Tag Archives: thermal conductivity

Applications of Vapor Chambers in Thermal Management of Electronics

The use of vapor chambers in the thermal management of electronics has grown exponentially since Advanced Thermal Solutions, Inc. (ATS) first wrote seven years ago about their ability to spread heat uniformly across the base of a heat sink, reducing the spreading resistance and enhancing the heat sink’s heat transfer capabilities when applied to high-powered components.

In a two-part series published originally in 2010 and based on an article from Qpedia Thermal eMagazine entitled, “Vapor Chambers and Their Use in Thermal Management,” it was explained that “a vapor chamber (VC) is basically a flat heat pipe that can be part of the base of a heat sink. It is vacuumed and then injected with just enough liquid (e.g. water) to wet the wick.” [1]

Similar to heat pipes, “The heat source causes the liquid to vaporize on the evaporator side. The resulting pressure increase in this area forces the vapor into the condenser side, which is the base of the heat sink. Here, the vapor transfers the heat to the heat sink, and it then condenses back to liquid. The liquid is pumped back to the base through the capillary action of the wick structure.”

In Fig. 1, two heat sinks are shown. One has a solid base and the other has a vapor chamber in its base and it is clear from the temperature distribution that the vapor chamber spreads out the heat across the base and distributes heat to a larger portion of the heat sink.

As the original article explained, “The very high equivalent thermal conductivity of the vapor chamber has spread the heat uniformly, leading to more efficiency from the heat sink.”

Vapor Chambers

Figure 1. Schematic View of Heat Sinks with (a) Solid Base and (b) Vapor Chamber Base. [1]

The second article runs through some of the equations that define the effective thermal conductivity of the wicking structure inside the vapor chamber and the impact that changing the wick material can have on its efficiency.

“This article shows that while a vapor chamber presents exciting technology, some calculations should be made to justify its use,” it continued. “In some situations, a solid copper block might provide better thermal performance than a vapor chamber. To use a vapor chamber instead of solid copper must be justified, for example, to reduce weight.

Another issue with vapor chambers presented by the article was that “some vapor chambers have a power limit of 500 watts. Exceeding this value might cause a dry out, as with a heat pipe, and could increase the vapor temperature and the pressure. The increase in internal pressure can deform the VC surfaces, or cause leakage from the welded joints.”

The study of vapor chambers has developed in the past seven years and, although some of the same issues remain, they are now thinner and lighter than ever and engineers are finding many new ways of incorporating them into cooling systems. Vapor chambers are now frequently used in applications ranging from hard drive disk cooling, PC cooling (not just for gamers and overclockers, but also for office computers), graphic card cooling, server cooling, high heat flux chips (IGBT and MOSFET), LED, and in consumer products (particularly mobile devices such as cell phones and tablets).

In addition to the benefits explained above, vapor chambers are critical in applications where height is limited, which is an increasing problem in today’s era of miniaturization, and where power densities are high. Vapor chambers are also important in applications where there are hotspots, where weight is a concern, and where there is a high ambient temperature or low airflow.

Hard Drive Disk Cooling

Several manufacturers in the hard drive market have turned to vapor chambers because of increases in spindle speed. In the past, many manufacturers and designers limited the thermal management of hard drives to using the aluminum case as a heat sink to dissipate the excess heat from the device, but as drives began working at 7,200 RPM and higher another option was required to ensure the reliability and longevity of the drive. [2]

A 2013 study that was published in International Communications in Heat and Mass Transfer explored the use of vapor chambers to cool hard drives in personal computers. The researchers found that adding vapor chambers to the cooling system could reduce the hard drive temperature by as much as 15.21%. [3]

Gaming, Overclocking, Personal Computing

The gaming and overclocking community has turned towards liquid cooling in recent years, as evidenced by a recent survey from KitGuru that showed 51% of its readers had already or would shortly be using liquid cooling for their personal computers. [4] While there is a trend in that direction, just under half (49%) of the respondents were also sticking with convection cooling options and many companies are incorporating vapor chamber technology in elaborate cooling devices (many with fans and heat pipes) for the PC market.

Cooler Master has introduced the V8 GTS CPU Air Cooler, which strongly resembles a car engine and has a horizontal vapor chamber and eight heat pipes. [5] The vapor chamber spreads the heat evenly from hotspots in the CPU and the heat pipes draw that heat into the tower’s heat sink.

