Vapor Chamber Heat Spreaders Research Articles

Thermal performance of a pin-finned vapor chamber heat spreader for CPU cooling applications

Thermal performance of a pin-finned vapor chamber heat spreader for CPU cooling applications

Experimental and numerical investigation on thermal characteristics of the heat sink integrating 3D vapor chamber heat spreader and liquid cooling fins

The traditional vapor chamber/ heat pipe heat sink in high-power chip heat dissipation structures consists of two separate parts. The contact thermal resistance between these parts leads to an increase in thermal resistance during the heat transfer process. To mitigate the contact thermal resistance and improve the heat dissipation performance of vapor chamber heat sinks, this paper proposes a heat sink integrating a 3D vapor chamber heat spreader and liquid cooling fins. Experiments and simulations were carried out to analyze the heat transfer characteristics under different heating powers. The pressure drop and thermal resistance were calculated to analyze the internal phase-change heat transfer process and capillary heat transfer limit. The simulation results were in close agreement with the experimental data for heating powers ranging from 397 to 795 W, with the thermal resistance of the proposed heat sink of about 0.04℃/W. Compared to three traditional vapor chamber heat sinks operating under the same heat flux, the proposed type exhibited a noteworthy enhancement in the heat transfer performance, leading to a reduction in thermal resistance by approximately 44% to 67%.

Performance improvement of heat sink with vapor chamber base and heat pipe

The heat sink with vapor chamber heat spreader has been the emerging design as it offers tremendous reduction in spreading resistance. However, this type of heat sinks suffer from the fin side temperature non-homogeneity, which hinders the heat transfer enhancement. The present study aimed to develop parallelogram fins integrated with heat pipes to increase the heat transfer performance of the heat sink with the vapor chamber heat spreader. The experimental and numerical investigations were performed on different heat sinks with different heat spreaders, including flat plate, vapor chamber, and vapor chamber combined with heat pipe. The air-cooled heat sink features a parallelogram fin configuration is proposed to tailor the temperature non-uniformity alongside the fins. The results showed that the overall thermal resistance of the heat sink with the vapor chamber and the vapor chamber combined with heat pipe spreader (no-cut fins) is about 50% and 55% lower than in the flat plate case. The proposed heat sink with vapor chamber combined with heat pipe spreader having a parallelogram fin structure offered about 61% reduction in thermal resistance and 45–50 °C reduction in chip temperature over the heat sink with flat plate heat spreader.

A two-phase liquid immersion cooling strategy utilizing vapor chamber heat spreader for data center servers

• A two-phase liquid immersion cooling strategy for servers was developed. • Gradient capillary wicking structures were employed in the vapor chamber. • Thermal performance of the vapor chamber was carefully evaluated. • A minimum total thermal resistance of 0.05 °C/W was obtained at 500 W. The 5th generation communication technology dramatically increases the cooling demands of data center servers, and the thermal management strategies should possess the crucial characteristics of relatively compact structure, high cooling efficiency, and reliable operation as well as energy saving. In general, cooling electronics effectively could significantly drive the efficient power utilization for the electronic devices. As an efficient heat dissipation solution, liquid immersion cooling is a potential and prominent candidate to cool high-power-density electronics by submerging them into a pool of dielectric liquid. In this paper, a two-phase liquid immersion cooling strategy using vapor chamber heat spreader was developed and investigated to tackle the continuously increasing demands in heat dissipation of data center servers. In order to promote the circulation of the working liquid and improve the heat transfer performance, gradient capillary wicking structures were innovatively employed in the vapor chamber. Systematic experimental investigations were conducted to fully study the thermal performance of the vapor chamber. The results indicated that the vapor chamber could effectively transport 500 W heat load with the heat source temperature below 85 °C, and could cope with a maximum heat load up to 900 W without dry-out, at which a vapor chamber thermal resistance of 0.046 °C/W was attained. Additionally, compared to other advanced cooling methods reported in literatures, the proposed two-phase liquid immersion cooling strategy using the vapor chamber exhibited a higher heat transfer performance, providing a promising means for high-power data center servers cooling.

