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Review Article

Two-Phase Thermal Ground Planes: Technology Development and Parametric Results OPEN ACCESS

[+] Author and Article Information
Avram Bar-Cohen

Fellow ASME
Defense Advanced Research Project
Agency (DARPA)/Microsystems Technology Office (MTO),
675 North Randolph Street,
Arlington, VA 22203
e-mail: abc@darpa.mil

Kaiser Matin

Mem. ASME
System Planning Corporation,
3601 Wilson Blvd,
Arlington, VA 22201
e-mail: kaiser.matin.ctr@darpa.mil

Nicholas Jankowski

Mem. ASME
U.S. Army Research Laboratory,
2800 Powder Mill Rd,
Adelphi, MD 20783
e-mail: Nicholas.Jankowski@us.army.mil

Darin Sharar

General Technical Services, LLC,
3100 New Jersey 138,
Wall Township, NJ 07719
e-mail: darin.j.sharar.ctr@mail.mil

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received August 4, 2014; final manuscript received October 10, 2014; published online November 14, 2014. Assoc. Editor: Ashish Gupta.This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Electron. Packag 137(1), 010801 (Nov 14, 2014) (9 pages) Paper No: EP-14-1069; doi: 10.1115/1.4028890 History: Received August 04, 2014

Defense Advanced Research Project Agency's (DARPA's) thermal ground plane (TGP) effort was aimed at combining the advantages of vapor chambers or two-dimensional (2D) heat pipes and solid conductors by building thin, high effective thermal conductivity, flat heat pipes out of materials with thermal expansion coefficients that match current electronic devices. In addition to the temperature uniformity and minimal load-driven temperature variations associated with such two phase systems, in their defined parametric space, flat heat pipes are particularly attractive for Department of Defense and commercial systems because they offer a passive, reliable, light-weight, and low-cost path for transferring heat away from high power dissipative components. However, the difference in thermal expansion coefficients between silicon or ceramic microelectronic components and metallic vapor chambers, as well as the need for a planar, chip-size attachment surface for these devices, has limited the use of commercial of the shelf flat heat pipes in this role. The primary TGP goal was to achieve extreme lateral thermal conductivity, in the range of 10 kW/mK–20 kW/mK or approximately 25–50 times higher than copper and 10 times higher than synthetic diamond, with a thickness of 1 mm or less.

As electronic system technology continues to advance, thermal management has become a key bottleneck. This trend often leads to the operation of electronic systems at the limits of the thermal management technology. In response in 2008, DARPA launched the TGP effort, as a subset of the larger DARPA thermal management technologies (TMT) program. The TGP effort, completed in 2012, focused on developing thin, lightweight, heat spreaders for single chip packages, and multichip modules (MCM) utilizing micro- and nano-structured materials to achieve exceptional thermal transport and enable increased power density in defense electronic systems. In wick-based TGPs also known as a vapor chamber, heat is carried away from the evaporator by vapor flow under an induced evaporator-to-condenser pressure difference, while capillary forces in the wick structure pump the condensate back to the evaporator as shown in Fig. 1 [1].

The TGPs developed in this program are engineered to maintain their thermal performance at high inertial loads, an especially important consideration for electronics on military platforms. Successful operation under high inertial loading requires robust wick structures capable of replenishing the fluid from the condenser back to the evaporator, against the inertial load. If the capillary force is not adequate, partial or full dry-out of the TGP evaporator may result, leading to a severe deterioration in the two-phase heat and mass transfer benefits of the TGP.

Based on these requirements, DARPA established several performance metrics to use as barometers for success in each of three distinct phases. The Phase I and Phase II metrics are summarized in Table 1. Eight teams participated in the TGP effort, each with a unique approach and targeted application. The approaches and applications are summarized in Table 2. Phase I focused on development of 2 mm thick wick structures that can operate at inertial loads as large as 2 g. Phase II focused on development of 3 cm × 3 cm × 3 mm thick functional TGP devices that can operate at 10 g inertial loads with a 25% improvement in thermal conductivity over bulk copper (400 W/mK). TGP III performers were encouraged to demonstrate effective lateral thermal conductivities approaching 20 kW/mK (50 times higher than copper), while targeting application-specific metrics, including the relevant g-loading and other ancillary performance metrics such as hermeticity, coefficient of thermal expansion (CTE), and long-term performance stability required to ensure application success.

Thus, Northrop Grumman Electronic Systems, working with the University of Missouri–Columbia and –Sandia, pursued an oscillating heat pipe approach, using water as the working fluid and both metallic and ceramic casings, to provide heat spreaders for arrays of radio frequency components. The University of California–Los Angeles (UCLA), working with advanced cooling technologies (ACT) and University of Michigan, developed a TGP with an aluminum nitride casing and powdered copper sintered biporous wicks, using water/inorganic aqueous solution as the working fluid for laser diode cooling.

Raytheon targeted cooling power amplifiers used in radar applications and worked with Purdue, Thermacore, and the Georgia Tech Research Institute to develop a composite copper TGP case with nanostructured copper wicks and water as the working fluid. General Electric, along with University of Cincinnati and the Air Force Research Laboratory, developed an aluminum nitride casing TGP with nano-superhydrophilic/phobic wicks, using water as the fluid, for cooling graphical processing unit (GPU) units in video cards. Teledyne partnered with Boston College, Duke University, and British Aerospace (BAe) to develop a TGP with a silicon casing and nanostructured carbon nano tube (CNT) wicks, using water and Heptanol as the working fluid, to cool gallium nitride power amplifiers. University of California–Santa Barbara (UCSB) targeted their TGP development to cool phased array radars using a titanium-cased TGP with a micropatterned wick structure and water as the fluid. The University of Colorado, together with Lockheed Martin and University of South Carolina, developed flexible TGPs (FTGPs) using polymer material for the case and atomic layer deposition (ALD) coated nanomesh structure wicks for capillary pumping of water to meet airborne computer cooling needs. Cooling of light-emitting diode (LED) arrays was targeted by University of California (UC)–Berkeley using a silicon TGP with a coherent porous silicon (CPSi) wick and an interline optimized evaporator and methanol/deionized water as the working fluid [1] (examples of these different approaches are to be illustrated in Sec. 3.1).