Vapor Chambers

The Cooler Master GTS V8 has a distinct car engine look and uses vapor chambers, heat pipes, and heat sinks to cool PCs. (Cooler Master/YouTube)

ID Cooling has introduced several products that boast vapor chamber technology, including the HUNTER, and FI (which stands for Finland) Series CPU coolers. [6] Even gaming systems have gotten into the act with the recently announced, high-powered Xbox One Scorpio expected to include a vapor chamber array as part of its thermal management. [7] Microsoft’s announcement that it was using vapor chambers in Project Scorpio was not surprising because of the technology’s ability to fit into the tight confines of the gaming system.

Microsoft’s Project Scorpio introduced a new, higher-powered gaming system that required an array of vapor chambers to keep it cool. (Microsoft)

Also, the increasing capabilities and power of next-generation graphics cards has led to a trend in the industry to use vapor chambers as part of a package to cool these components. Nvidia is one of the biggest names in graphic cards and for both the Titan X and the GeForce GTX 1080 (each launched in 2016) vapor chamber are used with a blower to dissipate the increased power of the devices. [8]

It is not only the gaming community that is benefiting from vapor chamber cooling. Hewlett Packard (HP) has also explored using vapor chambers for multiple purposes. HP released a white paper last year about using 3-D vapor chambers in its Z Coolers to enhance their thermal efficiency as well as reduce the acoustic impact of the fans. [9] Also last year, HPE Labs released a study of vapor chambers for cooling multiple chip modules dissipating 250 W and operating temperatures up to 45°C and found that “VC (vapor chamber) performs better for: high power, power density, off center or asymmetric heat sources.” [10]

Server Cooling

Much like in graphic cards or gaming systems, vapor chambers are increasingly used in server cooling applications because their size and weight allows them to fit into tight spaces, particularly in applications with high component density. For example, Rugged has released an M120 1-U server rack that includes vapor chambers to spread the heat evenly and high-speed fans to pull the heat out of the system. [11]

A study by Aavid Thermacore from the 2007 ASME InterPACK Conference explained that in blade processors that need to dissipate 100-300 W with heat sinks lower than 30 mm, vapor chambers could be used as the base of the heat sink to improve effective spreading and improve performance by 25-30%. [12] Radian’s Intel Skylake heat sink that is intended for server chips installed in a 1-U chassis put this into practice with a vapor chamber in its base that enhances the effective thermal conductivity of the stamped aluminum fins. [13]

Radian’s Intel Skylake heat sink uses a vapor chamber in the base to evenly spread the heat and improve the heat transfer through the fins of the heat sink. (Radian)

For more on the topic of vapor chambers as heat sink bases, read https://www.qats.com/cms/2017/07/26/vapor-chambers-solid-material-base-high-power-devices.

LED Cooling

A more recent development in the use of vapor chambers is their inclusion in LED packages. A 2016 study from the 37th International Electronic Manufacturing Technology Conference outlined the use of vapor chambers along with finned heat sinks in the thermal management of LED to enhance the thermal performance and provide a “more economical” process than making the heat sink larger or using more expensive materials. [14]

Advanced Cooling Technologies (ACT) also released a case study about cooling high-powered LED applications, such as ultraviolet (UV) cutting devices, which said, “Vapor Chambers are an important tool in LED thermal management, since they act as flux transformers, spreading the high input heat flux over the entire surface of the vapor chamber. This allows the heat to be removed from the vapor chamber by conventional cooling methods.” [15] ACT added that it developed C.T.E matched vapor chambers that allow for direct bonding with the LED and “dissipate heat fluxes as high as 700 W/cm2 and 2kW overall.”

Vapor chambers are also being used in automotive LED applications to prevent failures by spreading the heat quickly from the source. A study from the 2011 International Heat Pipe Symposium found that a vapor chamber with distilled water dropped the LED temperature from 112.7°C to 80.7°C, reduced thermal resistance by 56%, and reached steady state faster than conventional systems. [16]

Mobile Devices

The most obvious market for vapor chambers is mobile devices. Last fall, the news was filled with stories about Samsung cell phone batteries reaching thermal runaway and airplane passengers being forced to turn off the phones for concern about a midair fire. With their thin design and low weight, vapor chambers can be used to spread the heat quickly from batteries or high-powered processors in phones, laptops, tablets, etc. and reduce the risk for catastrophic failures.

