Tag Archives: qpedia thermal emagazine

What are the benefits of using Pin Fin Heat Sinks in thermal management of electronics

Engineers tasked with designing modern electronics face a number of issues. Expectations are for more functionality, more power, and more components in ever-smaller packages but also with quick turnaround for production and staying within tight budget parameters.

Thermal management is a critical aspect of the design process and, as demand for component-density and miniaturization continues to increase, engineers need cooling solutions that fit into small spaces, will not cause project cost overruns, and will provide the best heat transfer possible for today’s modern,  processors.

Heat sinks and convection cooling remain the go-to solutions for most systems and high-efficiency Pin Fin heat sinks are designed to meet the requirements of modern electronics cooling with little extra cost added. In particular, the pin fin heat sink geometry is designed to provide increased surface area for heat transfer, low thermal resistance from base to fins at high airflow (200-plus LFM), and work in environments where the direction of airflow is ambiguous.

Pin Fin Heat Sinks

Pin fin heat sinks from Advanced Thermal Solutions, Inc. (ATS). Pin fin heat sinks provide low thermal resistance at high LFM. (Advanced Thermal Solutions, Inc.)

How does the Pin Fin geometry work?

Barry Dagan, an engineer at Cool Innovations, Inc., wrote a piece for New Electronics in 2009 that explained how the pin fin structure uses the ambient airflow to enhance its thermal performance. [1]

“Any heat sink removes heat by ‘breaking’ the boundary layers of still air that are wrapped around its surface because still air is a very good thermal insulator,” Dagan explained. “The boundary layers are broken by accelerating the flow of air into the heat sink – either using fans and forced airflow or via the chimney effect. In either case, the faster the airstream, the more likely the boundary layers are to break and the more effective the heat sink will be.”

He added, “The round, aerodynamic pin design reduces resistance to surrounding airstreams that enter the pin array, while simultaneously increasing air turbulence. The omnidirectional pin configuration, which allows air to enter and exit the heat sink in any direction, exposes the heat sink to the fastest possible air speed.”

In an earlier article for EE Times, Dagan also noted that the pin fin geometry “allows for a high degree of customization.” Engineers can make adjustments to the overall height, pin height, base thickness, footprint, pin diameter, and pin density to find an optimal cooling solution for their particular project. [2]

“Pin fins can also be catered for situations where both footprint and height are restricted,” Dagan wrote. “For example, the pin fin technology enables the design of heat sinks with a footprint of half an inch squared and a total height as low as 0.15 in.”

A study conducted by Younghwan Joo and Sung Jin Kim that was published in the International Journal of Heat and Mass Transfer indicated that the heat dissipation per mass of optimized pin fin heat sinks was greater than optimized plate-fin heat sinks in most applications. [3]

Pin Fin Heat Sinks

Pin Fin heat sinks on a PCB. (Advanced Thermal Solutions, Inc.)

In a comparison of heat sinks conducted at Advanced Thermal Solutions, Inc. (ATS) and published in Qpedia Thermal eMagazine, a 33-mm tall elliptical pin fin heat sink under forced convection had the lowest thermal resistance of the 10 heat sinks that were tested. [4]

The ATS family of pin fin heat sinks, made from extruded aluminum, range in sizes from 10 mm by 10 mm to 60 mm by 60 mm. Heights range from 2-25 mm. Through testing in ATS wind tunnels, the pin fin heat sinks demonstrated thermal resistance as low as 2.5°C/W and added little weight to the board. [5]

How are pin fin heat sinks attached to a board?

Pin fin heat sinks are versatile and can be attached to a variety of component packages, including BGA, QFP, LCC, LGA, CLCC, TSOP, DIP, LQFP, and many others. Because pin fin heat sinks are lightweight, standard thermal tape or epoxy can be used to securely attach them to components.

In addition, pin fin heat sinks work with mechanical attachments such as z-clips and ATS maxiGRIPTM or superGRIPTM, which are two-component attachment systems that provide secure hold without damaging the PCB and only minimal addition to the component footprint.

Pin Fin Heat Sinks

Pin fin heat sinks attached to a PCB with ATS maxiGRIP heat sink attachment system. (Advanced Thermal Solutions, Inc.)

How do pin fin heat sinks provide cost savings?

