Nanothermal Interface Materials: Technology …electronicpackaging.asmedigitalcollection.asme.org/data/Journals/...Nanothermal Interface Materials: Technology Review and Recent Results

Avram Bar-CohenFellow ASME

Defense Advanced Research Project

Agency (DARPA)/Microsystems

Technology Office (MTO),

675 North Randolph Street,

Arlington, VA 22203

e-mail: [email protected]

Kaiser MatinMem. ASME

System Planning Corporation,

3601 Wilson Blvd,

Arlington, VA 22201


Sreekant NarumanchiMem. ASME

National Renewable Energy Laboratory,

15013 Denver West Parkway,

Golden, CO 80401


Nanothermal InterfaceMaterials: Technology Reviewand Recent ResultsThermal interface materials (TIMs) play a critical role in conventionally packaged elec-tronic systems and often represent the highest thermal resistance and/or least reliableelement in the heat flow path from the chip to the external ambient. In defense applica-tions, the need to accommodate large differences in the coefficients of thermal expansion(CTE) among the packaging materials, provide for in-field reworkability, and assurephysical integrity as well as long-term reliability further exacerbates this situation.Epoxy-based thermoplastic TIMs are compliant and reworkable at low temperature, buttheir low thermal conductivities pose a significant barrier to the thermal packaging ofhigh-power devices. Alternatively, while solder TIMs offer low thermal interface resistan-ces, their mechanical stiffness and high melting points make them inappropriate for manyof these applications. Consequently, Defense Advanced Research Projects Agency(DARPA) initiated a series of studies exploring the potential of nanomaterials and nano-structures to create TIMs with solderlike thermal resistance and thermoplasticlike com-pliance and reworkability. This paper describes the nano-TIM approaches taken andresults obtained by four teams responding to the DARPA challenge of pursuing the devel-opment of low thermal resistance of 1 mm2 K/W and high compliance and reliabilityTIMs. These approaches include the use of metal nanosprings (GE), laminated solderand flexible graphite films (Teledyne), multiwalled carbon nanotubes (CNTs) with layeredmetallic bonding materials (Raytheon), and open-ended CNTs (Georgia Tech (GT)). Fol-lowing a detailed description of the specific nano-TIM approaches taken and of the me-trology developed and used to measure the very low thermal resistivities, the thermalperformance achieved by these nano-TIMs, with constant thermal load, as well as undertemperature cycling and in extended life testing (aging), will be presented. It has beenfound that the nano-TIMs developed by all four teams can provide thermal interfaceresistivities well below 10 mm2 K/W and that GE’s copper nanospring TIMs can consis-tently achieve thermal interface resistances in the range of 1 mm2 K/W. This paper alsointroduces efforts undertaken for next generation TIMs to reach thermal interface resist-ance of just 0.1 mm2 K/W. [DOI: 10.1115/1.4031602]

Introduction

The nanothermal interfaces (NTI) thrust of the Thermal Man-agement Technologies (TMT) program kicked off in mid-2009[1]. The main objective was to develop TIMs based on novelmaterials and nanostructures that can provide significant reduc-tions in the thermal resistance of the interface layer between thebackside of an electronic device and the next layer of the package,typically a heat spreader, heat sink, or coldplate. Increased heatflux of DoD electronics has required significant TIM improve-ments. Many DoD systems operate at voltages, temperatures, fre-quencies, or other parameters that are outside of the rangespecified for reliable long-term operation. As a result, the failurerate for electronic devices under DoD field conditions is oftenhigher than in commercial equipment and, therefore, the NTI pro-gram also requires the TIMs to be reworkable. Requiring compo-nents to be replaced and “reworked” in the field denotes that theTIMs must have the ability to be applied at modest temperaturesand in conventional environments. In addition to providing high

thermal conductivity in the through-thickness direction, the NTIsmust also accommodate nonplanarity and roughness on the matingsurfaces. Finally, the NTIs were required to have long-term reli-ability (with a minimum of 100 demonstrated temperature cycles)and consistency from chip to chip.

Thermal greases or epoxies, mixed with highly conductivefillers, have relatively low effective thermal conductivity, as thegrease or epoxy insulates the filler material. Increasing the con-centration of the fillers along the heat flow path may establish pre-ferred paths for heat flow and improve the effective thermalconductivity, but such concentrations strongly increase the viscos-ity of the grease or paste, as well as the stiffness of the solidifiedcomposite and reduce the compliance of the TIM [2,3]. SolderTIMs, on the other hand, offer high thermal conductivity and lowthermal interface resistance. But, in typical thicknesses of25–50 lm, they have low mechanical compliance, which makesthem prone to high stress failure in the presence of thermallyinduced differential expansion caused by a “mismatch” in theCTE across the TIM. In order to meet the compliance require-ment, a minimum indium thickness of approximately 200 lm isneeded, significantly impeding thermal performance.

Figure 1 displays the TIM thermal resistivity versus thicknessmeeting the compliance for four NTI technologies, several com-mercial off-the-shelf (COTS) products, and two theoretical curvesfor high-conductivity TIM materials. The green lines and points inthe figure represent selected thermal greases and epoxies and thered sloped lines on the plot pertain to theoretical resistivities for

Contributed by the Electronic and Photonic Packaging Division of ASME forpublication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received July 1,2015; final manuscript received September 11, 2015; published online October 9,2015. Assoc. Editor: Ashish Gupta.

The United States Government retains, and by accepting the article forpublication, the publisher acknowledges that the United States Government retains, anon-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce thepublished form of this work, or allow others to do so, for United States Governmentpurposes.

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Indalloy and Indium solder. As shown in the plot, thin bond linesof Indium (80 lm) and Indalloy (35 lm) solder can reach theNTI’s thermal resistance goal of 1 mm2 K/W. However, the twocommonly bonded packaging materials: silicon, with a CTE of2.6� 10�6/K, and copper, with a CTE of 16.7� 10�6/K, create alarge CTE difference and, with thin bond lines, make these TIMsprone to high thermal stress and fatigue failure. The NTI program,therefore, addresses both low thermal resistance and high mechan-ical compliance, so that the TIMs provide superior thermal per-formance while maintaining robustness under thermal cycling andaging.

Technology transition is a critical goal of the NTI effort, as wellas the overall TMT program, and the final program goals reflect thetransition partner needs of each NTI team. Thus, each team hasworked with their transition partners to identify a military systemthat would uniquely benefit from the thermal management techno-logy under development. Toward that end, the transition partnershave provided their respective team with the requisite product/system specifications, including form factors, configurations, per-formance metrics, acquisition standards, and technology qualifi-cation procedures. Therefore, modeling, analysis, design, andfabrication of the NTI prototypes were carried out according tothese specifications. The prototypes were expected to utilize theunique features of the specific NTI technology and provide per-formance that materially exceeds the state of the art (SOA) in keyparameters. Finally, the projects’ experimental and modeling resultsserve to articulate the TMT “value proposition,” to provide theDoD with the greatest performance value for its investment.