A 2016 study from the International Journal of Heat and Mass Transfer described vapor chambers being used to reduce hotspots to improve the comfort of users, which is a problem unique to mobile devices. [17] The researchers proposed a “biporous condenser-side wick design” 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.”

A recent story from EE Times noted that the combined shipments of mobile devices was expected to decline in 2017, marking the third straight year of reduced shipments [18], but with companies expending resources to develop 5G technology there is still a need for superior cooling options moving forward and vapor chambers appear to be a perfect fit in mobile thermal management systems.

References
[1] “Vapor Chambers in Thermal Management”, Qpedia Thermal eMagazine, Sept. 2007.
[2] http://www.pcguide.com/ref/hdd/op/packCooling-c.html
[3] P. Naphon. S. Wongwises, and S. Wiriyasart, “Application of two-phase vapor chamber technique for hard disk drive cooling of PCs,” International Communications in Heat and Mass Transfer, January 2013.
[4] https://www.kitguru.net/components/cooling/andrzej/majority-of-kitguru-readers-now-planning-on-liquid-cooling/
[5] http://www.coolermaster.com/cooling/cpu-air-cooler/v8-gts/
[6] http://www.idcooling.com/Product/series/category_parent/25/name/AIR%20COOLING
[7] https://www.youtube.com/watch?v=RE2hNrq1Zxs
[8] http://www.pcworld.com/article/3102027/components-graphics/nvidias-monstrous-new-titan-x-graphics-card-stomps-onto-the-scene-powered-by-pascal.html and https://www.nvidia.com/en-us/geforce/products/10series/geforce-gtx-1080/
[9] http://www8.hp.com/h20195/v2/getpdf.aspx/4AA6-1205ENW.pdf?ver=2.0
[10] https://www.labs.hpe.com/techreports/2016/HPE-2016-85.pdf
[11] http://www.coresystemsusa.com/filedata/prod/443m120_1u_rugged_rackmount_computer.pdf
[12] http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1595384
[13] https://www.radianheatsinks.com/intel-skylake-heatsink/
[14] K.S. Ong, C.F. Tan, K.C. Lai, K.H. Tan, and R. Singh, “Thermal management of LED with vapor chamber and thermoelectric cooling,” 37th International Electronic Manufacturing Technology Conference, 2016.
[15] https://www.1-act.com/led-thermal-management-case-study-cte-matched-vapor-chamber/
[16] Ji Won Yeo, Hyun Jik Lee, Soo Jung Ha, et al., “Development of Cooling System of LED Headlamp for Vehicle Using Vapor Chamber Type Heat Pipe,” 10th International Heat Pipe Symposium, November 2011.
[17] GauravPatankar, Justin A.Weibel, and Suresh V.Garimella, “Patterning the condenser-side wick in ultra-thin vapor chamber heat spreaders to improve skin temperature uniformity of mobile devices,” International Journal of Heat and Mass Transfer, October 2016.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

Vapor Chambers and Solid Material as a Base for High Power Devices

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

Microelectronics components are experiencing ever-growing power dissipation and heat fluxes. This is due to dramatic gains in their performance and functionality. To cope with the heat issues of tomorrow’s technology, more efficient cooling systems will be required. It should be noted that as computer systems continue to compact, the components adjacent to the processors are experiencing an increase in power dissipation.

As a result, ambient temperatures local to the microprocessor heat sinks have increased, and temperatures in excess of 45°C have been reported. [1] Improvements are needed in all aspects of the cooling solution design, i.e., packaging, thermal interfaces, and air-cooled heat sinks. The article discusses the use of vapor chamber technology as a heat spreader to help cool high-power devices.

Introduction

Spreading resistances exist whenever heat flows from one region to another in a different cross-sectional area. For example, with high performance devices, spreading resistance occurs in the base plate when a heat source with a smaller footprint is mounted on a heat sink with a larger base plate area. The result is a higher temperature where the heat source is placed.

The impact of spreading resistance on a heat sink’s performance must not be ignored in the design process. One way to reduce this added resistance is to use highly conductive material, such as copper, instead of aluminum. Other solutions include using heat pipes, vapor chambers, liquid cooling, micro thermoelectric cooling, and the recently developed forced thermal spreader from Advanced Thermal Solutions, Inc. (ATS).