In his article for EE Times, Dagan explained, “Pin fin technology provides cost-effective heat sink solutions for medium and high-volume applications due to low associated tooling charges and minimal waste of raw materials.” [2]

For example, the ATS family of standard and custom pin fin heat sinks are all available for less than $2.00, with the vast majority of heat sinks available for less than a dollar. This means that engineers can find high-efficiency heat sinks and save money in the budget, which can be put to other design considerations, such as higher-powered fans to increase airflow, better heat sinks attachments, or additional chips and other board components. [5]

This is particularly beneficial for the growing maker market, which is working on new technology or enhancing current technology but generally with far smaller budgets than traditional OEM.

A 2012 article from The Economist, entitled “A Third Industrial Revolution,” discussed the impact of additive manufacturing techniques and how it was now possible to make parts through processes like 3-D printing that are cheaper and faster than traditional methods. According to the article, this will not just affect large manufacturers but also trickles down to a community of makers and smaller companies, what the article labeled “social manufacturing.” [6]

The article added, “As manufacturing goes digital, a third great change is now gathering pace. It will allow things to be made economically in much smaller numbers, more flexibly and with a much lower input of labour, thanks to new materials, completely new processes such as 3D printing, easy-to-use robots and new collaborative manufacturing services available online. The wheel is almost coming full circle, turning away from mass manufacturing and towards much more individualised production.”

Pin Fin Heat Sinks

Pin fin heat sinks provide cost-effective cooling solutions for small manufacturers and the maker market. (Advanced Thermal Solutions, Inc.)

A study, also from 2012, from MAKE magazine and Intel surveyed the maker community to get data about the (self-proclaimed) hobbyists, builders, tinkerers, and engineers. Out of the total respondents, 79 percent said that they worked in hardware and software, with electronics in second place at more than 60 percent. Thirty-four percent of respondents said that they were involved in making products for income and 19 percent of the total said that they paid for projects with outside funding. [7]

Crowdfunding can only take a project so far and for makers trying to earn money from designs, it is crucial to find cost-effective solutions both to ensure a project comes in under budget and to maximize profits from the sale of the design.

Pin fin heat sinks can be added at low-cost and provide the necessary thermal performance to push a design process along. For the maker market and its (at times) limited resources, high-efficiency pin fin heat sinks provide thermal performance on a budget with the versatility to fit into a variety of systems and designs.

References
[1] http://www.newelectronics.co.uk/electronics-technology/pin-fin-heat-sinks-point-the-way-to-more-efficient-cooling/18641/
[2] http://www.eetimes.com/document.asp?doc_id=1204099
[3] Younghwan Joo and Sung Jin Kim, “Comparison of thermal performance between plate-fin and pin-fin heat sinks in natural convection,” International Journal of Heat and Mass Transfer, April 2015, 345-356.
[4] https://www.qats.com/cms/wp-content/uploads/2014/03/HowAirVelocityAffects_Qpedia08.pdf
[5] https://www.qats.com/News-Room/Press-Releases-Content/1184.aspx
[6] http://www.economist.com/node/21552901
[7] http://www.nyu.edu/social-entrepreneurship/speaker_series/pdf/Maker%20Market%20Study%20FINAL.pdf

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.

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.

Industry Developments: Heat Exchangers for Electronics Cooling

By Norman Quesnel, Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc.

(This article will be featured in an upcoming 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. To read other stories from Norman Quesnel, visit https://www.qats.com/cms/?s=norman+quesnel.)

Heat exchangers are thermal management tools that are widely used across a variety of industries. Their basic function is to remove heat from designated locations by transferring it into a fluid. Inside the heat exchanger, the heat from this fluid passes to a second fluid without the fluids mixing or coming into direct contact. The original fluid, now cooled, returns to the assigned area to begin the heat transfer process again.

The fluids referred to above can be gases (e.g. air), or liquids (e.g. water or dielectric fluids), and they don’t have to be symmetrical. Therefore, heat exchangers can be air-to-air, liquid-to-air, or liquid-to-liquid. Typically, fans and/or pumps are used to keep these heat transfer medium in motion and heat pipes may be added to increase heat transfer capabilities.

Figure 1 shows a basic heat exchanger schematic. A hot fluid (red) flows through a container filled with a cold fluid (blue) but the two fluids are not in direct contact.

