majority of recentresearch works is focused on the development of thermalinterface materials with carbon fillers, among which carbonnanotube is the most chosen filling material. Noteworthyplayers in the field of carbon-based TIM developmentinclude research groups from Purdue University [6, 23-25,49, 50], Hongkong University of Science and Technology[17-19], and Tsinghua University [31] etc. The researchwork performed at Purdue University is noticeable due to thevarious growth methods of CNTs and test approachesapplied in TIM development. At Hongkong University ofTechnology, interesting works on “lift-off” transfer of CNTfilm and development of CNT-Cu composite have beenperformed. Unique approach of making CNT-PDMScomposite by inject molding has invented at TsinghuaUniversity.Despite the massive research work, these new thermalinterface materials still only exist in laboratories and are faraway from real application. The performance of most thesenovel TIMs is still not high enough to overtake the currenthigh-end commercial products. Many of them even generatelower results compared to commercial materials. Severaltechnical barriers can be identified. Although theoretical©EDA Publishing/THERMINIC 2008 157ISBN: 978-2-35500-008-9