The Army Research Laboratory (ARL) power components branch at the Adelphi Laboratory Center was commissioned as a third-party testing facility and provided evaluation for a variety of TGP performers throughout the three phases of the program. A photograph of the ARL testing apparatus is shown in Fig. 2.

Static and dynamic testing was accomplished by affixing the TGPs to a centrifuge. The centrifuge provided inertial loads ranging from 0 to 20 g, flexibility in heat spreader mounting, fluid interconnections for liquid cooling, electrical interconnects for power supply, onboard diagnostics, and data acquisition. Most of the TGP structures in all three phases of the program were tested in this apparatus, which was configured to evaluate wick performance under 2 g inertial loads for Phase I, TGP characteristics up to 10 g of acceleration for Phase II, and varied customer-based geometries and loads for Phase III. The acceleration tested Phase II TGPs were 3 cm by 3 cm and 3 mm thick, as per the program metrics. Commercial vapor chambers were also tested by ARL up to 10 g. General Electric (GE) was tested at the Air Force Research Laboratory (AFRL), Teledyne in house, and the others at ARL. UCSB and UC–Berkeley were not tested. In most of the Phase II TGPs, as well as the commercial off-the-shelf (COTS) vapor chambers, thermal conductivity degraded under 10 g's. None of the TGPs failed at 10 g, and Northrop, GE, and Teledyne showed minimal degradation.

Figure 3 is a bar graph showing the effect of acceleration on the effective lateral thermal conductivity achieved by TGP II's of different performers. Most significantly, all the tested Phase II TGP's displayed thermal conductivities at 10 g that were above the minimum targeted value of 400 W/mK (copper). For most Phase II TGPs as well as the COTS vapor chambers provided by Vapro (8 cm × 8 cm × 3 mm) and Celsia (3 cm × 3 cm × 3 mm)—thermal conductivity was degraded under 10 g inertial loading. It is noted that Celsia TGPs were not optimized. The Vapro vapor chambers tested were “thicker” versions but they also offer thin vapor chambers. Teledyne and Northrop Grumman (NG) oscillating loop heat pipe (OHP) TGP IIs performed the best under 10 g loads and with negligible reduction in performance. It is thus apparent that many of these TGP II designs had relatively high capillary pumping limits, capable of working against enhanced (10 × ) gravitational acceleration.

Figure 4 displays the effective lateral thermal conductivities and thicknesses of the Phase III TGPs, along with the values for commercial graphite and diamond heat spreaders, as well as for several typical COTS products, all for a gravity-neutral, horizontal orientation.

In addition to providing elevated thermal conductivity, each team's TGP has a unique feature and detailed in Sec. 3.1. It is noted that effective lateral thermal conductivity is not the only performance metric to define the success of a TGP. Each TGP has its own merit as maximum heat transfer is inversely proportional to the working length. Depending upon specific requirements, other factors, e.g., low cost, high heat flux, thin and flexible, CTE matched substrate, very large area, and reliability should be taken into account for system thermal application. As detailed in Table 2, the case materials include copper, aluminum nitride, silicon carbide, silicon, titanium, and metal matrix composites. The unique wick structure and material include etched silicon, Cu nanorods/nanomesh, sintered metallic powder wick, ALD coating, patterned CNT, nano-superhydrophilic/phobic wick, CNT/Si structures with adaptive coating, and CPSi nucleating evaporator. Table 3 shows the Phase III TGP dimensions for each performer having various form factors. It may be seen that, depending on the targeted application requirements, the TGPs range in area from 4.5 cm2 to 75 cm2, with thickness from 0.75 mm to 3 mm.

The DARPA-funded TGPs are seen to display thermal conductivity values of 0.7 kW/mK for a 3 mm thick TGP (NG) with 9 cm2 to 3 kW/mK for a 2 mm thick Teledyne TGP with 29 cm2 and 20 kW/mK for a 1 mm thick TGP (GE) over 9 cm2 area. Out of eight, four of the TGP performers did achieve the program goal of an effective thermal conductivity between 10 kW/mK and 20 kW/mK and nearly all the performers reached the target thickness of 1 mm for a high conductivity TGP, with at least one of their prototypes. Two of the performers, namely, GE and Teledyne, achieved the combined goal of providing an effective thermal conductivity between 10 kW/mK and 20 kW/mK with a thickness of approximately 1 mm. The University of Colorado TGP was the thinnest, at 0.75 mm, with an effective thermal conductivity of 2 kW/mK over a 4 cm2 area.

The Phase III TGPs are thus seen to generally exhibit an advantage over the solid heat spreaders, easily exceeding the thermal conductivity of copper (∼0.4 kW/mK), generally reaching lateral effective thermal conductivities at least as high as commercial synthetic diamond and pyrolitic graphite (∼1.5 kW/mK), and demonstrating that “extreme” lateral conductivity values of 10 kW/mK–20 kW/mK, in a horizontal orientation, could be achieved in a 1 mm thick planar structure fabricated from a variety of case materials and working fluids. Synthetic diamonds are costly and pyrolitic graphites are fragile. The COTS vapor chambers are capable of achieving conductivities approaching the highest TGP values, but are generally thicker and not CTE-matched to microelectronic chips and/or substrates.

Specific TGP III Designs.

Examples of specific TGP III designs are presented here. These examples are approved for public release through publications. It should be noted that these are only examples and they do not represent the best results achieved by the TGP program.

