Chapter Four - Recent Advances in Vapor Chamber Transport Characterization for High-Heat-Flux Applications

The efficiency and heat transfer characteristics of a newly designed flat polymer heat pipe that uses copper micro thermal-via was investigated using a fabricated laboratory model to measure the amount of heat that could be removed from a given heat source (a heater or a light emitting diode (LED) module) under a range of different operating conditions. The heat pipe consists of a copper frame 1 mm thick which is sandwiched between top and bottom sheets of FR4 polymer to form a vapor chamber. Two layers of copper mesh, one fine (#200) on top of another of coarser mesh (#100) form a wick structure in the vapor chamber. The design also includes an array of thermal via, formed by 0.5 mm holes drilled through the polymer, which are copper plated and filled with resin similar to the FR4 polymer. This novel design enhances heat conduction through the wall of the polymer heat pipe. A transient dual interface method (TDIM) was used to measure the thermal resistance of the LED module mounted on the flat heat pipe. Experimental results showed that use of the thermal via design reduced the lateral thermal resistance by 20~25%. The thermal resistance of the flat heat pipe was also affected by the filling ratio of working fluid and the tilt angle. When the flat heat pipe with thermal via was used as a mounting substrate with the LED module located in the center of the top surface, experimental results showed the thermal resistance of the substrate was reduced by 54%.

A novel flat polymer heat pipe with thermal via for cooling electronic devices

The current work is a survey of applied applications of passive 2-phase technologies, such as heat pipe and vapor chamber, in heat sink designs with thin base for electronic cooling. The latest improvements of the technologies and manufacturing processes allow achievable heat sink base thickness of 3 mm as compared to around 5 mm previously. The key technical challenge has been on maintaining structural integrity for adequate hollow space for the working fluid vapor in order to retain high performance while reducing the thickness of the overall vapor chamber or flattened heat pipe. Several designs of thin vapor chamber base heat sink and embedded heat pipe heat sink from different vendors are presented for a moderate power density application of a 60 W, 13.2 mm square heat source. Numerous works have been published by both academia and commercial applications in studying the fundamental science of passive 2-phase flow technologies; their performance has been compared to solid materials, like aluminum and copper. These works have established the merits of using heat pipes and vapor chambers in electronic cooling. The intent of this paper is to provide a methodical approach to help to accelerate the process in evaluating the arrays of different commercial designs of these devices in our product design cycle. In this paper, the trade-offs between the different types of technologies are discussed for parameters such as performance advantages, physical attributes, and some cost considerations. This is a bake-off evaluation of the complete heat sink solutions from the various vendors and not a fundamental research of vapor chambers and heat pipes — for that, it is best left to the vendors and universities.

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

A novel flat heat pipe is put forward. The novel flat heat pipe is characteristic of its integral wick structure of microgrooves, which is made of a series of thin aluminum foils folded side by side. The thermal performance of the novel flat heat pipe under the different heat loads and incline angles has been investigated experimentally. It is found that the equivalent thermal conductivity of the novel flat heat pipe can be 12.3 times higher than that of the heat pipe material. Moreover, the novel flat heat pipe with integral micro-grooved wick has good temperature uniformity. The novel flat heat pipe can play a pronounced role in heat transfer enhancement, and be expected to be good candidates for thermal management of electronic devices.

Recently energy saving is most important concept for all electric products and production. Especially, in Data-Center cooling system, power consumption of current air cooling system is increasing. For not only improving thermal performance but also reducing electric power consumption of this system, liquid cooling system has been developed. This paper reports the development of cold plate technology and vapor chamber application by using micro-channel fin. In case of cold plate application, micro-channel fin technology is good for compact space design, high thermal performance, and easy for design and simulation. Another application is the evaporating surface for vapor chamber. The well-known devices for effective heat transfer or heat spreading with the lowest thermal resistance are heat pipes and vapor chamber, which are two-phase heat transfer devices with excellent heat spreading and heat transfer characteristics. Normally, vapor chamber is composed of sintered power wick. Vapor chamber container is mechanically supported by stamped pedestal or wick column or solid column, but the mechanical strength is not enough strong. So far, the application is limited in the area of low strength assembly. Sometime the mechanical supporting frame is design for preventing deformation. In this paper, the testing result of sample is described that thermal resistance between the heat source and the ambient can be improved approximately 0.1°C/W by using the micro-channel vapor chamber. Additionally, authors presented case designs using vapor chamber for cooling computer processors, and proposed ideas of using micro-channel vapor chamber for heat spreading to replace the traditional metal plate heat spreader.

Heat pipes and vapor chambers have been widely utilized for the thermal management of electronic devices due to their effective heat transport, passive cooling operation, and high reliability. In these devices, a wick structure transports a working fluid from the heat sink to the heat source via capillary action in the wick structure. This paper provides a broad overview of the latest studies on the development of Micro-/Nanostructured wicks for passive cooling systems. Micro/nanopillar-based wick structures provide a high capillary pressure, a large permeability, and larger areas for evaporation, resulting in a significantly higher heat removal capability and dryout heat flux. A special emphasis is placed on the various types and geometries of wick structures and their performance. Additionally, limitations and recommendations for future investigations are discussed.