Several materials, such as epoxies and solders, were investi-gated, but ultimately the performers found that solder was theonly acceptable attachment method. Thus, in evaluating the resis-tivity of the TIM attention must be directed to three distinct“thermal stack” resistances: the “bulk” TIM and an interfacematerial (i.e., solder) on either side. In order to meet the stringentthermal resistance goals of the NTI program, several teams pur-sued the use of low temperature solder materials, includingpure indium, for attaching their nanostructured TIMs. The use ofthese materials and indium-based solders, in particular, requiresadditional testing and analysis before transition of the NTI tech-nologies into fieldable DoD platforms.

Table 1 shows the material sets, identifying the interface mate-rial, as well as the conducting structure, and TIM approach of thefour NTI teams, led by Raytheon, Teledyne, GT, and GE, respec-tively. The specific approaches taken and the applications targetedare more fully described in a later section of this paper, NTI Tech-nologies and Issues. Each NTI is engineered with unique materi-als, structures, and configurations to provide maximum benefit totheir targeted military system application. The method or materialused for attaching the NTIs was a matter of concern throughoutthe NTI program.

The NTI metrics, listed in Table 2, are seen to consist of ther-mal resistivity, temperature cycles, stability, reworkability, appli-cation time, shear force, etc., with progressively more difficultmetrics as the development teams progress from Phase I to PhaseII and then to the application form factor of Phase III. It is to beseen that Phase III goals include surface roughness tolerance,maximum processing temperature and area applied.

Phase III resistivity goal, of less than 1 mm2 K/W, posed a sig-nificant metrological challenge to the performer teams. The GTteam developed a method using infrared microscopy for meas-uring the total thermal resistance across multiple interfaces. Themethod is capable of measuring samples of wide ranging resistan-ces with thicknesses varying from 50 lm to 250 lm. The NationalRenewable Energy Laboratory (NREL), within the DOE, wasselected to perform TIM characterization for the NTI program andhave developed significant expertise and infrastructure for thethermal characterization of low resistivity TIMs. The NRELperformed the following tasks to investigate the performance ofeach NTI technology under conditions from Table 2: (1) one-dimensional steady-state resistance measurement of NTI samplesheld between copper blocks, (2) thermal cycling of the NTI sam-ples, and (3) thermal aging of the NTI samples held between cop-per and silicon, with both the one-dimensional steady-state testingand acoustic microscopy, before and after the extended “aging.”

As part of the characterization—and “qualification”—of theNTI samples, all the teams were asked to evaluate the reworkabil-ity of their NTI’s in the following two ways: (1) re-attach themating surfaces five or more times with “fresh” NTI’s and

Fig. 1 NTI thermal resistance versus thickness compared toCOTS technologies

Table 1 NTI team approach

Interface Ti/Pd Bismuth solder Molecular coupling layer Indium solderConduction

Double-sided CNT foil Raytheon, Purdue,Georgia TechEnd: 1/2012/2013

Diamond/graphite Teledyne, MIT,BAE SystemsEnd 12/2012

Open-ended MWCNT Georgia Tech,Northwestern University,Rockwell CollinsEnd: 10/21/2012

Metal nanosprings/nanowires GE, University of Illinois,Urbana-Champaign, ILEnd: 3/14/2013

040803-2 / Vol. 137, DECEMBER 2015 Transactions of the ASME


document the resulting sequence of thermal resistances and (2)attempt to reuse a single NTI sample up to five times—dismantling and re-attaching the mating surfaces each time—andreport the results. It was realized that some of the NTI technolo-gies might not be suited for reuse and that each attempt to reusethe NTI might result in either (a) the destruction of the sampleupon removal or (b) the failure to reattach. However, the teamswere asked to attempt to reuse their samples and document thechanges in the thermal resistance, so that the features and limita-tions of their approaches were understood.

NTI Technologies and Issues

Raytheon Corporation. In collaboration with the GeorgiaInstitute of Technology and Purdue University, the Raytheon NTIteam developed nanothermal interface materials (nTIMs) basedupon metallically bonded, vertically aligned carbon nanotubes(VACNTs), of the type shown in Fig. 2. The short, verticallyaligned CNTs are grown on both sides of a thin interposer foil andinterfaced with substrate materials via metallic bonding andattached using Sn63, Indalloy 121, or Indalloy 256 solder on bothsides [4–9]. These nTIMs provide the thermal performance of asolder joint while maintaining the compliance typical of a low-conductivity, filled-epoxy, or grease. The Raytheon team has

achieved a factor of three improvement in interfacial resistancerelative to the SOA commercial TIMs.

A high-precision, one-dimensional, steady-state conduction testfacility was utilized to measure the performance of the nTIM sam-ples and, more importantly, to correlate performance to the con-trollable parameters. Hundreds of samples have been tested,utilizing myriad permutations of these parameters, and contribut-ing to a deeper understanding and optimization of the CNTgrowth characteristics and application processing conditions.

With the nTIM, junction temperature reductions of 30 �C ormore are possible. This increases the device efficiency by approxi-mately 10%, reducing prime power consumption. It also increasesthe power output by approximately 10%, improving range andsensitivity. Finally, the expected 30 K temperature reduction canincrease reliability by approximately 10� , reducing life-cycle/ownership cost.

Figure 3 shows that the Raytheon nTIM offers benefits at alllevels electronic device stack-up. An nTIM1 will offer thermalperformance equivalent to the SOA in die-attach but withimproved mechanical compliance/die stresses. An nTIM2 willsignificantly improve heat flow from the chip carrier to the heatspreader and enable an increased device footprint for a givenstackup CTE mismatch. Use of an nTIM3 to attach the heatspreader to the heat sink will offer improved thermal performance,enhanced surface conformability, and will allow process repeat-ability and rework.