University of Colorado–Boulder:

The Colorado team has developed four major prototypes to demonstrate their FTGP. The first one is a PCB-based, 3 cm × 3 cm TGP with effective thermal conductivities higher than 1500 W/mK. Operation for this device is effective under 10 g inertial loads. The second prototype is a Mylar-aluminum, 5 cm × 9.5 cm TGP with desired flexibility (highly bendable) achieved with simplified fabrication and assembly. This was not tested under any inertial loads. The third prototype is a copper-cladded Kapton, 5 cm × 9.5 cm TGP with the desired flexibility and durability and excellent thermal performance. The fourth prototype is an all-polymer 5 cm × 9.5 cm TGP. The outer casing is Kapton and the inside has nylon spacers and polymer wicking structure. In addition, the Colorado team has conducted basic studies that are critical to supporting the current prototypes and future improvements [26]. These studies are: (1) ALD for hydrophilic coating and hermetic sealing of polymer layers; (2) evaporation heat transfer with micromeshes; (3) fabrication and heat transfer of nanowires and nanomeshes; (4) system design models; and (5) molecular layer deposition and heat transfer characterization for TGP-device interfaces.

Figure 5 shows an example of the layout of the flexible TGP showing the copper mesh wicking structure, nylon vapor material [3]. The printed circuit board-based FTGP offer a promising potential to meet these needs. They have demonstrated 0.25 mm thin flexible TGP that can be fabricated by existing printed circuit board vendors. From this technology transfer case, it is also clear that the effective thermal conductivity of a TGP is only one of the performance measures. For some applications, e.g., smartphones or tablets, an effective thermal conductivity in the range of 2000–5000 W/mK is good enough. However, the TGP's thickness and cost are more important performance measures in order for it to compete with other thermal management solutions.

Figure 6 shows the thermal resistance of this flexible polymer TGP at different bend radius compared to copper reference sample. The thermal resistance of the copper sample remains constant at 4.6 K/W. Thermal resistance for the flexible TGP begins to increase beyond 18 W due to beginning of dry out condition in the evaporator. For the bending cases, the 90 deg bends show more increase than the 45 deg bend.

The targeted application areas are avionics modules as well as cellphones. Professor Y. C. Lee, the PI of the University of Colorado team, has received a new grant from the State of Colorado's advanced industries accelerator program for technology transfer. He is improving the FTGP for mobile systems, e.g., smartphones, tablets, and wearable electronics. For such an application, however, the thickness of the TGP itself has to be extremely thin and the cost has to be extremely low.

UCLA:

A low CTE vapor chamber for heat transport and spreading was developed by the UCLA team for thermal management of high-power, high heat flux silicon, gallium arsenide, and/or gallium nitride microelectronics chips. The development effort focused on innovative hybrid wick structures and low CTE materials, specifically aluminum nitride ceramic with direct bond copper and metal matrix nanocomposites with tailorable CTEs. The TGP's high performance capability stems from its unique hybrid wick and low CTE case [711]. The wick consists of dedicated liquid supply structures and a thin liquid spreading layer made of either sintered metal powder or micropost structures. The thin liquid spreading layer is optimized to reduce evaporator resistance and the dedicated liquid supply structures are critical for handling high heat flux over large heating areas (i.e., high total power). The low CTE construction allows for direct die attach, eliminating the thermal resistance of the die substrate and associated interface.

UCLA has transitioned this TGP to their partner, ACT, a leading provider of thermal management solutions for commercial and military electronic systems. The UCLA TGP has been integrated into ACT's product line so that customers can now purchase this high flux heat spreader, with direct die attach capability for thermal management of solid-state lasers and other high powered electronic cooling applications. This is the first technology from the TMT program that has successfully achieved technology transition. The UCLA TGP demonstrated 250 W/cm2 across 1 cm × 1 cm of heating area without reaching dryout.

Figure 7 shows the effect of three hybrid wick structures, on applied heat flux and resultant superheat [11]. At 250 W/cm2, the wall superheat is almost identical at 15 K, indicating the evaporation is independent of the three liquid supply wick structure and low thermal resistance liquid spreading layers mainly plays the role of evaporative heat transfer. At higher heat fluxes, the liquid supply structures are more effective.

Teledyne:

The Teledyne TGP vapor chambers are made of silicon that has a thermal expansion coefficient matching nicely with most semiconductor materials. These TGPs can be readily integrated with microelectronics and microsystems. The patterned multiscale wick structures enable the vapor chamber to operate at high heat flux and high-g conditions. Compared to other passive heat-spreading technologies, such as solid plates and conventional heat pipes, this technology offers high-g wicking, a high thermal conductivity wick layer, high overall effective thermal conductivity, a thin profile, and high heat flux capability [1215]. A novel triple stack process has been developed to enable the massive fabrication of large scale but thin silicon vapor chambers which are formed through bonding three etched silicon wafers. Using glass-frit bond technology, they demonstrated hermetic seals on 3.8 cm × 3.8 cm TGP devices. The effective density of the device is less than 1.5 × 103kg/m3. The targeted application is power amplifier module cooling. Figure 8 shows several biwick structures that have been developed by Teledyne [13]. The advantages of CNT biwick structures are that the large pores provide low viscous flow facilitating vapor escape during nucleate boiling while the small pores enhance thin film evaporation. Figure 9 indicates a hysteresis caused by a biwick dryout and can create a temperature difference of up to 40 °C at the same heat flux [13]. A large hysteresis shows different curve paths during the wick dryout and rewetting process.

UCSB:

The UCSB TGP is fabricated using multiscale UCSB Titanium (Ti) and Titania (TiO2) processing technology including nanostructured titania (NST). The UCSB TGP features NST (a super-hydrophilic wick material) on microscale, deep etched Ti grooves. The array of Ti/NST grooves forms a super-hydrophilic wick structure for the water-filled TGP [16,17]. The NST wick array can be tailored to the application by changing the density, position, and the height of the deep etched Ti grooves. The TGP consists of a Ti sheet with an integrated array of Ti microscale grooves coated with NST and cavities to support the chips; this top sheet is bonded or laser welded to a second sheet of Ti. Figure 10 shows SEM image of Ti microstructure [17].