In the case of vapor chambers (VC), the general perception has been that phase change technologies provide more effective thermal conductivity than solid metals. The spreading resistance of the base for both solid metal and conventional VC heat spreaders is defined as:

Vapor Chamber (1)

Where Ts [°C] is the temperature of the hottest point on the base, and Tb,top [°C] is the average temperature of the base top surface. [1]

Vapor Chambers

Figure 1. Schematics of a) Heat Pipe and b) Vapor Chamber [2], with c) Photo of Vapor Chambers. [3]

Table 1 shows the thermal conductivity of different materials in spreading the heat at the base. Heat pipes and VC emerged as the most promising technologies and cost effective thermal solutions due to their excellent uniform heat transfer capability, high efficiency, and structural simplicity. Their many advantages compared to other thermal spreading devices are that they have simple structures, no moving parts, allow the use of larger heat sinks, and do not use electricity. This article’s emphasis is on vapor chambers.

Is a heat pipe considered a material? Should we include vapor chambers in this table?

The principle of operation for VC is similar to that of heat pipes. Both are heat spreading devices with highly effective thermal conductivity due to phase change phenomena. A VC is basically a flat heat pipe that can be part of the base of a heat sink. Figure 1 shows the schematics of a typical heat pipe and VC. [2]

A VC is a vacuum vessel with a wick structure lining its inside walls. The wick is saturated with a working fluid. The choice of this fluid is based on the operating temperature of the application. In a CPU application, operating temperatures are normally in the range of 50-100°C. At this temperature range water is the best working fluid. [3]

As heat is applied, the fluid at that location immediately vaporizes and the vapor rushes to fill the vacuum. Wherever the vapor comes into contact with a cooler wall surface it condenses, releasing its latent heat of vaporization. The condensed fluid returns to the heat source via capillary action, ready to be vaporized again and repeat the cycle.

The capillary action of the wick enables the VC to work in any orientation, though its optimum performance is orientation dependent. The pressure drop in the vapor and the liquid determines the capillary limit or the maximum heat carrying capacity of the heat pipe. [4] For electronics applications, a combination of water and sintered copper powder is used. [2]

A VC, as shown in Fig.1 (b), is different from a heat pipe in that the condenser covers the entire top surface of the structure. In a VC, heat transfers in two directions and is planar. In a heat pipe, heat transmission is in one direction and linear.

The VC has a higher heat transfer rate and lower thermal resistance. In the two-phase VC device, the rates of evaporation, condensation, and fluid transport are determined by the VC’s geometry and the wicks’ structural properties. These properties include porosity, pore size, permeability, specific surface area, thermal conductivity, and the surface wetability of the working fluid. [5] Thermal properties of the wick structure and the vapor space are described in the next section.

Effective Thermal Conductivity

Wick Structure

Heat must be supplied through the water-saturated wick structure, at the liquid-vapor interface, for the evaporation process to happen. With water and sintered copper powder, the water becomes a thermal barrier due to its much lower thermal conductivity compared with the copper. [2]

There are several ways to compute the effective thermal conductivity of the wick structure.

For parallel assumption:

(2)

For serial assumption:

(3)

For sintered wick structure, Maxwell gives: [2]

(4)

Chi gives: [2]

(5)

Where:

(6)

In the equations above, Kl and Ks are the thermal conductivities of water and copper, respectively, ε is the porosity of the wick, rc and rs are the contact radius (or effective capillary radius) and the particle sphere radius, respectively.

Table 2 shows a comparison of effective thermal conductivity (W/m°C) for the wick using equations 2—5. It appears that Equation 5 gives a more realistic value. This is also the typical value used in Vadakkan et al. [6]

Table 2. Effective Thermal Conductivity for the Wick Structure.

Vapor Space

Effective thermal conductivity for vapor chambers used in remote cooling applications has been derived from Prasher [4], based on the ideal gas law, and from the Clapeyron equation for incompressible laminar flow conditions.

(7)

Where Hfg is the heat of vaporization (J/Kg), P is pressure (N/m2), ρ is density (kg/m3), d is the vapor space thickness (mm), R is the gas constant per unit mass (J/K.Kg), μ is the dynamic viscosity (N.s/m2), and T is the vapor temperature (°C).

As shown in Equation 7, effective thermal conductivity is a function of thermodynamic properties and vapor space thickness. Larger vapor space thickness reduces the flow pressure drop, and thus increases the effective thermal conductivity. Note that the effective thermal conductivity is relatively low at low temperatures. This has significant implications for low heat flux applications or start-up conditions [2].