Table 2 NTI metrics

Metric Unit SOA Phase I Phase II Phase III

Thermal resistivitya mm2 -K/W 9 <7 <4 <1Temperature cyclesb #, �40 to 150 1000 >10 >50 >100

0–70 �C �40 to 80 �C �40 to 125 �CReworkc # 3–5 1 >3 >10Stabilityd hrs and % degradation >1000 >100 >300 >1000

<25% <25% <10% <5%Application timee hrs 2.5 10 5 2Shear forcef kg 2.5 2.5 3.5 5

Surface roughness tolerance lm 25 5 10 10Area cm2 1 0.25 >1 >1

Maximum processing temperature �C 170 240 240 240

Maximum processing pressure psi 50 <30 <30 <30

Note: Additional comments: device/substrate is assumed to be Si/Cu, although other combinations are allowed for specific applications. Surface prepara-tion should be no more complex than simple O2 plasma. Storage should not be more complex than temperature of �40 �C in dry package. After process-ing, TIM shall not require additional long-term static pressure beyond requirements for assembly and fixtures.aUnits defined as thermal resistance (C/W) normalized by area. Results must include Rth of TIM and both interfaces.bTen samples tested, all below thermal resistance specification for phase at all times. Ramps at< 3 �C/min (slow) and> 25 �C/min (fast).cDisassemble and reassemble substrate/TIM/substrate N times and meet all other program requirements for Phase.dSteady operation at 130 �C and meet Rth requirement for Phase. Degradation is based on Rth measurements before and after test for specified duration at130 �C.eApplication time includes all surface prep, attachment, and curing.f1 cm2 die must sustain shear force requirement while maintaining 100% attach.

Fig. 2 Raytheon nTIM device overview. Multiwalled CNTs grown on both sides of a grapheneor metallic foil; ends of CNTs are metalized to enhance adhesion.

Journal of Electronic Packaging DECEMBER 2015, Vol. 137 / 040803-3


Figure 4 shows that the nTIM’s thermal performance, at lessthan 0.02 cm2 �C/W at 100 lm thickness, is an order of magnitudebetter than a gap pad, ten times better than thermal grease, andapproximately five times better than silver-filled epoxy at anincreased bondline thickness than the latter two TIMs. This CNT-based nTIM has also demonstrated stability after thermal cyclingand high-temperature baking with multiple samples. During PhaseIII, the Raytheon team improved bonding methods and increasedthe nTIM footprint. They demonstrated the nTIM in Raytheonproduct representative settings and explored potential collabora-tion with commercial ventures for larger-scale production.

GT. The GT NTI team developed processing techniques thatuse separated steps of CNT synthesis and low-temperature trans-fer to a mating surface. Due to their high thermal conductivity (upto 3000 W/mK), GT’s CNTs are a natural choice for the TIM forelectronic components with high heat dissipation [10–20]. Early

NTI attempts used CNTs as fillers, to form high thermal conduc-tivity fluids or TIM composites. However, this approach has pro-ven not to be effective, due to the random dispersion and theintermittent contact among the CNTs. A more advanced approachinvolves growing CNTs vertically on a silicon wafer. These verti-cally aligned CNTs can then be attached to a copper heat spreaderto form a thermal interface structure.

GT’s nano-TIM is based on well-aligned, open-ended VACNTsthat have superior thermal conductivity in the heat transfer direc-tion. The approach is different than Raytheon’s as it has no inter-poser. GT applied extensive measurement capability to their effort,includes the use of infrared microscopy for thermal resistancemeasurements [14]. Thermally conductive adhesives are used asthe attachment material for the NTI examples shown in Fig. 5, butuse has been made of Ag ink, indium, and other materials. TheGT’s VACNTs were grown by chemical vapor deposition on bothsides of a compliant interposer foil and interfaced with various sub-strates via a metallic bond. Two metallic bonding techniques wereexplored: (1) palladium nanoparticle and (2) gold diffusion.

General Electric (GE) Global Research. GE’s compliantnTIM allows for thin solder bondlines by using a compliantstructure of copper nanosprings, fabricated via glancing angledeposition (GLAD) on W or Si with a Cu capping layer, toaccommodate the thermal expansion difference of their matedmaterials [21–23]. Process parameters that control the springstructure are: substrate rotation speed controlling spring radius,growth time controlling spring height, and the rotation speedand growth time controlling the number of turns and, thereby,stiffness. The capping layers are electroless plated Ni to mini-mize corrosion and oxidation. They are attached to the substrateusing solder on both sides: Indium and Sn62 have been

Fig. 3 Raytheon nTIM application

Fig. 4 Comparison of Raytheon NTI performance with alternate TIMs

Fig. 5 GE Compliant NTI TIM [19]



explored. Nanosprings are 100� more compliant than solders,so the thermal stresses are carried by the nanosprings ratherthan the solders, enabling thinner solder layers. TIMs generallyfail due to thermal strain during thermal cycling but nanospringscan stretch under thermal strain.

The GE nanospring nTIM is more than 100� higher in compli-ance than the solder bond, allowing for thin bondlines with thermalresistance less than 0.01 cm2 �C/W. The best performance amongcommercially available TIMs is that of an indium solder layer thatis reflowed and wetted to the heat sink and silicon surfaces. In typi-cal applications the resistance is limited by the bondline thickness,which, due to thermal fatigue, must be kept to around 200 lm. Athermally conductive, thermal stress relieving layer within thebondline, i.e., the approximately 100 lm high nanospring structure,enables much thinner solder bondlines. The GE NTI’s bondlinesuse thin mating layers with the CTE close to the respective sub-strates to minimize thermal strain. A schematic figure of the struc-ture is shown in Fig. 6 where the TIM is used to bond a silicon dieto a copper heat sink. The nanospring length is the factor that needsto be changed for variation in the amount of thermal strain that hasto be managed for a reliable interface material. Larger bondingarea, CTE mismatch, and temperature cycling range will requirelonger nanospring extension but this has not been characterized forreliability. The nanospring length is varied by the spring pitch andturns. The pitch and radius are fixed by the deposition angle so thepitch and radius increase linearly. The stiffness decreases withincreasing height and with material modulus.

Integrated with GE’s thermal ground plane (TGP) [20], the GENTI is designed to be integrated into GE’s Intelligent PlatformsGRA111 VPX line-replaceable module. Integration of the NTIand TGP will yield a 40% reduction in temperature rise, enable66.7% higher power and processing speed, and provide lowthermal resistance and reliable bonding to a low-cost copper lid.TGP and NTI together could improve the thermal performance ofthe GRA111 and increase its mean time between failures.

The GE team was the first to reach Phase III resistivity goal of1 mm2 K/W. However, this remarkably low value necessitatedcareful calibration and repeated measurements, with laserflash equipment, before this result could be confirmed. More in-formation about performance testing is in the NTI Performancesection of this paper.