Experimental investigation of a vapor chamber featuring wettability-patterned surfaces

Vapor-chamber heat spreaders are hermetically-sealed systems that rely on metal wicks to circulate a phase-changing liquid and spread heat more efficiently than solid-metal heat sinks. But metal wicks also impose capillary limitations due to their high pressure drop. Surface-wettability patterning does not pose the same limitations and, as shown in this work, could replace metal wicks in vapor chambers to transport the condensate faster and more efficiently. Combining the fast condensate transport characteristic of wettability patterning with the added advantage of domain-regulated dropwise condensation (DWC) and filmwise condensation (FWC) towards enhanced condensation performance, an intriguing hypothesis is explored here by incorporating surface-wettability patterning inside vapor chambers made of copper with and without wicking posts. Wettability patterns with different ratios of superhydrophilic (FWC-promoting) to hydrophobic (DWC-promoting) domains are explored and compared with a control case of unpatterned mirror-finish copper, all located on the condenser side. The heat-spreading capability of the devices from a heated spot of ~0.9 cm2 to an effective vapor chamber area of ~25.8 cm2 is tested at a horizontal orientation where gravity acts perpendicular to the vapor chamber plane and at a vertical orientation. The lowest thermal resistance achieved is 0.24 K/W at 87 W heat load (heat flux of 97 W/cm2) with a horizontally-placed vapor chamber that has a 0.5 mm-thick evaporator wick, a wettability-patterned condenser plate and no wicking posts. The work offers quantitative arguments for incorporating wettability patterning in vapor-chamber technology, and motivates further optimization towards achieving the full potential of this approach.

Optimization design of grooved condenser wick structures in a vapor chamber for electronic cooling applications

This article focuses on the layout design of grooved condenser wick structures to improve the surface temperature uniformity for vapor chamber heat spreaders. The design is formulated as a general “area-to-point” (AP) heat conduction problem in disk-shaped configuration. The fundamental relationship between the heat transfer performance and the grooved wick layouts is discussed from the perspective of constructal optimization. The first-order constructs of the heat generating disk are theoretically optimized when the constraint of minimum length scale is imposed for manufacturability of the high conductivity channels (HCCs) (wick grooves). According to the first-order constructs, the disk domain is divided into a number of identical sectors whose boundaries are all adiabatic. A new growth simulator is then developed to obtain the full constructal layouts, where the HCCs are treated as being alive and the optimization is viewed as an adaptive growth procedure. In each sector, the HCCs sprout from the disk center, extend toward the perimeter, and finally converge at a unique topology that exhibits dichotomous, hierarchical, and branching characteristics. Numerical comparison indicates that the new wick structure improves both the temperature uniformity and the capillary pressure, which can contribute to the vapor chamber heat spreaders with better thermal performance.

On the transient thermal response of thin vapor chamber heat spreaders: Optimized design and fluid selection

Vapor chambers provide highly effective heat spreading to assist in the thermal management of electronic devices. Although there is a significant body of literature on vapor chambers, most prior research has focused on their steady-state response. In many applications, electronic devices generate inherently transient heat loads and, hence, it is critical to understand the transient thermal response of vapor chambers. We recently developed a semi-analytical transport model that was used to identify the key mechanisms that govern the thermal response of vapor chambers to transient heat inputs (Int. J. Heat Mass Trans. 136 (2019) 995–1005). The current study utilizes this understanding of the governing mechanisms to develop design guidelines for improving the performance of vapor chambers under transient operating conditions. Two key aspects of vapor chamber design are addressed in this study: first, a parametric optimization of the wall, wick, and vapor-core thicknesses; and second, the selection of the working fluid. A protocol is demonstrated for selecting these parameters given the external vapor chamber envelope dimensions and boundary conditions. The study helps provide a framework for designing vapor chambers subject to transient heat loads, and to differentiate such design from the practices followed traditionally for steady-state operation.