Drawbacks

There are a few drawbacks to using a VC instead of solid copper. Some VC have a power limit of 500 watts. Exceeding this temperature might cause a dry out and could increase the vapor temperature and pressure.

An increase in internal pressure can deform the VC surfaces or cause leakage from the welding joints. Other factors to be addressed include cost, availability, and in special cases, the vapor chamber’s manufacturability.

When to Use a Vapor Chamber

The early design stages are when to decide if it makes sense to use a heat pipe/VC instead of copper or other solid materials to better spread heat. To predict the minimum thermal spreading resistance for a VC, a simplified model was developed by Sauciuc et al. [1]. Their model assumes that the minimum VC spreading resistance θsp is approximately the same as the evaporator (boiling) resistance θev.

(8)

Here, hev [W/m2K] is the boiling heat transfer coefficient and Aev [m2] is the area of the evaporator (heat input area). It is also assumed that the boiling regime inside the VC is nucleate pool boiling. This is a conservative assumption, since in reality the spreading resistance in a VC is greater than just the boiling resistance. If the spreading resistance calculated from this simplified model is higher than that of a solid copper base, then a VC should not be used. [1]

The boiling model is based on Rohsenow’s equation for nucleate pool boiling on a metal surface, and is given by: [7]

(9)

Where μf is the dynamic viscosity of the liquid, hfg is the latent heat, g(ρf – ρg) is the body force arising from the liquid-vapor density difference, σ is the surface tension, cp,f is the specific heat of liquid, Cs,f and n are constants that depend on the solid-liquid combination, Prf is the liquid’s Prandtl number, and ΔT = [Ts – Tsat], which is the difference between the surface and saturation temperatures.

It can be seen that the heat flux is mainly a function of fluid properties, surface properties, and the fluid/material combination, and that superheat is required for boiling. For electronics cooling applications, it is widely accepted that water/copper is the optimum combination for VC fabrication [1].

The evaporator heat transfer coefficient definition is:

(10)

The ratio of phase change spreading over copper spreading can be estimated for the base of conventional rectangular heat sinks using Rohensaw’s equation and conventional modeling tools, Figure 2 from [1] shows the relationship of this ratio versus base thickness (solid metal heat sink only) for different footprint sizes. The heat input area is kept constant for this plot. This figure shows that for spreading resistance ratios greater than 1.0, the ratio decreases with increasing condenser size.

This implies that the VC type base is better situated for larger condenser sizes. The figure also indicates that ratio 1.0 occurs at greater base thickness for larger condensers. For example, with a 200×200 mm footprint, a VC would outperform a corresponding copper base heat sink (with a thickness of 10 mm or less). However, with a 50×50 mm footprint the sink’s base thickness would have to be less than about 2.5 mm for the VC to make the same claim. [1]

Figure 2 also shows that there is a “worse case point” when comparing the thermal performance of a VC and a solid copper base heat sink. This is identified by the maximum in the curve for the 50×50 mm footprint at a base thickness of 10 mm. At this point the spreading resistance ratio is at its largest value, which indicates the worst performance for the VC (when compared with the corresponding solid copper base). In general, there will be a maximum base thickness (dependent on heat source size and footprint) in considering a VC base.

Unless weight is a major concern, with a base thickness above this maximum, a VC base should not be considered. Conversely, for a heat sink base thickness below this maximum, a VC base is a viable option.

Figure 2: Ratio of Phase Change Resistance (Rohensaw’s Equation) Versus Solid Metal Resistance. [1]

Summary

Although a VC enhances heat spreading through high effective thermal conductivity, some modeling needs to be considered early in the design stage. Because a VC is a liquid filled device, cautions need to be exercised in its deployment in electronics. The dry out or loss of liquid due to poor manufacturing will render the VC as a hollow plate, thus adversely impacting device thermal performance.

In some situations as shown earlier, a solid copper base might provide better spreading of heat without the potential pitfalls of a VC.