This hairlike NST structures enhances wetting of the Ti pillars. Figure 11 shows the prototype of a first generation laser welded Ti-based hermetically sealed TGP [17]. Laser welding ensures more reliability. The thickness of the TGP can be as thin as 1 mm, and the area can be less than 1 cm × 1 cm to greater than 7.6 cm × 40 cm, the Phase III device dimensions. The TGPs are to be used in radar module cooling. The UCSB team formed a start-up company named Pi-MEMS led by Payam Bozorgi.

UC–Berkeley:

TGP is a microscale silicon loop heat pipe, which can interface directly with densely packed electronic chips to provide enhanced localized cooling and improve device performance. Unlike vapor chamber and standard heat pipe solutions currently commercially available, the Berkeley TGP combines both a vapor chamber/heat pipe element in its much enhanced evaporator and a loop heat pipe to transport the heat away from the evaporator to a separate condenser [18,19]. This condenser can be close to, or an extended distance away from, the evaporator which provides flexibility in the design. It has a silicon case and a CPSi wick, with an interline optimized evaporator. Target application is LED arrays and circuits. They are also working on spinning out this technology by THC3 Enterprises, LLC, and their startup company.

GE:

The General Electric Global Research TGPs are flat, thin (external thickness of 1 mm) heat pipes which utilize two-phase cooling. The goal is to utilize TGPs as thermal spreaders in a variety of microelectronic cooling applications. GE examined TGP substrates fabricated of aluminum nitride (AlN) and copper. AlN is a unique heat pipe casing material in that it is closely CTE-matched with many semiconductor materials. As a result, high power components with these substrate materials can in principle be reliably integrated with TGP heat spreaders using thin thermal interface bond-lines [2022]. Copper is a lower risk material with fabrication advantages and a shorter path to impact on operational systems, though it is not CTE matched to semiconductors and requires additional components in the overall thermal solution. A nanostructured wick will enable the TGP to operate in an adverse gravity environment of up to 20 g. The GE TGP charging setup is shown in Fig. 12 [22].

Charging in small packages is a challenge as the working fluid needs to be dispensed accurately. This also affects the thermal conductivity of the TGP. The fill volume can be in the order of 10–100 μl. In the calculation of effective thermal conductivity they used a shape factor of 0.6 to account for the miniaturized device. The accuracy objective was set below 10% for relative error in thermal conductivity measurement due to uncertainty in temperature measurements as shown in Fig. 13 [22]. The GE TGP is designed to be integrated into GE Intelligent Platforms' GRA111 VPX line replaceable module for GPU cooling. The GE TGP has an effective thermal conductivity of 20,000 W/mK (for 20 cm × 3 cm × 1 mm) and will provide 56% increase in GPU power, leading to 2 x imaging processing capability for UAV ISR and advanced sensor fusion applications.

The formation of noncondensable gases (NCGs) has been a challenge for many of the TGP teams. The gases tend to form as a result of corrosion, when the TGP fluid comes in contact with the wick or case material due to chemical reaction. NCGs occupy a portion of the condenser and thus affect the thermal performance of the TGPs by reducing the effective thermal conductivity [21]. In some cases, e.g., OHPs, NCGs were not an issue. NCGs can be mitigated by ALD surface coating of possible reactive species in the TGP packages [22].

Raytheon:

The Raytheon radio frequency thermal ground plane (RFTGP) program was focused on the development of high effective thermal conductivity (keff > 1 kW/mK), low CTE (CTE, 5–7 ppm/K), thin (1–3 mm) heat spreaders for high heat flux (100 W/cm2) cooling. RFTGPs were developed to spread heat from multiple small (10 s of mm2) device sources over larger (10 s of cm2) areas in high packaging density applications. Efforts focused on the development of CTE-matched vapor chamber designs incorporating high heat flux bearing, low thermal resistance evaporator wick structures [2326]. The TGP utilizes capillary driven two-phase heat transfer and combines scaled-down sintered Cu powder and nanostructured materials in the vapor chamber wick to achieve low thermal resistance. Cu-coated vertically aligned carbon nanotubes is the nanostructure of choice in this development. Unique design and construction techniques are employed to achieve CTE-matching with a variety of device and packaging materials in a low-profile form-factor. The Raytheon TGP is targeted for radar module cooling. Four test vehicles were employed in this study as shown in Fig. 14 by varying heat sink resistance (HSR) [26].

Two of these incorporated a 15/85 CuMo heat spreader and two incorporated 1.4 mm thick TGPs. The average benefit of the Raytheon TGP was 0.6 C/W or 26% for the high HSR configuration using 508 μm gap pads as shown in Fig. 15. Figure 16 shows that Raytheon TGP is insensitive up to 10 g loading.

NG:

The NG TGP is an OHP which relies on the formation, growth, and collapse of vapor bubbles to drive fluid flow, converting thermal energy into kinetic energy and back again. The rapid evaporation and condensation produce a chaotic oscillating motion. The heat transfer is thought to be due largely to forced convection, associated with the rapidly oscillating liquid plugs (sensible) and some phase change heat transfer due to liquid vaporization and vapor condensation (latent). In liquid-filled, oscillating heat pipe TGPs, bubble growth, and collapse drives periodic liquid flow from the heated to the cooled sections of the TGP [2729].