Vapor chambers and heat pipes are often used as standalone heat spreaders for electronics cooling and in other applications. Metal additive manufacturing (AM) techniques have an intrinsic ability to form the porous structures and internal cavities required to fabricate a vapor chamber or a heat pipe. Additive manufacturing techniques thereby have potential for fabricating vapor chambers with complex geometries and locally tailored wick structures to improve performance as well as to monolithically embed vapor chamber heat spreaders within other components. The focus of the present work is to conduct an experimental assessment of the viability of additively manufactured vapor chambers. A direct metal laser sintering (DMLS) technique was used to fabricate a monolithic stainless steel vapor chamber heat spreader with a 39% porous, 0.5-mm thick wick and a 1.5-mm thick internal vapor core. The functionality of the as-printed vapor chamber was evaluated based on characterization of the heat spreading behavior with and without fluid charge. Charging with water decreased the effective thermal resistance of the device while spreading heat from a central hot spot, confirming functionality of the additively manufactured vapor chamber.

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

A review of small heat pipes for electronics

Effect of evaporation and condensation section length ratio on thermal performance of aluminum flat plate heat pipe with different micro grooved wicks

Silicon carbide (SiC) semiconductors have been identified to have potential to replace silicon devices due to superior electrical and thermal properties for a range of power conversion applications. For electrified vehicle applications, the device configuration leads to high current rates, and this in turn leads to high heat fluxes (~1 kW/cm2) over large bare dies (~1 cm2). SiC devices are capable of operating at higher junction temperatures than Si devices, which warrants revisiting air-cooling solutions that are more simple and reliable than liquid cooling. To enable air cooling for high heat flux dissipation, transformative heat spreading technologies must be developed to increase the heat sink footprint area considering the relatively low heat transfer coefficients available. An advanced vapor chamber technology is being investigated for spreading the high heat fluxes generated by next-generation wide band-gap power devices. For vapor chamber heat spreaders to operate at very high heat fluxes over large areas, the internal wick layer at the evaporator must simultaneously minimize the device temperature rise and the flow resistance to liquid resupply by capillary action during boiling. In this study, a vapor chamber is investigated having an embedded two-layer evaporator wick designed to decouple the functions of liquid resupply (through a cap layer) and capillary-fed boiling heat transfer (within a base layer). The performance of a 50 mm × 50 mm × 5.5 mm vapor chamber with an embedded two-layer evaporator wick is evaluated as the heat spreader under a straight pin fin heat sink cooled via air jet impingement for a 1 cm2 area heat source. The maximum dryout heat flux and thermal resistance are compared with that of a vapor chamber having a traditional monolayer evaporator wick. At a power dissipation of ~500 W, the air-cooled two-layer wick vapor chamber provides a 12% reduction in the thermal resistance compared to the monolayer wick vapor chamber assembly. The results indicate that the design of the evaporator wick of the vapor chamber plays a critical role in determining the overall thermal resistance of the heat sink plus spreader assembly.

Heat pipes are heat transfer devices capable of transferring large quantities of heat effectively and efficiently. A vapor chamber (VC) is a flat heat pipe. A novel VC with hollow condenser tubes embedded on the top of it is proposed. This paper reports on the experimental thermal performance of three VC devices embedded with hollow tubes and employed as heat sinks. The first device consisted of a VC with a single hollow tube while the other two VCs had an array of multi-tubes with different tube lengths. All three devices were tested under natural and force air convection cooling. An electrical resistance heater was employed to provide power inputs of 10 and 40 W. Surface temperatures were measured with thermocouple probes at different locations around the devices. The results show that temperatures increased with heater input while total device thermal resistances decreased. Force convection results in lower temperatures and lower resistance. Dry-out occurs at high input power and with too much condensi...

Spreading of high-flux electronics heat is a critical part of any packaging design. This need is particularly profound in advanced devices where the dissipated heat fluxes have been driven well over 100W/cm2. To address this challenge, researchers at Raytheon, Thermacore and Purdue are engaged in the development and characterization of a low resistance, coefficient of thermal expansion (CTE)-matched multi-chip vapor chamber heat spreader, which utilizes capillary driven two-phase heat transport. The vapor chamber technology under development overcomes the limitations of state-of-the-art approaches by combining 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. This paper describes the materials, design, construction and characterization of these vapor chambers. Results from experiments conducted using a unique high-heat flux capable 1DSS test facility are presented, exploring the effects of various microscopic wick configurations, CNT-functionalizations and fluid charges on thermal performance. The impacts of evaporator wick patterning, CNT evaporator functionalization and CNT condenser functionalization on performance are assessed and compared to monolithic Cu wick configurations. Thermal performance is explained as a function of applied heat flux and temperature through the identification of dominant component thermal resistances and heat transfer mechanisms. Finally, thermal performance results are compared to an equivalent solid conductor heat spreader, demonstrating a >40% reduction in thermal resistance. These results indicate great promise for the use of such novel vapor chamber technology in thickness-constrained high heat flux device packaging applications.

Development and evaluation of a supersized aluminum flat plate heat pipe for natural cooling of high power telecommunication equipment