References:
1. Sauciuc, I. Chrysler, G., Mahajan, Ravi, and Prasher, Ravi, “Spreading in the Heat Sink Base: Phase Change Systems or Solid Metals?”, IEEE Transactions on Components and Packaging Technologies, December 2002, Vol. 25, No. 4.
2. Wei, X., Sikka, K., Modeling of Vapor Chamber as Heat Spreading Devices, 10th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronics Systems, 2006.
3. Wuttijumnong, V., Nguyen, T., Mochizuki, M., Mashiko, K., Saito, Y., and Nguyen, T., Overview Latest Technologies Using Heat Pipe and Vapor Xhamber for Cooling of High Heat Generation Notebook Computer, Twentieth Annual IEEE Semiconductor Thermal Measurement and Management Symposium, 2004.
4. Prasher, R, A Simplified Conduction Based Modeling Scheme for Design Sensitivity Study of Thermal Solution Utilizing Heat Pipe and Vapor Chamber Technology, Journal of Electronic Packaging, Transactions of the ASME, 2003, Vol. 125, No. 3.
5. Lu, M., Mok, L., Bezama, R. A Graphite Foams Based Vapor Chamber for Chip Heat Spreading, Journal of Electronic Packaging, December 2006.
6. Vadakkan, U., Chrysler, G., and Sane, S., Silicon/Water Vapor Chamber as Heat Spreaders for Microelectronic Packages, IEEE SEMI-THERM Symposium, 2005.
7. Incropera, F., Dewitt, D., Bergman, T., and Lavine, A., Introduction to Heat Transfer, Wiley, Fifth Edition, 2007.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

ATS Heat Sink Hammer Test

Advanced Thermal Solutions brings the wacky and insane experiments of Crazy “Lenny”. Watch as he attempts to smash ATS’s patented maxiGRIP and superGRIP heat sink attachments with an assortment of hammers.

To learn more go to: www.qats.com/Heat-Sink/Attachments

 

Digital News Gets Support from ATS

With Thermal Engineer day approaching (7/24) we here at ATS would like to thank all of the PR firms and digital news magazines who covered our new clipKIT campaign.

thermal technology - digital news - electronics businessAs a token of our appreciation, we have provided a link to our customers and viewers to download our clipKIT data sheet for all your attachment needs. HERE.

 

What are Heat Pipes and What Characteristics Make Them Helfpul for Electronics Cooling?

Heat Pipes, the Super Conductors
Heat pipes are transport mechanisms that can carry heat fluxes ranging from 10 W/cm2 to 20 KW/cm2 at a very fast speed. Essentially, they can be considered as heat super conductors. Heat pipes can be used either as a means to transport heat from one location to another, or as a means to isothermalize the temperature distribution.

The first heat pipe was tested at Los Alamos National Laboratory in 1963. Since then, heat pipes have been used in such diverse applications as laptop computers, spacecraft, plastic injection molders, medical devices, and lighting systems. The operation of a heat pipe is described in Figure 1.


Figure 1. Schematic View of a Heat Pipe [1].

A heat pipe has three sections: the evaporator, adiabatic, and condenser. The interior of the pipe is covered with a wick, and the pipe is partially filled with
a liquid such as water. When the evaporator section (L ) is exposed to a heat source, the liquid inside vapor- izes and the pressure in that section increases. The increased pressure causes the vapor to flow at a fast speed toward the condenser section of the heat pipe (L ). The vapor in the condenser section loses heat to the integral heat sink and is converted back to liquid by the transfer of the latent heat of vaporization to the condenser. The liquid is then pumped back to the evaporator through the wick capillary action. The middle section
of the heat pipe (La), the adiabatic portion, has a very small temperature difference.

figure2Figure 2. Pressure Drop Distribution in a Heat Pipe [1].

Figure 2 shows the pressure drop distribution inside a heat pipe. In order for the capillary force to drive the vapor, the capillary pressure of the wick should exceed the pressure difference between the vapor and the liquid at the evaporator. The graph also shows that if the heat pipe is operated against the force of gravity, the liquid undergoes a larger pressure drop. The result
is less pumping of the wick with reduced heat transfer. The amount of heat transfer decrease depends on the particular heat pipe.

figure3Figure 3. Different Wick Structures

A typical heat pipe is made of the following:
1. Metallic pipe  The metal can be aluminum, copper or stainless steel. It must be compatible with the working fluid to prevent chemical reactions, such as oxidation.

2. Working fluid  Several types of fluids have been used to date. These include methane, water, ammonia, and sodium. Choice of fluid also depends on the
operating temperature range.