Optimization design of grooved evaporator wick structures in vapor chamber heat spreaders

This article proposes a novel optimization method for designing the layout of grooved evaporator wick structures in vapor chamber heat spreaders. Firstly, the design is transformed into a general ‘Area-to-Line’ (AL) constructal problem with conductive heat transfer in a disc configuration. Then, constructal optimization of the first order high conductivity channels (HCCs) with strict length scale is carried out based on entransy dissipation rate (EDR) minimization. According to the optimal constructs, the uniformly heated disc is divided into a series of identical sectors whose perimeter is set as the heat sink and other boundaries are all adiabatic. Based on this, a new growth simulator is developed from the inspiration of natural branching phenomena, and applied to obtain the full constructal layouts of the HCCs. In such simulation, the HCCs are described by parameterized level-set surfaces, which start at the heat sink line and grow toward the disc center according to the design sensitivities. The evaporator wick structure derived by growth simulation shows a unique topology exhibiting dichotomous, hierarchical and branching characteristics, which make the capillary pressure improve, and the temperature gradient more homogeneous. The proposed method contributes to the vapor chamber heat spreaders with better thermal performance.

Design, Fabrication, and Performance Evaluation of a Hybrid Wick Vapor Chamber

The growing electrification of transportation systems is dramatically increasing the waste heat that must be dissipated from high-density power electronics. Transformative embedded heat spreading technologies must be developed in next-generation systems to enable air cooling of power semiconductors with heat fluxes exceeding 500 W/cm2 over large hotspot areas up to 1 cm2. In this study, vapor chamber heat spreaders, or thermal ground planes (TGPs), with customized wick structures are investigated as one possibility. A 10 cm × 10 cm TGP with hybrid wick, which is a blend of a biporous wick with a standard monoporous wick, was designed. The TGP was tested in combination with a straight pin fin heat sink under air jet impingement and a 1 cm2 size heat source. The experimental performance of the hybrid wick TGP was compared under the same air-cooled conditions with an off-the-shelf TGP of the same size from a commercial vendor and a TGP with a biporous wick only. The customized hybrid wick TGP exhibits ∼28% lower thermal resistance compared with a traditional commercial TGP, and the capillary limit heat flux is measured as 450 W/cm2. Technical challenges in extending this capillary limit heat flux value and TGP integration into packaged electronics are described.

On the transient thermal response of thin vapor chamber heat spreaders: Governing mechanisms and performance relative to metal spreaders

Vapor chambers can offer a passive heat spreading solution for thermal management in electronics applications ranging from mobile devices to high-power servers. The steady-state operation and performance of vapor chambers has been extensively explored. However, most electronic devices have inherently transient operational modes. For such applications, it is critical to understand the transient thermal response of vapor chamber heat spreaders and to benchmark their transient performance relative to the known behavior of metal heat spreaders. This study uses a low-cost, 3D, transient semi-analytical transport model to explore the transient thermal behavior of thin vapor chambers. We identify the three key mechanisms that govern the transient thermal response: (1) the total thermal capacity of the vapor chamber governs the rate of increase of the volume-averaged mean temperature; (2) the effective in-plane diffusivity governs the time required for the spatial temperature profile to initially develop; and (3) the effective in-plane conductance of the vapor core governs the range of the spatial temperature variation, and by extension, the steady-state performance. An experiment is conducted using a commercial vapor chamber sample to confirm the governing mechanisms revealed by the transport model; the model accurately predicts the experimental measurements. Lastly, the transient performance of a vapor chamber relative to a copper heat spreader of the same external dimensions is explored as a function of the heat spreader thickness and input power. The mechanisms governing the transient behavior of vapor chambers are used to explain the appearance of key performance thresholds beyond which performance is superior to the copper heat spreader. This work provides a foundation for understanding the benefits and limitations of vapor chambers relative to metal heat spreaders in transient operation and may inform the design of vapor chambers for improved transient performance.