Teledyne. Teledyne’s laminated, carbon-based NTIs using ver-tically aligned graphite–metal composites is shown in Fig. 7.They are based on highly oriented two-dimensional graphite nano-platelets (GNPs) embedded in Indalloy 121 solder. The GNPs arecompressed into flexible graphite sheets and are then introducedinto the solder, with the high-conductivity planes aligned alongthe primary heat transfer direction. With this structure, it is easyto form straight, highly conductive paths along the desired heatflow direction [24]. Moreover, the metallic host matrix has amuch higher thermal conductivity than that of a polymer host ma-trix and, due to its excellent flow characteristics in the molten

Fig. 6 GT NTI overview

Fig. 7 Teledyne NTI overview



state, can drastically improve the quality of physical contactbetween the fillers and the substrate surfaces. The bondingbetween the graphite and the metallic host is also stronger thanthe bonding between GNPs and nonmetallic materials. Further-more, compared to organic hosts, solders have much lower pho-non spectra mismatch with the GNPs and CNTs, and thus, offer asignificantly higher interface conductance which greatly improvesthe overall thermal performance [25]. CTE mismatch can be con-trolled by ratio of GNP: solder, CNT spacing, or GNP volumefraction. Also solder and graphite sheet thickness can be adjustedto control compliance [26]. Teledyne reported that their NTIachieved an overall thermal resistivity as low as 1 mm2 K/W, at abonding temperature approximately 230 �C, and compressionpressure of 30 PSI using improved fabrication processes for thegraphite-based TIMs [27]. Due to its unique construction, the Tel-edyne NTI can function as both a TIM and an effective heatspreader, eliminating the need for costly high conductivity spread-ers, made of diamond or Boron Nitride, and simplifying the pack-aging process.

NTI Testing Methods

To experimentally characterize the thermal properties and ther-mal performance of TIMs, a variety of techniques have beendeveloped and utilized, including both steady-state and transienttechniques [NREL paper]. In this section, we mainly focus on theTIMs which were characterized by metrology measurementsprovided at NREL using several different techniques.

Xenon Flash Technique. The xenon flash technique was usedto evaluate the performance of NTI bonded samples, using aNetzsch LFA 447 Nanoflash instrument. A xenon flash pulse irra-diates one surface of the test sample and the generated thermalenergy then penetrates and flows toward the underside of thesample as shown in Fig. 8. An infrared detector with a 7.8-mm

aperture records the sample’s temperature rise as a function oftime. The technique is demonstrated in Fig. 8. Under an adiabaticcondition, the thermal diffusivity is directly calculated using thetemperature rise profile via the following equation:

a ¼ 0:1388l2

t50

(1)

where a is the thermal diffusivity, l is the thickness of the testsample, and t50 is the time at which 50% of the temperature risehas occurred on the back side of the sample.

To determine the thermal conductivity, the density and heatcapacity of the test samples must also be known. Using measuredvalues or from previous knowledge of a sample’s bulk density andspecific heat, its thermal conductivity can be calculated, as shownin the following equation:

kðTÞ ¼ aðTÞ � qðTÞ � CpðTÞ (2)

where T is the temperature, k is the thermal conductivity, q is thebulk density, and Cp is the specific heat.

When the thermal conductivity is known and w with knowledgeof the sample’s bondline thickness, the thermal resistance of theinterface layer can be calculated

R ¼ x

k(3)

where R is the thermal resistance, x is the bondline thickness, andk is the thermal conductivity.

Steady-State ASTM Test Stand. NREL has developed asteady-state test stand for measuring the thermal resistance ofTIMs (TIMs and NTIs). The operation of this steady-state teststand follows the method outlined in ASTM D5470-12.

Fig. 8 Xenon flash technique

Fig. 9 Steady-state ASTM test stand



The basic configuration of the test apparatus is shown in Fig. 9,along with the picture of an actual test apparatus that was used totest interface materials. The heater cartridges are embedded intoan aluminum hot plate, while silicone oil is circulated through analuminum cold plate.

Four thermistors (or resistance temperature detectors) areembedded in the metering blocks which have the TIM/NTIbetween them. The metering block is made of oxygen-free copperwith a thin nickel coating to prevent oxidation and also to preventthe block from erosion or corrosion. The average roughness of thesurface of the blocks (facing the TIM/NTI) is about 0.5 lm.Through measurement of heat flux and temperature differenceacross the sample, the thermal resistance is computed.

Phase-Sensitive Transient Thermoreflectance (PSTTR). TheThe PSTTR technique employs two localized fast lasers tohelp determine the critical thermal properties of materials and

interfaces, such as thermal conductivity and thermal resistance. Amodulated pump laser is directed on the sample surface to inducea temperature rise and the consequent thermal wave travelsthrough the sample. A probe laser is used to detect the temporaltemperature variation on the back surface, which responds to theperiodicity of the heating on the front surface of the sample. Byextracting the inherent phase information, the thermal propertiesare derived. Compared with the xenon flash technique and steady-state ASTM test stand, the PSTTR technique is sensitive to smalltemperature changes (within a few degrees) and has a high resolu-tion which enables measurement of thermal resistances lowerthan 1 mm2 �K/W [28,29]. The experimental configuration of thePSTTR is depicted in Fig. 10.

Reliability Testing and Characterization. The NTI samplesprovided by the four DARPA teams were subjected to temperaturecycling as well as thermal aging at an elevated temperature. In

Fig. 10 PSTTR technique experimental setup



thermal aging tests, the samples were exposed to a temperatureof 130 �C for 300 hrs. In thermal cycling tests, the samples weresubjected to temperatures from �40 �C to 80 �C at low (3 �C/min)and high (25 �C/min) ramp rates. The thermal cycle testing array,shown in Fig. 11, was used to study thermally induced stresses inthe novel bonded interfaces. The chambers contain workspacevolumes of 50� 28� 30 cm and are capable of operating from�70 �C to 180 �C.

Acoustic microscopy images were taken of the NTI bondlinesfor qualitative evaluations of the interfaces, before and duringcycling and aging tests. The C-mode scanning acoustic micro-scope (C-SAM), shown in Fig. 12, is an instrument that uses ultra-sound in the frequency range of 5–230 MHz to nondestructivelyinspect samples for defects.

The technique relies on the acoustic impedance mismatch ofmaterials, and can generate images within a sample based on scat-tering, refraction, and/or absorption of the acoustic signal withinvarious material layers. Ultrasound cannot travel through air or avacuum; therefore, it is capable of finding defects within a samplesuch as voids, cracks, and/or delaminations.

NTI Performance

The full suite of NREL measurement techniques was used toevaluate the thermal and reliability characteristics of the sampleNTI’s produced by the DARPA performers. They include alignedCNTs (Raytheon and GT), a laminated graphite and solder

composite (Teledyne), and copper nanosprings (GE). GE hasestablished an assembly technique that forms metal nanospringsby the GLAD process. The number of springs, diameter of springwire, radius of winding, number of windings, and overall springlength can be controlled by the GLAD process. This makes it pos-sible to engineer the desired shear and compressive compliancewithin the nano-interface material while also optimizing for mini-mal thermal resistance. Teledyne developed a bonding processthat vertically aligns graphite platelets within the contact areabetween two surfaces. The platelets are first aligned and com-pressed into thin layers before a solder binds the graphite layers toeach other and to the surfaces. The Georgia Institute of Technol-ogy led an effort to develop a low-temperature process that growsand aligns CNTs as a thin interface material.