Heat Exchangers

Figure 1. In a Simple Heat Exchanger Heat Transfers from the Hot (Red) Fluid to the Cold (Blue) Fluid, and the Cooler After Fluid Re-Circulates to Retrieve More Heat. [1]

One example of a common heat exchanger is the internal combustion engine under the hood of most cars. A fluid (in this case, liquid coolant) circulates through radiator coils while another fluid (air) flows past these coils. The air flow lowers the liquid coolant’s temperature and heats the incoming air.

Applied to electronics enclosures, heat exchangers draw heated air from a cabinet, cool it, and then return the cooled air to the cabinet. These heat exchangers should be designed to provide adequate cooling for expected worst case conditions. Typically, those conditions occur when the ambient is the highest and when electrical loads through the enclosure are very high. Under typical conditions, heat exchangers can cool cabinet interiors to within 5°F above the ambient air temperature outside the enclosure.

Air-to-Air

Air-to-air heat exchangers have no loops, liquids or pumps. Their heat dissipation capabilities are moderate. Common applications are in indoor or outdoor telecommunications cabinetry or in manufacturing facilities that don’t have a lot of dust or debris.

Air-to-air heat exchangers provide moderate to good cooling performance. They don’t allow outside air to enter or mix with the air inside the enclosure. This protects the enclosure’s contents from possible contamination by dirt or dust, which could damage sensitive electronics and electrical devices and cause malfunctions.

An example of higher performance, air-to-air heat exchangers is the Aavid Thermacore HX series. These heat exchangers feature rows of heat pipes that add effective, two-phase heat absorbing properties when moving hot air away from a cooling area. The liquid inside the heat pipes turns to vapor. This transition occurs inside a hot cabinet. (See Figure 2)

The vapor travels to the other end of the heat pipe, which is outside the cabinet. Here it is cooled by a fan, transitions back to liquid form, and cycles back inside the cabinet environment.

Heat Exchangers

Figure 2. An Air-to-Air Heat Exchanger with Heat Pipes Extending Inside (top) and Outside (bottom) a Cabinet. Internal Heat is Transferred Outside the Enclosure. (Aavid Themacore) [1]

Other air-to-air heat exchangers feature impingement cooling functionality that can provide better performance than using heat pipes. Aavid Thermacore’s HXi Impingement core technology uses a folded fin core that separates the enclosure inside and outside. A set of inside fans draws in the hotter, inside air and blows it toward the fin core. This inside impingement efficiently transfers the heat to the fin core. Similarly, a set of outside fans draws in the cooler, ambient air and blows it toward the outer side of the fin core removing the waste heat. See Figure 3 below.

Heat Exchangers

Figure 3. Air-to-Air Heat Exchangers with Double-Sided Impingement Cooling Technology Can Move Twice the Heat Load of Conventional Exchangers. (Aavid Themacore) [3]

Liquid-to-Air

In some electronic cabinets, high power components can’t be cooled by circulating air alone or the external ambient air temperature is not cool enough to allow an air-to-air heat exchanger to solve the problem unaided. In these applications, liquid-to-air heat exchangers provide additional cooling to maintain proper cabinet temperatures.

For example, in a situation where heat is collected through a liquid-cooled cold plate attached directly to high power components. Even with the cold plate, the ambient air external to the cabinet is not cool enough to maintain the internal cabinet temperature at an acceptable or required level. Here, a liquid coolant in an active liquid-to-air heat exchanger can be used to cool the enclosure.

Heat Exchangers

Figure 4. Tube-to-Fin, Liquid-to-Air Heat Exchangers Provide High-Performance Thermal Transfer. [4] (Advanced Thermal Solutions, Inc.)

Advanced Thermal Solutions, Inc. (ATS) tube-to-fin, liquid-to-air heat exchangers have the industry’s highest density of fins. This maximizes heat transfer from liquid to air, allowing the liquid to be cooled to lower temperatures than other exchangers can achieve. All tubes and fins are made of copper and stainless steel to accommodate a wide choice of fluids.