3. Wick  The wick structure comes in different shapes and materials. Figure 3 shows the profiles of common wick types: axial groove, fine fiber, screen mesh, and sintering. Each wick has its own characteristics. For example, the axial groove has good conductivity, poor flow against gravity, and low thermal resistance. Conversely, a sintering wick has excellent flow in the opposite direction of gravity, but has high thermal resistance.

Table 1. Heat Pipes with Different Structures and Operating Conditions [1]table1Table 1 shows experimental data for the operating temperature and heat transfer for three different types of heat pipes [1].

Certain factors can limit the maximum heat transfer rate from a heat pipe.

These are classified as follows:
1. Capillary limit
  Heat transfer is limited by the pumping action of the wick.
2. Sonic limit  When the vapor reaches the speed of sound, further increase in the heat transfer rate can only be achieved when the evaporator temperature
increases.
3. Boiling limit  High heat fluxes can cause dry out.
4. Entrainment limit  High speed vapor can impede the return of the liquid to the condense.

A heat pipe has an effective thermal conductivity much larger than that of a very good metal conductor, such as copper. Figure 4 shows a copper-water heat pipe and a copper pipe dipped into an 80°C water bath. Both pipes were initially at 20°C temperature. The heat pipe temperature reaches the water temperature in about 25 seconds, while the copper rod reaches just 30°C after 200 seconds. However, in an actual application when a heat pipe is soldered or epoxied to the base of a heat sink, the effective thermal conductivity of the heat pipe may be drastically reduced due to the extra thermal resistances added by the bonding. A rule of thumb for the effective thermal conductivity of a heat pipe is 4000 W/mK.

figure4
Figure 4. Experiment Comparing Speed of Heat Transfer Between a Heat Pipe and a Copper Pipe [1].

Heat pipe manufacturers generally provide data sheets showing the relationship between the temperature difference and the heat input. Figure 5 shows the temperature difference between the two ends of a heat pipe as a function of power [2].

figure5Figure 5. Temperature Difference Between the Evaporator and the Condenser in a Heat Pipe [2].

figure6Figure 6. Typical Round Heat Pipes in the Market.

There are many heat pipe shapes in the market, but the most common are either round or flat. Round heat pipes can be used for transferring heat from one point to another. They can be applied in tightly spaced electronic components, such as in a laptop. Heat is transferred to a different location that provides enough space to use a proper heat sink or other cooling solution. Figure 6 shows some of the common round heat pipes available in the market.

Flat heat pipes (vapor chambers) work conceptually the same as round heat pipes. Figure 7 shows a flat pipe design, they can be used as heat spreaders. When the heat source is much smaller than the heat sink base, a flat heat pipe can be embedded in the base of the heat sink, or it can be attached to the base to spread the heat more uniformly on the base of the heat sink. Figure 8 shows some common flat heat pipes.

figure7Figure 7. Conceptual Design Schematic of a Flat Heat Pipe [1].

 

figure8Figure 8. Commonly-used Flat Heat Pipes.

Although a vapor chamber might be helpful in minimizing spreading resistance, it may not perform as well as a plate made from a very high conductor, such as diamond. A determining factor is the thickness of the base plate. Figure 9 shows the spreading resistance for 80 x 80 x 5 mm base plate of different materials with a 10 x 10 mm heat source. The vapor chamber has a spreading resistance that is better than copper, but worse than diamond. However the price of the diamond might not justify its application. Figure 9 also includes the spreading
resistance from the ATS Forced Thermal Spreader (FTS), which is equal to that of diamond at a much lower cost. The FTS uses a combination of mini and
micro channels to minimize the spreading resistance by circulating the liquid inside the spreader.

figure9Figure 9. Thermal Spreading Resistances for Different Materials. [3] – ATS

Heat pipes have a very important role in the thermal management arena. With projected lifespans of 129,000-260,000 hours (as claimed by their manufacturers), they will continue to be an integral part of some new thermal systems. However, with such problems as dry out, acceleration, leakage, vapor lock and reliable performance in ETSI or NEBS types of environments, heat pipes should be tested prior to use and after unsatisfactory examination of other cooling methods.

References:
1. Faghri, A. Heat Pipe Science and Technology Taylor & Francis, 1995.
2. Thermacore Internation, Inc., www.thermacore.com.
3. Xiong, D., Azar, K., Tavossoli, B., Experimental Study on a Hybrid Liquid/Air Cooling System, IEEE, Semiconductor Thermal Measurement and Management Symposium 2006.

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