Two rounds of testing, in which standardized test samples werebonded by the performers for accelerated thermal cycling, wereconducted by NREL. The first round of testing was performed onthe samples from GT, GE, and Teledyne. NREL did not receivesamples from Raytheon for the first round of testing. The meas-ured thermal resistance results warranted improvements in mate-rial performance, and hence, the performers were given anopportunity to modify their materials before a second round oftesting was conducted. For the second round of testing, NRELreceived samples from Raytheon, GE, and Teledyne while GTopted not to participate in this phase of the testing. In each roundof testing, initial thermal performance was characterized by thexenon flash transient measurement technique. Xenon flash meas-urements were taken at periodic intervals during and after thecompletion of accelerated testing. First round testing of GT willbe presented here only.

First Round Results. The thermal performance and reliabilityof the performers’ interface materials were characterized by utiliz-ing 10-mm� 10-mm cross-sectional footprint samples of siliconbonded to copper via the NTIs, as shown in Fig. 13. The surfaceroughness was not measured. The silicon diodes used for creatingthe bonded samples were 350 lm thick and were provided with abackside metallization of aluminum/titanium/nickel/silver. Thecopper coupons were 1 mm thick and were not provided with anymetallization. Surface preparation and additional metallizationprocessing were allowed for the teams to optimize the bondstrength with their interface materials. After bonding, bondlinethicknesses varied amongst the performers’ samples from 70 to325 lm. These samples were made to simulate actual thermos-mechanical behavior of electronic packages.

Bonded samples were evaluated for thermal performance, usinga Netzsch LFA 447 Nanoflash instrument. The Nanoflash operatesfollowing the ASTM E-1461-13 test standard [18]. A xenon flashpulse directs energy toward the underside of a test sample. Aninfrared detector with a 7.8 mm aperture records the sample’stop-side rise in temperature as a function of time.

Fig. 11 Thermal cycling and aging testing array

Fig. 12 C-SAM

Fig. 13 Silicon and copper coupons



Prior to testing, all samples were sprayed with DGF-123 DryGraphite Film Spray. This uniform graphite coating allows forconsistent absorptivity of the xenon flash pulse and emissivity tothe infrared detector between test samples. Initial thermal resist-ance measurements are summarized in Table 3 and Fig. 14.

The GT NTI design was targeted to produce a thermal resistiv-ity of 3.6 mm2 K/W. Approximately half of the samples weremeasured to have thermal resistances lower than 5 mm2 K/W;however, a significant number of samples were measured withmuch higher resistivity values. This result indicates that whileGT’s approach has the potential to produce a low thermal resist-ance material, significant synthesis variations are present in theproduction process.

In addition to transient thermal measurements with the nano-flash apparatus, acoustic microscopy images were taken of thebondlines for qualitative evaluations of the interfaces. In general,darker areas indicate a strong bond between the silicon and coppercoupons while lighter areas denote the likely presence of voidingor delamination. The GT NTI samples typically had large areas ofdiscontinuity in bond quality (Fig. 15). The presence of lighter,poorer bond areas in these samples correlated with higher thermalresistance measurements. For reference, a sample bonded withlead-solder is shown with a high percentage of voiding.

Reliability Testing and Characterization. The bonded sam-ples were subjected to accelerated tests in the form of temperaturecycling as well as thermal aging at an elevated temperature. Intotal, 15 samples were tested as shown in Table 3. In thermalaging tests, the samples were exposed to a temperature of 130 �Cfor 300 hrs. In thermal cycling tests, the samples were subjected totemperatures from �40 �C to 80 �C at low (3 �C/min) and high(25 �C/min) ramp rates. Transient thermal measurements with theNanoflash apparatus were performed to characterize the thermalperformance of all samples prior to, during, and after accelerated

testing. Acoustic microscopy was used to monitor the conditionof the interfaces during the same analysis intervals. Samplesthermally aged at 130 �C were inspected every 100 hrs.

Aging Tests. After 300 hrs, the GT NTI samples all showed anincrease in thermal resistance, as shown in Table 4 and Fig. 16,though 2 of the 4 samples (#2 and #4) experienced just a 50%

Table 3 Initial sample thermal resistance (mm2 K/W)

Sample number GT

1 28.82 16.73 13.34 4.65 13.46 9.87 3.68 4.39 4.910 46.811 15.612 4.913 3.914 4.715 4.4

Fig. 14 Initial sample thermal resistance (mm2 K/W)

Fig. 15 Acoustic images of samples from GT (left) and leadsolder as a reference (right)

Table 4 GT thermal aging typical results (mm2 K/W) 300 hrs at130 �C

GT samples

Number of hours 1 2 3 4

0 28.8 16.7 13.3 4.6100 68.3 30.7 27.9 5.1200 72.2 22 36.6 6.5300 86.5 24.3 43.3 7

Fig. 16 GT thermal aging results (mm2 K/W)



increase in resistivity over that time period. Thus, the lowestresistivity sample that initially was measured at 4.6 mm2 K/Wonly reached 7 mm2 K/W when the aging test concluded. Thethermal resistance of the bondline within samples approached100 mm2 K/W for just one case, suggesting that a failure of theinterface would occur shortly, if thermal aging continued.

Thermal Cycling—Low Ramp Rate Tests. Under low ramp ratethermal cycling conditions, samples were subjected to tempera-ture variations from �40 �C to 80 �C and transitioned between theextremes at a low, 3 �C/min, ramp rate. Samples were inspectedevery ten cycles with transient thermal measurements and acousticimaging. The GT NTI samples showed an approximately 50%increase in thermal resistance for samples that were initially meas-ured at 4 mm2 K/W. Samples that were initially measured at10 mm2 K/W or higher showed a significantly larger increase inthermal resistivity, approaching three times the initial value. TheGT typical sample results are summarized in Table 5 and Fig. 17.

Thermal Cycling—High Ramp Rate Results. The NTI sampleswere subjected to a second accelerated test with temperature var-iations again cycling between �40 �C and 80 �C, but with ramprates greater than 25 �C/min to impart a more severe thermal

shock condition onto the samples. All the samples from GTshowed a significant increase in thermal resistivity after thermalcycling under high ramp conditions, as shown in Table 6 andFig. 18. However, it is to be noted that the GT samples with ahigher initial resistivity appeared to deteriorate less, with sample#11, for example experiencing only a 30% increase, while thesamples with the lower initial resistivity (#9 and #12) deterioratedby a factor of ten and 3.5, respectively.