During Phases I and II, the NG team pursued two parallel approaches to building TGPs: NG etched microchannels and wicks in Si and SiC and developed anodic bonding processes of Pyrex lids over these channels, while the University of Missouri–Columbia worked on advanced TGP designs in Cu, including specialized devices that could operate against 10 g loads. Both efforts explored the utilization of combined wicks along with flow channels and also compared acetone versus water as the working fluid. By the end of Phase II, the copper heat pipes had demonstrated that it was possible to achieve an effective thermal conductivity of 10,000 W/m K. The 3 cm × 3 cm Phase II SiC device contained channels that were unevenly distributed to ensure that the evaporator was wetted at high inertial loads (up to 10 g) [30]. The SiC oscillating heat pipes passed the Phase II effective thermal conductivity metric, the 100 h duration test, and the Phase III hermeticity metric. In Phase III, the NG team spent the majority of its efforts developing OHPs in two form factors: a 4 in. × 4 in. power amplifier module, exploring both Cu and SiC embodiments, and a 14 in. × 14 in. Line replaceable unit capable of dissipating 500 W. The 14 in. designs proved to be quite successful, passing the 1000 h Phase III duration test and displaying a relatively constant effective thermal conductivity against 10 g inertial loads (760 W/mK to 699 W/mK). The 14 in. × 14 in. aluminum OHP proved to be the most successful, and has prompted considerable interest in future applications at the chassis level. Three styles of 14 in. × 9.5 in. aluminum OHPs capable of handling 500 W were tested. These OHPs are targeting a two-rail, edge-cooling configuration which requires 5 °C uniformity across the devices.

Figure 17 shows effective thermal conductivity of a three-dimensional flat plate oscillating heat pipe (3D FP-OHP) with heat input [29]. These TGPs have no wick. The maximum effective conductivity achieved was 14,000 W/mK at a cooling temperature of 60 °C and heat input of 260 W.

Parametric Trends.

Evaporator Heat Flux: Fig. 18 displays the variation of the maximum observed evaporator heat flux with the effective length of the TGP III, for the TGP III for which this data was available. The variety of case and wick materials, as well as working fluid, across the family of TGP III, makes it difficult to obtain a rigorous, monotonic trend line. Nevertheless, due to the capillary pumping limits, which—for a specified wick structure—reduce the liquid flow rate and, consequently, heat carrying capacity of the TGP, it might be expected that the evaporator heat flux will be higher for short TGPs and lower for long TGPs. This trend is, in fact, observed in Fig. 18, where the evaporator heat fluxes are seen to vary from 330 W/cm2 for the 1 cm Raytheon TGP, through 250 W/cm2 for the 4 cm UCLA design, and just 5 W/cm2 for the 15 cm long GE TGP III. This trend noted here is not a correlation.

Figure 19 displays the variation of thermal resistance with design power dissipation, as reported by seven of the TGP teams, namely Raytheon, UCLA, UCSB, Colorado, NG, Teledyne, and GE for their Phase III prototypes. Although each TGP III shows a rather complex dependence of resistance on power dissipation despite the variety of case materials, size, wick structures, and working fluids used. The overall trend shows a near-monotonic decline in resistance with increasing power dissipation for UCLA, UCSB, and Raytheon TGPs while for the Teledyne and Colorado TGPs it is increasing. For GE and NG TGPs, it is more or less constant. As the TGP IIIs also varied greatly in overall size, as well as the size of the evaporator and heaters, it is difficult to draw conclusions from this plot and there is no correlation derived from this plot. Nevertheless, it is to be noted that despite all these variations, and orders-of-magnitude differences in power and resistance, these seven TGPs were designed to display a 10–20 K temperature difference from the evaporator to the condenser.

DARPA TGP program is an overall success. The program now boasts of successful technology maturation and initiation of startup companies by several performers. The program presented several hermetically sealed TGP configurations that can be used effectively to spread heat with minimum temperature difference from power intensive devices. Several microfabrication methods involving various nanostructure materials have been developed ensuring good wicking as well as hermeticity of the TGP devices carrying high heat fluxes. A common theme among many of the TGP approaches is the use of nanostructures in the TGP wicks. In theory, if more liquid can be pumped, the TGP power limit can be raised. The nanostructures were also intended to increase hydrophilicity—if the liquid is spread out in a thinner film over the evaporator area, it would provide improved heat transfer coefficients and more efficient heat transport. The nanostructures were expected to improve capillary pressure and help the TGPs maintain performance under high g-loading. Next generation thermal management solutions involving TGPs will benefit from these very high performance, flexible, CTE matched, and light-weight heat spreaders.

The authors want to acknowledge Dr. Thomas Kenny (former PM/MTO) for his efforts in defining, initiating and guiding the early stages of DARPA's TGP program, under BAA 7-36. We also want to thank Dr. Kristen Bloschock currently at Lockheed Martin Corporation and the principal investigators of the DARPA TGP program: Q. Cai from Teledyne, Scott Miller from GE, David Altmen from Raytheon, Larry Greenberg from NG, Y. Sungtaek Ju from UCLA, Albert Pisano from UC–Berkeley, Carl Meinhart from UCSB, and Y. C. Lee from the University of Colorado–Boulder, for their contributions to this paper. The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense. Distribution Statement A, Approved for Public Release, Distribution Unlimited.