Thermal spreading resistance of grooved vapor chamber heat spreader

Thermal spreading resistance problem is encountered when the heat spreader is subjected to a concentrated heat source, which has attracted researchers’ attention for decades with the increasing heat flux of electronic packages or power electronic devices. Vapor chamber heat spreader has been regarded as one promising solution to this problem for its excellent thermal performance. In this work, two kinds of groove wick structure, i.e., radial groove and cross groove, copper-water vapor chamber heat spreader were fabricated and tested for their thermal performance. The dimension of the vapor chamber heat spreader was 120 mm × 120 mm × 7.5 mm. It was sandwiched between a heater copper block and an air-cooled aluminum heat sink. The effective heating area was 30 mm × 30 mm. The power input varied from 60 W to 350 W. Test results demonstrated that the thermal spreading resistance played an important role in the total thermal resistance of the vapor chamber heat spreader. In addition, the influence of charging ratio on the thermal spreading resistance of the vapor chamber was also numerically investigated.

Design of an Area-Scalable Two-Layer Evaporator Wick for High-Heat-Flux Vapor Chambers

A hybrid two-layer evaporator wick is proposed for passive, high-heat-flux dissipation over large areas using a vapor chamber heat spreader. For such applications, the evaporator wick layer must be designed to simultaneously minimize the device temperature rise and minimize the flow resistance to a capillary feeding of the wick. This requires a strategy that exploits the benefits of a thin wick for reduced thermal resistance and a thick wick for liquid feeding. In the present design, a thick cap layer of wick material evenly routes liquid to a thin, low-thermal-resistance base layer through an array of vertical liquid-feeding posts. This two-layer structure decouples the functions of liquid resupply (cap layer) and capillary-fed boiling heat transfer (base layer), making the design scalable to heat input areas of ~1 cm2 for operation at 1 kW/cm2. A reduced-order model is developed to demonstrate the potential performance of a vapor chamber incorporating such a two-layer evaporator wick design. The model comprises simplified hydraulic and thermal resistance networks for predicting the capillary-limited maximum heat flux and the overall thermal resistance, respectively. The reduced-order model is validated against a higher fidelity numerical model and then used to analyze the performance of the vapor chamber with varying two-layer wick geometric feature sizes. The two-layer wick design is found to sustain liquid feeding at higher heat fluxes, without reaching the capillary limit, compared to single-layer evaporator wick designs.

Solid and Vapor Chamber Integrated Heat Spreaders: Which to Choose and Why

The performance and reliability of semiconductor electronics require that thermal management hardware be used to adequately cool devices, components, and packages. A vital component of many devices, including but not limited to PAs, CPUs, GPUs, and LEDs, is the integrated heat spreader (IHS). IHSs are implemented to spread the heat from the small electronic component to a larger area in order to facilitate sufficiently low thermal resistance active or passive cooling to the final fluid-cooled heat sink in the stack. One question that still needs to be addressed for electronics packaging is “Under what circumstances do vapor chamber heat spreaders outperform solid IHSs?” This paper is aimed at providing a straightforward approach to addressing this question, specifically during the early “scoping” design stage. Sufficiently accurate steady-state thermal network models for both solid and vapor chamber IHSs are proposed for estimating the overall stack resistance of a package, including TIM1, IHS, TIM2, and convective heat sink. In this way, the performance of both heat spreading technologies can be compared and contrasted over a range of geometric and operating parameters without having to resort to complex numerical models, which may require specialized expertise and/or be too expensive and time-consuming for early-stage design phases of electronic packages.