Second Round Results. Despite the encouraging resultsachieved in the thermal testing of the NTI samples supplied to

Table 5 GT thermal cycling (low ramp rate) results (mm2 K/W)

GT samples

Number of cycles 5 6 8 13

0 13.4 9.8 4.3 3.910 20.9 13.1 3.8 4.320 23.2 16.3 4.4 4.930 31 22.5 5.4 5.540 28.4 23.6 5 5.850 37.9 29.5 6.1 6.3

Fig. 17 GT thermal cycling (low ramp rate) results (mm2 K/W)

Table 6 GT thermal cycling (high ramp rate) results (mm2 K/W)

GT samples

Number of cycles 9 10 11 12

0 4.9 46.8 15.6 4.910 6.8 72.3 17.3 5.820 13 91 17.9 7.430 19.4 80.6 22.1 8.140 26.1 73.2 24.9 11.950 51 85.9 20.7 17.7

Fig. 18 GT thermal cycling (high ramp rate) results (mm2 K/W)

Table 7 Initial thermal resistance results

Sample number Raytheon GE Teledyne

1 39.4 0.8 4.02 89.1 0.9 3.33 23.1 0.9 4.04 64.5 0.7 2.35 164.7 0.9 1.86 7.8 0.9 3.27 64.6 0.8 2.88 92.7 1.1 4.19 25.2 1.1 3.710 25.0 0.9 3.111 40.5 0.8 2.412 27.5 0.9 2.213 25.0 1.0 2.414 15.3 0.9 2.215 24.4 3.7 2.416 20.9 — 2.217 20.8 — 2.318 17.0 — —19 20.3 — —20 20.3 — —21 5.2 — —22 11.1 — —23 32.5 — —24 5.5 — —25 5.4 — —26 5.9 — —27 9.2 — —28 8.0 — —29 2.9 — —

Breakdown of Raytheon samplesSample numbers Team

01–08 Purdue SiC/CuMo09–20 GT Si/Cu21–27 GT SiC/CuMo28 and 29 Purdue SiC/CuMo



NREL, the accelerated testing results for these samples did notmeet the DARPA NTI performance criteria. Consequently, the par-ticipating teams were given an opportunity to improve their NTIperformance through changes in the synthesis process. Samples

were obtained from Raytheon, GE, and Teledyne for this secondround of testing. The GT team declined to participate in this secondround of testing. Raytheon sent two sets of samples—one in whichthe NTI was bonded between the Si and Cu coupons, and anotherin which the NTIs were bonded between SiC and CuMo coupons.GE made adjustments to their samples by using a different metalli-zation on the Cu and the NTI, which would prevent formation of abrittle intermetallic layer. The authors are not aware of the changes

Fig. 19 Initial sample thermal resistance (mm2 K/W)

Fig. 20 Acoustic images of samples from Raytheon (left), GE (center), and Teledyne(right)

Table 8 Raytheon thermal aging results (mm2 K/W) 300 hrs at 130 �C

Raytheon samples

Number of hours R-SiC-06 R-SiC-07 R-Si-17 R-Si-18 R-Si-19 R-Si-20 R-SiC-27 R-SiC-28 R-SiC-29

0 7.8 64.6 20.8 17.0 20.3 20.3 9.2 8.0 2.950 11.4 153.6 21.7 22.3 50.1 20.0 10.4 86.5 2.8100 10.0 209.8 19.5 24.3 49.4 21.0 11.6 120.0 3.2200 13.7 251.4 20.9 24.1 49.6 19.7 11.8 153.3 3.4300 16.4 270.3 19.1 25.1 43.9 19.9 13.8 129.0 3.3

Fig. 21 Raytheon thermal aging results (mm2 K/W)

Table 9 GE thermal aging results (mm2 K/W) 300 hrs at 130 �C

GE samples

Number of hours GE-27 GE-28 GE-29 GE-30

0 1.1 0.9 0.8 0.950 1.0 0.9 1.0 1.0100 1.1 1.0 0.9 1.1200 1.1 1.3 1.2 1.8300 1.5 2.3 1.7 3.2



made by Teledyne in the synthesis process in order to enhance thethermal performance and reliability of the Teledyne NTIs.

Initial Thermal Resistivity Measurements. The initial ther-mal resistance results of all samples for the second round of test-ing are summarized in Table 7, breakdown of Raytheon samplesfrom different sources are also listed.

It is clear from the above figure (Fig. 19) that the initial thermalresistivity of the GE and Teledyne samples significantlyimproved, as compared to the first round. Almost all of the GEsamples had less than 1 mm2 K/W initial thermal resistivity withjust two out of 13 being slightly higher than the target value of1 mm2 K/W. The Teledyne NTI samples exhibited resistance val-ues between 1.8 and 4.1 mm2 K/W. However, a significant incon-sistency was noted with the Raytheon samples, with the bestsample displaying a thermal resistivity of 2.9 mm2 K/W, but morethan half of the samples exhibiting thermal resistivities greaterthan 10 mm2 K/W. Acoustic images representative of the generalcondition of samples from each performer are shown in Fig. 20.GE (center) shows full coverage bonding while Teledyne andRaytheon show voiding or delamination. Teledyne shows morebonding dark areas.

Fig. 22 GE thermal aging results (mm2 K/W)

Table 10 Teledyne thermal aging results (mm2 K/W) 300 hrs at130 �C

Teledyne samples

Number of hours T-09 T-10 T-11 T-12

0 3.7 3.1 2.4 2.250 3.6 15.1 5.2 Fail100 3.8 35.9 8.2 Fail200 4.1 40.5 9.4 Fail300 4.9 Fail Fail Fail

Fig. 23 Teledyne thermal aging results (mm2 K/W)

Table 11 Raytheon thermal cycling (low ramp rate) results (mm2 K/W)

Raytheon samples

Number of cycles R-SiC-03 R-SiC-04 R-Si-13 R-Si-14 R-Si-15 R-Si-16 R-SiC-24 R-SiC-25 R-SiC-26

0 23.1 64.5 25.0 15.3 24.4 20.9 5.5 5.4 5.910 113.2 195.3 77.8 39.1 919.1 20.8 8.9 6.8 13.420 182.4 240.6 122.1 45.8 1624.0 21.7 9.8 7.0 14.930 146.9 339.5 121.7 46.0 Fail 21.6 10.4 7.1 17.140 127.4 348.5 111.8 49.2 Fail 21.8 10.1 7.7 17.150 131.7 356.2 140.8 68.1 Fail 20.7 12.3 7.2 19.7