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Nam, Y. , Sharratt, S. , and Ju, Y. S. , 2011, “Characterization and Modeling of the Heat Transfer Performance of Nanostructured Cu Micropost Wicks,” ASME J. Heat Transfer, 133(10), p. 101502. [CrossRef]
Dussinger, P. , Ju, Y. S. , Catton, I. , and Kaviany, M. , 2012, “High Heat Flux, High Power, Low Resistance, Low CTE Two-Phase Thermal Ground Plans for Direct Die Attach Applications,” Government Microcircuit Applications and Critical Technology Conference (GOMACTech-12), Las Vegas, NV, March 19–22, Paper No. 22.4.
Ju, Y. S. , Kaviany, M. , Nam, Y. , Sharratt, S. , Hwang, G. S. , Catton, I. , and Fleming, E. , 2013, “Planar Vapor Chamber With Hybrid Evaporator Wicks for the Thermal Management of High-Heat-Flux and High-Power Optoelectronic Devices,” Int. J. Heat Mass Transfer, 60, pp. 163–169. [CrossRef]
Cai, Q. , Bhunia, A. , Tsai, C. , Kendig, M. W. , and DeNatale, J. F. , 2013, “Studies of Material and Process Compatibility in Developing Compact Silicon Vapor Chambers,” J. Micromech. Microeng., 23(6), p. 065003. [CrossRef]
Cai, Q. , and Chen, Y. C. , 2012, “Investigations of Bi-Porous Wick Structure Dryout,” ASME J. Heat Transfer, 134(2), p. 021503. [CrossRef]
Cai, Q. , Chen, B. C. , Tsai, C. , and Chen, C. L. , 2010, “Development of Scalable Silicon Heat Spreader for High Power Electronic Devices,” ASME J. Therm. Sci. Eng. Appl., 1(4), p. 041009. [CrossRef]
Cai, Q. , and Chen, L. C. , 2010, “Design and Test of CNT Biwick Structure for High Heat Flux Phase Change Heat Transfer,” ASME J. Heat Transfer, 132(5), p. 052403. [CrossRef]
Srivastava, N. , Din, C. , Judson, A. , MacDonald, N. C. , and Meinhart, C. D. , 2010, “A Unified Scaling Model for Flow Through a Lattice of Microfabricated Posts,” Lab Chip, 10(9), pp. 1148–1152. [CrossRef] [PubMed]
Ding, C. , Soni, G. , Bozorgi, P. , Piorek, B. D. , Meinhart, C. D. , and MacDonald, N. C. , 2010, “A Flat Heat Pipe Architecture Based on Nanostructured Titania,” J. Microelecmech. Syst., 19(4), pp. 878–884. [CrossRef]
Dhillon, N. S. , Chan, M. W. , Cheng, J. C. , and Pisano, A. P. , 2011, “Noninvasive Hermetic Sealing of Degassed Liquid Inside a Microfluidic Device Based on Induction Heating,” International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS), Seoul, South Korea, Nov. 15–18, pp. 197–200.
Dhillon, N. S. , Hogue, C. , Cheng, J. C. , and Pisano, A. P. , 2011, “Experimental Investigation of Thin-Film Evaporation in an Open-Loop Columnated Micro-Evaporator,” International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS), Seoul, South Korea, Nov. 15–18, pp. 205–208.
De Bock, H. P. , Chauhan, S. , Chamarthy, P. , Eastman, C. , Weaver, S. , Whalen, B. P. , Deng, T. , Russ, B. , Gerner, F. M. , Johnson, D. , Courson, D. , Leland, Q. , and Yerkes, K. , 2008, “Development and Experimental Validation of a Micro/Nano Thermal Ground Plane,” ASME Paper No. AJTEC2011-44646.
Varanasi, K. K. , Deng, T. , Chamarthy, P. , Chauhan, S. , Bock, P. , Kulkarni, A. , Mandrusiak, G. , Rush, B. , Russ, B. , Denault, L. , Weaver, S. , Gerner, F. , Leland, Q. , and Yerkes, K. , 2009, “Engineered Nanostructures for High Thermal Conductivity Substrates,” Nanotech Conference and Expo, Houston, TX, May 3–7, pp. 505–508.
De Bock, H. P. , Chauhana, S. , Chamarthya, P. , Stanton, E. , Weaver, S. E. , Denga, T. , Gerner, F. M. , Ababnehb, M. T. , and Varanasi, K. , 2010, “On the Charging and Thermal Characterization of a Micro/Nano Structured Thermal Ground Plane,” 12th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Las Vegas, NV, June 2–5.
Weibel, J. A. , Kim, S. S. , Fisher, T. S. , and Garimella, S. V. , 2012, “Carbon Nanotube Coatings for Enhanced Capillary-Fed Boiling From Porous Microstructures,” Nanoscale and Microscale Thermophys. Eng., 16(1), pp. 1–17. [CrossRef]
Weibel, J. A. , Kousalya, A. S. , Fisher, T. S. , and Garimella, S. V. , 2012, “Characterization and Nanostructured Enhancement of Boiling Incipience in Capillary-Fed Ultra-Thin Sintered Powder Wicks,” 13th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), San Diego, CA, May 30–June 1, pp. 119–129.
Ranjan, R. , Murthy, J. Y. , Garimella, S. V. , Altman, D. H. , and North, M. T. , 2012, “Modeling and Design Optimization of Ultrathin Vapor Chambers for High Heat Flux Applications,” IEEE Trans. Compon. Packag. Manuf. Technol., 2(9), pp. 1465–1478. [CrossRef]
Altman, D. H. , Gupta, A. , Dubrowski, T. E. , Sharar, D. J. , Jankowski, N. R. , and North, M. T. , 2013, “Analysis and Characterization of Thermal Expansion-Matched Wick-Based Multi-Chip Passive Heat Spreaders in Static and Dynamic Environments,” ASME Paper No. InterPACK2013-73087.
Givler, R. C. , and Martinez, M. J. , 2009, “Modeling of Pulsating Heat Pipes,” Sandia National Laboratory, Albuquerque, NM, Report No. SAND2009-4520.
Ma, H. , Thompson, S. M. , Hathaway, A. A. , Smoot, C. D. , Wilson, C. , Young, R. M. , Greenberg, L. , Osick, B. R. , Campen, S. , Morgan, B. C. , Sharar, D. , and Jankowski, N. , 2011, “Experimental Investigation of a Flat-Plate Oscillating Heat Pipe During High-Gravity Loading,” ASME Paper No. IMECE2011-64821.
Thompson, S. , Tessler, B. S. , Ma, H. , Smith, D. , and Sobel, A. , 2013, “Ultrahigh Thermal Conductivity of Three-Dimensional Flat-Plate Oscillating Heat Pipes for Electromagnetic Launcher Cooling,” IEEE Trans. Plasma Sci., 41(5), pp. 1326–1331. [CrossRef]
Thompson, S. M. , Hathaway, A. A. , Smoot, C. D. , Wilson, C. A. , Ma, H. B. , Young, R. M. , Greenberg, L. , Osick, B. R. , Van Campen, S. , Morgan, B. C. , Sharar, D. , and Jankowski, N. , 2011, “Robust Thermal Performance of a Flat-Plate Oscillating Heat Pipe During High-Gravity Loading,” ASME J. Heat Transfer, 133(10), p. 104504. [CrossRef]
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References