Available with or without fans, ATS heat exchangers are available in a range of sizes and heat transfer capacities up to 250W per 1°C difference between inlet liquid and inlet air temperatures. They can be used in a wide variety of automotive, industrial, HVAC, electronics and medical applications. [4]

Heat Exchanger

Figure 5. Small, Light-Weight Liquid-to-Liquid Heat Exchanger Provides Efficient Cooling Performance. [5]

Lytron’s liquid-to-liquid heat exchangers are only 10-20% the size and weight of conventional shell-and-tube designs. Their internal counter-flow design features stainless steel sheets stamped with a herringbone pattern of grooves, stacked in alternating directions to form separate flow channels for the two liquid streams. This efficient design allows 90% of the material to be used for heat transfer. Copper-brazed and nickel-brazed versions provide compatibility with a wide range of fluids. [5]

Nanofluids

The development of nanomaterials has made it possible to structure a new type of heat transfer fluid formed by suspending nanoparticles (particles with a diameter lower than 100nm). A mixture of nanoparticles suspended in a base liquid is called a nanofluid. The choice of base fluid depends on the heat transfer properties required of the nanofluid. Water is widely used as the base fluid. Experimental data indicates that particle size, volume fraction and properties of the nanoparticles influence the heat transfer characteristics of nanofluids. [5]

When compared to conventional liquids, nanofluids have many advantages such as higher thermal conductivity, better flow, and the pressure drop induced is very small. They can also prevent sedimentation and provide higher surface area. From various research, it has been found that adding even very small amounts of nanoparticles to the base fluid can significantly enhance thermal conductivity.

Heat Exchangers

Figure 6. 3D Design of Curved Tube Heat Exchanger. Increased Turbulence and Velocity Increases Heat Transfer Rate. [6]

A recent paper by Fredric et al. proposes a theoretical heat exchanger with curved tubes and with nanofluids as the coolant. Nanofluids in place of regular water provide improved thermal conductivity due to the increased surface area. The heat transfer rate is further improved using curved tubes in place of straight tubes because the used of curved tubes increases the turbulence and fluid velocity, which helps increase the heat transfer rate. [6]

References
1. Advanced Thermal Solutions, Inc., https://www.qats.com/Products/Liquid-Cooling/Heat-Exchangers.
2. Aavid Thermacore, http://www.thermacore.com/documents/system-level-cooling-products.pdf.
3. Aavid Thermacore, http://www.thermacore.com/products/air-to-air-heat-exchangers.aspx.
4. Advanced Thermal Solutions, https://www.qats.com/Products/Liquid-Cooling/Heat-Exchangers.
5. Kannan, S., Vekatamuni, T. and Vijayasarathi, P., “Enhancement of Heat Transfer Rate in Heat Exchanger Using Nanofluids,” Intl Journal of Research, September 2014.
6. Fredric, F., Afzal, M. and Sikkandar, M., “A Review on Shell & Tube Heat Exchanger Using Nanofluids for Enhancement of Thermal Conductivity,” Intl. Journal of Innovative Research in Science, Engineering and Technology, March 2017.

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

New Qpedia Thermal eMagazine Now Available for Download

Qpedia Thermal eMagazine, Volume 7, Issue 10, has just been released and can be downloaded at: http://www.qats.com/Qpedia-Thermal-eMagazine/Back-Issues

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Featured articles this month include:

The Impact of Orientation on a Heat Pipe’s Performance with Different Wick Structures

This article presents a review of experimental results obtained with heat pipes, in the shape of round cross sections, in different orientations. The results of the experimental study are very useful, showing how thermal performance can be dependent upon factors such as inclination angle, heat input and operating temperature. The findings are then compared with data published in the literature.

The Performance of a Heat Sink with Piezoelectric Fins

Some engineers have introduced piezoelectric fans and synthetic air jets into LED light cooling to enhance the convection heat transfer. Other researchers have proposed the use of heat sinks with piezoelectric fins. This paper discusses this novel concept of cooling LED heat sinks. This article shares experimental studies on the performance of piezoelectric fins with different fin heights, widths and pitches, and under different vibrating modes.

Industry Developments: Diamond-Based Thermal Management Systems

Adopting CVD diamond as a heat spreader for the semiconductor industry is still in its early stages. Demand for superior thermal management solutions for opto-electronics, power and RF devices will drive a broader adoption of this engineering material. When combined with the growth rates of these markets, the increased use of diamond should drive significant investments in synthetic diamond manufacturing capacity. These should in turn promote economies of scale which will enable diamond to operate in segments of the semiconductor market it previously could not compete in.

Technology Review: Techniques for Measuring the Thermal Performance of Heat Pipes, 2006 to 2012

In this issue our spotlight is on techniques for measuring the thermal performance of heat pipes. There is much discussion about this subject in the electronics industry, and these patents show some of the salient features that are the focus of different inventors.

Cooling News – Product News

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