Fig. 24 Raytheon thermal cycling (low ramp rate) results(mm2 K/W)

Table 12 GE thermal cycling (low ramp rate) results (mm2 K/W)

GE samples

Number of cycles GE-23 GE-24 GE-25 GE-26

0 0.9 0.9 0.8 1.110 1.0 0.8 0.9 1.020 0.9 0.9 1.0 1.130 0.9 0.8 1.0 1.140 1.0 1.0 1.0 1.150 1.0 0.9 1.0 1.2



Fig. 25 GE thermal cycling (low ramp rate) results (mm2 K/W)

Table 13 Teledyne thermal cycling (low ramp rate) results(mm2 K/W)

Teledyne samples

Number of cycles T-05 T-06 T-07 T-08

0 1.8 3.2 2.8 4.110 1.9 4.3 5.3 4.720 2.1 5.1 9.5 5.730 2.1 6.1 12.3 7.040 1.9 6.2 14.6 7.050 2.2 7.1 15.8 8.2

Fig. 26 Teledyne thermal cycling (low ramp rate) results(mm2 K/W)

Table 14 Raytheon thermal cycling (high ramp rate) results (mm2 K/W)

Raytheon samples

Number of cycles R-SiC-01 R-SiC-02 R-Si-09 R-Si-10 R-Si-11 R-Si-12 R-SiC-21 R-SiC-22 R-SiC-23

0 39.4 89.1 25.2 25.0 40.5 27.5 5.2 11.1 32.510 81.2 268.2 74.1 94.1 56.0 42.0 6.2 15.2 43.920 112.8 Fail 71.6 121.3 78.8 44.3 6.8 16.8 57.330 135.9 Fail 70.3 244.8 360.4 175.0 6.4 16.9 66.040 163.2 Fail 62.7 243.3 455.7 384.2 6.4 17.8 81.350 185.9 Fail 76.0 647.2 463.1 562.8 6.7 16.4 113.8

Fig. 27 Raytheon thermal cycling (high ramp rate) results(mm2 K/W)

Table 15 GE thermal cycling (high ramp rate) results (mm2

K/W)

GE samples

Number of cycles GE-19 GE-20 GE-21 GE-22

0 0.8 0.9 0.9 0.710 0.9 0.8 0.8 0.820 0.8 1.1 0.9 0.830 0.9 1.1 1.0 0.840 0.9 0.9 1.0 0.950 0.9 1.0 1.0 0.8

Fig. 28 GE thermal cycling (high ramp rate) results (mm2 K/W)



Reliability Testing and Characterization. The samples weresubjected to the exact same accelerated testing profiles as in thefirst round of testing—thermal aging, thermal cycling with a lowramp rate, and thermal cycling with a high ramp rate. Acousticmicroscopic images were taken at periodic intervals to monitorthe structural integrity of the interface layers under acceleratedtesting.

Thermal Aging Tests. The Raytheon NTI samples demonstrateda mixed response to thermal aging, as shown in Table 8 andFig. 21. In both the silicon–carbide-to-copper–molybdenum(R-SiC) samples and the silicon-to-copper (R-Si) samples, differ-ent rates of increase of thermal resistivity were observed. A cou-ple of R-SiC samples can be considered to have failed, with theirthermal resistivities measuring higher than 100 mm2 K/W after300 hrs of aging. It is to be noted that a single RaytheonSiC-CuMo NTI sample (#29) did display a low and nearly invari-ant resistivity, starting with an initial value of 2.9 mm2 K/W andending at 3.3 mm2 K/W.

Highly promising results were obtained for the GE samplesunder thermal aging (Table 9 and Fig. 22). Only after 100 hrs ofaging did the thermal resistances of all samples start to increase,albeit rather slightly, and the resistivity of one sample (#27)remained invariant through 200 hrs at 1.1 mm2 K/W. The thermalresistivity of all the GE NTI’s remained under 3.4 mm2 K/W after300 hrs of aging, in this second round of testing.

Apart from one sample, in this second round of testing, all theTeledyne NTI samples failed during or before thermal aging test-ing due to poor structural coherence within the bondline thickness,as shown in Table 10 and Fig. 23. The initial thermal resistancesof these samples did not serve as a predictive indicator of failureunder thermal aging test conditions. The best sample, T-09,showed nearly invariant behavior for the first 200 hrs varying inresistivity from 3.7 mm2 K/W to 4.1 mm2 K/W, but then rising to4.9 mm2 K/W at 300 hrs.

Thermal Cycling—Low Ramp Rate Tests. Under thermalcycling (low ramp rate) testing, most of the Raytheon samplesshowed a significant amount of degradation as the number ofcycles increased, as shown in Table 11 and Fig. 24. One sampleeach from the R-Si and R-SiC batches performed relatively betterthan other samples; however, the thermal resistances of both weremeasured above 5 mm2 K/W after 50 thermal cycles. It is to benoted that Raytheon sample #16, an NTI bonding silicon andcopper, and sample #25, an NTI bonding SiC and CuMo, dodisplay acceptable 50-cycle results, 20.9–20.7 mm2 K/W and5.4–7.2 mm2 K/W, respectively, but failed to meet the DARPAresistivity goal of 1 mm2 K/W.

Similar to what was observed with the thermal aging results,the GE samples again demonstrated an excellent thermal perform-ance and reliability under thermal cycling (low ramp rate). Of the

Table 16 Teledyne thermal cycling (high ramp rate) results(mm2 K/W)

Teledyne samples

Number of cycles T-01 T-02 T-03 T-04

0 4.0 3.3 4.0 2.310 22.6 3.9 8.0 3.120 46.9 4.6 22.1 4.130 63.2 4.7 27.6 5.040 65.2 5.1 36.2 4.350 82.1 5.4 42.4 6.8

Fig. 29 Teledyne thermal cycling (high ramp rate) results(mm2 K/W)

Fig. 30 High thermal conductivity fillers in metal matrix



four samples that underwent thermal cycling, the thermal resistan-ces of two samples remained under 1 mm2 K/W while that of theother two were only slightly higher after 50 thermal cycles. Also,it was observed that thermal cycling (low ramp rate) had very lit-tle impact on these samples as the thermal resistance valuesremained more or less the same as the initial ones (Table 12 andFig. 25).

Although the Teledyne samples did not perform as well as theGE samples under low ramp rate thermal cycling, the measuredthermal resistivity values were much better than achieved in thefirst round of samples. As shown in Table 13 and Fig. 26, after 50thermal cycles, three out of four samples had thermal resistivitieslower than 10 mm2 K/W, and sample T-05 varied from 1.8 to just2.2 mm2 K/W over the test period.