Bar-Cohen, A. , Matin, K. , Bloschock, K. , Jankowski, N. , and Darin, S. , 2014, “Two-Phase Thermal Ground Planes: Technology Development and Parametric Results Final Report,” DARPA Thermal Ground Plane (TGP) DOC Report No. 21479.
Oshman, C. , Shi, B. , Li, C. , Yang, R. G. , Lee, Y. C. , and Bright, V. M. , 2010, “Fabrication and Testing of a Flat Polymer Micro Heat Pipe,” 15th International Solid-State Sensors, Actuators and Microsystems Conference, (TRANSDUCERS 2009), Denver, CO, June 21–25, pp. 1999-2002.
Oshman, C. , Qian, L. , Li-Anne, L. , Ronggui, Y. , Bright, V. M. , and Lee, Y. C. , 2012, “Flat Flexible Polymer Heat Pipes,” J. Micromech. Microeng., 23(1), p. 015001. [CrossRef]
Yang, R. G. , Lee, Y. C. , Bright, V. M. , Li, C. , Peterson, G. P. , Oshman, C. , Shi, B. , and Cheng, J. , 2011, “Flexible Thermal Ground Plane and Manufacturing the Same,” U.S. Patent No. 20110017431 A1.
Oshman, C. J. , 2012, “Development, Fabrication, and Experimental Study of Flat Polymer Micro Heat Pipes,” Ph.D. dissertation, University of Colorado, Boulder, CO.
Oshman, C. , Shi, B. C. L. , Ronggui, Y. , Lee, Y. C. , Peterson, G. P. , and Bright, V. M. , 2011, “The Development of Polymer-Based Flat Heat Pipes,” IEEE J. Microelectromech. Syst., 20(2), pp. 410–417. [CrossRef]
Hwang, G. S. , Nam, Y. , Flemming, E. , Dussinger, P. , Ju, Y. S. , and Kaviany, M. , 2010, “Multi-Artery Heat Pipe Spreader: Experiment,” Int. J. Heat Mass Transfer, 53(13–14), pp. 2662–2669. [CrossRef]
Hwang, G. S. , Nam, Y. , Flemming, E. , Dussinger, P. , Ju, Y. S. , and Kaviany, M. , 2011, “Multi-Artery Heat-Pipe Spreader: Lateral Liquid Supply,” Int. J. Heat Mass Transfer, 54(11–12), pp. 2334–2340. [CrossRef]
Nam, Y. , Sharratt, S. , and Ju, Y. S. , 2011, “Characterization and Modeling of the Heat Transfer Performance of Nanostructured Cu Micropost Wicks,” ASME J. Heat Transfer, 133(10), p. 101502. [CrossRef]
Dussinger, P. , Ju, Y. S. , Catton, I. , and Kaviany, M. , 2012, “High Heat Flux, High Power, Low Resistance, Low CTE Two-Phase Thermal Ground Plans for Direct Die Attach Applications,” Government Microcircuit Applications and Critical Technology Conference (GOMACTech-12), Las Vegas, NV, March 19–22, Paper No. 22.4.
Ju, Y. S. , Kaviany, M. , Nam, Y. , Sharratt, S. , Hwang, G. S. , Catton, I. , and Fleming, E. , 2013, “Planar Vapor Chamber With Hybrid Evaporator Wicks for the Thermal Management of High-Heat-Flux and High-Power Optoelectronic Devices,” Int. J. Heat Mass Transfer, 60, pp. 163–169. [CrossRef]
Cai, Q. , Bhunia, A. , Tsai, C. , Kendig, M. W. , and DeNatale, J. F. , 2013, “Studies of Material and Process Compatibility in Developing Compact Silicon Vapor Chambers,” J. Micromech. Microeng., 23(6), p. 065003. [CrossRef]
Cai, Q. , and Chen, Y. C. , 2012, “Investigations of Bi-Porous Wick Structure Dryout,” ASME J. Heat Transfer, 134(2), p. 021503. [CrossRef]
Cai, Q. , Chen, B. C. , Tsai, C. , and Chen, C. L. , 2010, “Development of Scalable Silicon Heat Spreader for High Power Electronic Devices,” ASME J. Therm. Sci. Eng. Appl., 1(4), p. 041009. [CrossRef]
Cai, Q. , and Chen, L. C. , 2010, “Design and Test of CNT Biwick Structure for High Heat Flux Phase Change Heat Transfer,” ASME J. Heat Transfer, 132(5), p. 052403. [CrossRef]
Srivastava, N. , Din, C. , Judson, A. , MacDonald, N. C. , and Meinhart, C. D. , 2010, “A Unified Scaling Model for Flow Through a Lattice of Microfabricated Posts,” Lab Chip, 10(9), pp. 1148–1152. [CrossRef] [PubMed]
Ding, C. , Soni, G. , Bozorgi, P. , Piorek, B. D. , Meinhart, C. D. , and MacDonald, N. C. , 2010, “A Flat Heat Pipe Architecture Based on Nanostructured Titania,” J. Microelecmech. Syst., 19(4), pp. 878–884. [CrossRef]
Dhillon, N. S. , Chan, M. W. , Cheng, J. C. , and Pisano, A. P. , 2011, “Noninvasive Hermetic Sealing of Degassed Liquid Inside a Microfluidic Device Based on Induction Heating,” International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS), Seoul, South Korea, Nov. 15–18, pp. 197–200.
Dhillon, N. S. , Hogue, C. , Cheng, J. C. , and Pisano, A. P. , 2011, “Experimental Investigation of Thin-Film Evaporation in an Open-Loop Columnated Micro-Evaporator,” International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS), Seoul, South Korea, Nov. 15–18, pp. 205–208.
De Bock, H. P. , Chauhan, S. , Chamarthy, P. , Eastman, C. , Weaver, S. , Whalen, B. P. , Deng, T. , Russ, B. , Gerner, F. M. , Johnson, D. , Courson, D. , Leland, Q. , and Yerkes, K. , 2008, “Development and Experimental Validation of a Micro/Nano Thermal Ground Plane,” ASME Paper No. AJTEC2011-44646.
Varanasi, K. K. , Deng, T. , Chamarthy, P. , Chauhan, S. , Bock, P. , Kulkarni, A. , Mandrusiak, G. , Rush, B. , Russ, B. , Denault, L. , Weaver, S. , Gerner, F. , Leland, Q. , and Yerkes, K. , 2009, “Engineered Nanostructures for High Thermal Conductivity Substrates,” Nanotech Conference and Expo, Houston, TX, May 3–7, pp. 505–508.
De Bock, H. P. , Chauhana, S. , Chamarthya, P. , Stanton, E. , Weaver, S. E. , Denga, T. , Gerner, F. M. , Ababnehb, M. T. , and Varanasi, K. , 2010, “On the Charging and Thermal Characterization of a Micro/Nano Structured Thermal Ground Plane,” 12th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Las Vegas, NV, June 2–5.
Weibel, J. A. , Kim, S. S. , Fisher, T. S. , and Garimella, S. V. , 2012, “Carbon Nanotube Coatings for Enhanced Capillary-Fed Boiling From Porous Microstructures,” Nanoscale and Microscale Thermophys. Eng., 16(1), pp. 1–17. [CrossRef]
Weibel, J. A. , Kousalya, A. S. , Fisher, T. S. , and Garimella, S. V. , 2012, “Characterization and Nanostructured Enhancement of Boiling Incipience in Capillary-Fed Ultra-Thin Sintered Powder Wicks,” 13th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), San Diego, CA, May 30–June 1, pp. 119–129.
Ranjan, R. , Murthy, J. Y. , Garimella, S. V. , Altman, D. H. , and North, M. T. , 2012, “Modeling and Design Optimization of Ultrathin Vapor Chambers for High Heat Flux Applications,” IEEE Trans. Compon. Packag. Manuf. Technol., 2(9), pp. 1465–1478. [CrossRef]
Altman, D. H. , Gupta, A. , Dubrowski, T. E. , Sharar, D. J. , Jankowski, N. R. , and North, M. T. , 2013, “Analysis and Characterization of Thermal Expansion-Matched Wick-Based Multi-Chip Passive Heat Spreaders in Static and Dynamic Environments,” ASME Paper No. InterPACK2013-73087.
Givler, R. C. , and Martinez, M. J. , 2009, “Modeling of Pulsating Heat Pipes,” Sandia National Laboratory, Albuquerque, NM, Report No. SAND2009-4520.
Ma, H. , Thompson, S. M. , Hathaway, A. A. , Smoot, C. D. , Wilson, C. , Young, R. M. , Greenberg, L. , Osick, B. R. , Campen, S. , Morgan, B. C. , Sharar, D. , and Jankowski, N. , 2011, “Experimental Investigation of a Flat-Plate Oscillating Heat Pipe During High-Gravity Loading,” ASME Paper No. IMECE2011-64821.
Thompson, S. , Tessler, B. S. , Ma, H. , Smith, D. , and Sobel, A. , 2013, “Ultrahigh Thermal Conductivity of Three-Dimensional Flat-Plate Oscillating Heat Pipes for Electromagnetic Launcher Cooling,” IEEE Trans. Plasma Sci., 41(5), pp. 1326–1331. [CrossRef]
Thompson, S. M. , Hathaway, A. A. , Smoot, C. D. , Wilson, C. A. , Ma, H. B. , Young, R. M. , Greenberg, L. , Osick, B. R. , Van Campen, S. , Morgan, B. C. , Sharar, D. , and Jankowski, N. , 2011, “Robust Thermal Performance of a Flat-Plate Oscillating Heat Pipe During High-Gravity Loading,” ASME J. Heat Transfer, 133(10), p. 104504. [CrossRef]