Enhanced Capillary‐Fed Boiling in Copper Inverse Opals via Template Sintering

AbstractCapillary‐fed boiling of water from microporous metal surfaces is promising for low thermal resistance vapor chamber heat spreaders for hot spot management. Vapor transport through the void spaces in porous metals enables high heat fluxes at low evaporator superheat. In this work, the critical heat fluxes of capillary‐fed boiling in copper inverse opal (IO) wicks that consist of uniform pores with 3D periodicity is investigated. Template sintering is used to enlarge the “necks”, or hydraulic vias, that bridge adjacent IO pores of diameters from 0.6 to 2.1 µm. The enhanced neck size increases the hydraulic permeability for vapor extraction by an order of magnitude, and subsequently the CHF from 100 to 1100 W cm−2. Modeling of the boiling limit accounts for the vapor pressure drop through an IO wick using Darcy's law at a given bubble departure rate. This work links the effect of wick structure design on the boiling crises phenomenon in microporous surfaces and demonstrates material capabilities for ultrathin and low superheat thermal management solutions for high‐power‐density electronic devices.

Patterning the condenser-side wick in ultra-thin vapor chamber heat spreaders to improve skin temperature uniformity of mobile devices

Vapor chamber technologies offer an attractive approach for passive heat spreading in mobile electronic devices, in which meeting the demand for increased functionality and performance is hampered by a reliance on conventional conductive heat spreaders. However, market trends in device thickness mandate that vapor chambers be designed to operate effectively at ultra-thin (sub-millimeter) thicknesses. At these form factors, the lateral thermal resistance of vapor chambers is governed by the saturation temperature/pressure gradient in the confined vapor core. In addition, thermal management requirements of mobile electronic devices are increasingly governed by user comfort; heat spreading technologies must be designed specifically to mitigate hot spots on the device skin. The current work considers these unique transport limitations and thermal requirements encountered in mobile applications, and develops a methodology for the design of vapor chambers to yield improved condenser-side temperature uniformity at ultra-thin form factors. Unlike previous approaches that have focused on designing evaporator-side wicks for reduced thermal resistance and delayed dryout at higher operating powers, the current work focuses on manipulating the condenser-side wick to improve lateral heat spreading. The proposed condenser-side wick designs are evaluated using a 3D numerical vapor chamber transport model that accurately captures conjugate heat transport, phase change at the liquid–vapor interface, and pressurization of the vapor core due to evaporation. A biporous condenser-side wick design is proposed that facilitates a thicker vapor core, and thereby reduces the condenser surface peak-to-mean temperature difference by 37% relative to a monolithic wick structure.

Effect of powder size on capillary and two-phase heat transfer performance for porous interconnected microchannel nets as enhanced wick for two-phase heat transfer devices

In order to enhance the performance of vapor chamber heat spreaders, porous interconnected microchannel nets (PIMNs) were developed as an enhanced capillary wick by traditional copper powder sintering and wire-electric machining methods. The capillary performance of the PIMNs was characterized using a novel IR-thermal imaging method. The experimental results indicated that the gravitational effect was negligible at the initial capillary rise stage but became increasingly influential at the intermediate stage, implied by the transition of capillary rise kinetic from H(t)∼t1/2 to H(t)∼t1/3 where H(t) is the capillary height. The sample of 50–75μm powders exhibited the best capillary performance possibly due to the good balance between the capillary pressure and permeability. The two-phase heat transfer performance of the enhanced wicks was further characterized by pool boiling tests, which revealed that the sample of 50–75μm also maximized the heat transfer coefficient, and could be an optimum structural design of the PIMNs. Comparative studies with other enhanced wicks justified that the PIMNs yielded competitive capillary performance, making it a promising alternative as enhanced wick for two-phase devices.