Thermal Cycling—High Ramp Rate Tests. Under high ramprate thermal cycling, the Raytheon samples followed a similartrend to that observed under other accelerated test conditions,

with the thermal resistivities of most of the samples indicatingmajor signs of failure at the end of accelerated testing. The ther-mal resistivity values of the second-round Raytheon samples areshown in Table 14 and Fig. 27. Once again, however, two of theRaytheon samples—#21 and #22, bonding SiC to CuMo—displayed relatively robust resistivity behavior, increasing by lessthan 50% over the 50 high-ramp-rate cycles, but—in both cases—starting with resistivities of 5.2 and 11.1 mm2 K/W, respectively,that were already higher than the DARPA goal.

The thermal resistivity values of the second-round GE samples,shown in Table 15 and Fig. 28, confirmed their superior thermalperformance and reliability under various accelerated testconditions. All four samples met the thermal resistance target of1 mm2 K/W after 50 hrs of thermal cycling. Similar to the resultsobtained for thermal cycling under the low ramp rate conditions,the GE samples performed well under thermal cycling withthe high ramp rate, exhibiting minimal variation in thermalresistances.

The second round high-ramp-rate test of Teledyne NTIs revealedsimilar results to those achieved in the first round, with two samplesapproaching failure and displaying resistivities of 42 and 82 mm2

K/W, respectively, under thermal cycling loads, but two other sam-ples remaining under 10 mm2 K/W after 50 thermal cycles (Table16 and Fig. 29). On average, the second round of Teledyne sampleshad higher thermal performance than the first round of samples.But the small sample size and lack of consistency in these resultsmake it difficult to substantiate that conclusion.

Next Generation nTIMs. Current DARPA efforts are investi-gating TIMs with extremely low, solderlike thermal resistancealong with high, epoxylike mechanical compliance with the goalof providing 1 mm2 K/W. Next generation effort seeks to advanceSOA TIMs by reducing the thermal resistance by another factor of10, without compromising the mechanical compliance of theTIM. DARPA awarded two Young Faculty Awards (YFAs) fordeveloping next generation nano-enabled TIMs. The goal for thiswas more aggressive with a target of 0.1 mm2 K/W. TAMU(Texas A&M) and CMU (Carnegie Mellon) approaches andresults will be discussed in this section as they look verypromising.

Fig. 31 Comparison of Cu/fBNNS with SOA TIMS

Fig. 32 Metal nanowire array TIM [25]



TAMU Effort. The Texas A&M team used functionalizedBN/Copper nanoribbons/sheets in solder alloys for their next-generation TIM, as shown in Fig. 30. The functionalized nano-sheets are attached to the metallic solder and other metallic alloysby soft ligands. Exfoliated BN nanosheets were functionalized bysoft organic ligands and, using an electro-codeposition approach,functionalized BN nanosheets were dispersed in the coppermatrix and metal–inorganic–organic TIMs nanocomposites werereliably produced [30–33]. In systematic TTR testing at NREL,with several different ligands, TIM wafers with a 45–50 lm bond-line thickness the overall thermal resistivity was found to be0.33–0.45 mm2 K/W depending on the ligand, corresponding toan effective thermal conductivity of 240 W/m K to 280 W/m K.The Young’s modulus was found to equal 15 GPa and the Hard-ness equal to 117 MPa.

Figure 31 depicts the relationship of thermal conductivity andelastic modulus for a range of packaging materials. It may be seenthat the Cu/f-BNNS nanocomposite TIMs, produced by theTAMU team, occupy a previously unexplored part of this chart,offering very large improvements in thermal conductivity at lowvalues of elastic modulii, compared to the commercially availablepolymer(epoxy)-based, solder TIMs, and thermal greases.

CMU Effort. Carnegie Mellon’s next generation TIM is basedon the use of conductive and compliant ordered nanostructures,such as copper or silver nanowire arrays with embedded metalnanoparticles [27,34–37]. Figure 32 reflects the CMU approach,which displays large-scale anodic aluminum oxide templates withtunable sizes were used to grow copper and silver nanowire arraysvia electrochemical deposition. Scanning electron microscopystudies showed that the nanowires have a large aspect ratio(e.g., 100–1000) and a high packing density. The high thermalconductivity of the metal nanowires and use of an open array withfiller particles offers the promise of efficient heat transfer and highmechanical compliance and makes CMU a prime candidatefor achieving an overall nTIM thermal resistance of less than0.1 mm2 �K/W.

Preliminary thermal measurements have demonstrated a bestthermal resistivity of approximately 0.3 mm2 K/W. The AFM-measured Young’s moduli for the copper and silver nanowirearrays were found to range from 6 MPa to 25 MPa, and from5 MPa to 19 MPa, respectively, more than three orders of magni-tude smaller those of bulk copper and silver.

Conclusions

The goal of the NTI program was to improve the thermal bottle-neck in DoD systems by lowering TIM resistance to 1 mm2 K/Wensuring thermoplastic like compliance and reworkability. Thispaper summarized the work of four performers: GT, Raytheon,GE, and Teledyne from their samples tested at NREL. It is foundall the performers improved substantially from the SOA. GTachieved a thermal resistance best value of 3.9 mm2 K/W, Ray-theon best values are close to 2.9 mm2 K/W, Teledyne achieved avalue close to 1.8 mm2 K/W, and GE a best value of 0.7 mm2

K/W. To that end in goal four performers provided samples thatimproved substantially on the SOA. GE NTIs were the only onesthat consistently met the DARPA goals offering 1 mm2 K/W.YFA efforts are paving the way for further reductions in resistivitytoward 0.1 mm2 K/W.

Acknowledgment

The authors would like to thank DARPA for the financial sup-port for this work. The authors would also like to thank the Princi-pal Investigators: Dave Altman (Raytheon), Jack Moon (GT),Yuan Zhao (Teledyne), and Dave Shaddock (GE), as well as theirresearch teams, for their contributions to the NTI program andtheir collaboration with the authors in preparation of this paper.We would also like to thank Dr. Sheng Shen and Dr. Mustafa

Akbulut for their contributions as DARPA YFAs. The views,opinions, and/or findings contained in this article/presentation arethose of the author/presenter and should not be interpreted as rep-resenting the official views or policies, either expressed orimplied, of the Defense Advanced Research Projects Agency orthe Department of Defense. Distribution Statement A, Approvedfor Public Release, Distribution Unlimited.

Nomenclature

cp ¼ specific heat (J/g K)R ¼ thermal resistance (mm2 K/W)t ¼ time (s)

T ¼ temperature (�C)

Greek Symbols

a ¼ thermal diffusivity (cm2/s)k ¼ thermal conductivity (W/m K)q ¼ bulk density (g/cm3)

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