Figures

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Fig. 2

Photograph of ARL DARPA TGP acceleration testing system

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Fig. 3

Effective thermal conductivity of Phase III TGPs

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Fig. 4

Variation of maximum heat flux with length for Phase III DARPA TGPs

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Fig. 5

Flexible TGP construction by University of Colorado–Boulder

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Fig. 6

Thermal resistance of flexible polymer TGP at different loads and bends

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Fig. 10

SEM Image of Ti structure of UCSB TGP

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Fig. 11

Laser welded Ti-based TGP

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Fig. 13

Uncertainty in thermal conductivity measurement

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Fig. 12

GE TGP charging setup

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Fig. 14

Test vehicles for Raytheon TGP

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Fig. 15

Thermal resistance of Raytheon TGP at various loads

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Fig. 16

Raytheon TGP due to g-loading

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Fig. 7

Effect of applied heat flux versus wall superheat

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Fig. 8

Teledyne TGP biwick structures

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Fig. 9

Biwick dryout hysteresis

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Fig. 17

Effective thermal conductivity of NG 3D FP-OHP

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Fig. 18

Variation of thermal resistance with power dissipation for Phase III DARPA TGP

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Fig. 19

Effective thermal conductivity at rest and at 10 g for Phase II DARPA TGPs

Tables

Table Grahic Jump Location
Table 1 Performance metrics for DARPA TGP program—Phase I and Phase II
Table Grahic Jump Location
Table 2 DARPA TGP Phase III—teams, applications, and attributes
Table Grahic Jump Location
Table 3 Dimensions of Phase III DARPA TGPs

Errata

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