Performance-Governing Transport Mechanisms for Heat Pipes at Ultrathin Form Factors

Heat pipes and vapor chamber heat spreaders offer a potential solution to the increasing thermal management challenges in thin-form-factor mobile computing platforms, where efficient spreading is required to simultaneously prevent overheating of internal components and formation of hot regions on the device exterior surfaces. Heat pipe performance limitations unique to such ultrathin form factors and the key heat transfer mechanisms governing the performance must be characterized. A thermal resistance network model and a detailed 2-D numerical model are used to analyze the performance of heat pipes under these conditions. A broad parametric study of geometries and heat inputs using the reduced-order model helps delineate the performance thresholds within which the effectiveness of a heat pipe is greater than a comparable solid heat spreader. A vapor-phase threshold unique to ultrathin heat pipes operating at low-power inputs is observed. At this threshold, the vapor-phase thermal resistance imposed by the saturation pressure/temperature gradient in the heat pipe causes a crossover in the thermal resistance relative to a solid heat spreader. The higher fidelity numerical model is used to assess the accuracy of the resistance network model and to verify the validity and applicability of each assumption made regarding the transport mechanisms. Key heat transfer mechanisms not captured by the reduced-order thermal network models are identified. These include the effects of boundary conditions on the interface mass flux profile, convective effects on the vapor core temperature drop, and 2-D conduction on smearing of evaporation/condensation mass flux into the adiabatic section.

Experimental studies of thermal resistance in a vapor chamber heat spreader

In recent years, with the increasing heat dissipation of electrical appliances, efficient cooling systems are required to solve the elevated temperature problems encountered in certain interior areas of electrical devices. One solution is to use a vapor chamber heat spreader two-phase heat transfer device. A prototype vapor chamber heat spreader was built and tested for its thermal performance. The chamber had dimensions of 90 mm × 90 mm × 3.5 mm. It was sandwiched between a heater block and a water cooled plate. Heat was added to the bottom part of the vapor chamber heat spreader by a cartridge heater inserted in an aluminum block. The effective heating area was 12.7 mm × 8.9 mm. The spreader upper surface was attached to an aluminum block, which was compressed against a copper water cold plate from which the heat was removed. The power input varied from 5 W to 50 W. Test results demonstrated that spreading resistance played a vital role in the total thermal resistance. The influence of inclination on the vapor chamber performance and temperature uniformity were also investigated.

Experimental Characterization of Capillary-Fed Carbon Nanotube Vapor Chamber Wicks

The thermal performance of passive vapor chamber heat spreaders can be improved by enhancing evaporation from the internal wick structure. A wick structure that integrates conventional copper screen mesh and carbon nanotubes (CNTs) is developed and characterized for increased heat transport capability and reduced thermal resistance. The high-permeability mesh provides for a low-resistance liquid flow path while the carbon nanotubes, with their high thermal conductivity and large surface area, help reduce conduction and phase-change resistances. The wicks are fabricated by sintering a copper mesh on a multilayer substrate consisting of copper and molybdenum. CNTs are grown on to this mesh and a submicron layer of copper is evaporated on to the CNTs to improve wettability with water and wicking. Samples grown under varying degrees of positive bias voltage and varying thicknesses of post-CNT-growth copper evaporation are fabricated to investigate the effect of surface morphology variations. The resultant boiling curves indicate that micro/nano-integrated wicks fabricated with higher positive bias voltages during CNT synthesis coupled with thicker copper coatings produce lower wick thermal resistances. Notably, heat fluxes at the heater surface of greater than 500 W/cm2 were supported without a critical heat flux condition being reached.

TEMPERATURE UNIFORMITY ANALYSIS OF A MULTI-WELL VAPOR CHAMBER HEAT SPREADER

In this study, temperature uniformity and heating rate of heat spreader with multi-well are simulated and analyzed by CFD software under natural convection condition. Firstly, multi-well heat spreader made of aluminum, copper, silver and vapor chamber are simulated and compared, when dual and six heat sources are applied at a heating power of 1200W. Secondly, with six heat sources at heating power of 188, 300, 600 and 1200W, the heating rate of heat spreaders are studied. Vapor chamber heat spreader has the better temperature uniformity both in six sources and dual sources mode due to a higher thermal conductivity, followed by silver, copper and aluminum heat spreader. Aluminum and silver heat spreader show the higher heating rate, followed by copper heat spreader, vapor chamber heat spreader due to a larger heat capacity, heat up the slowest.