Category Archives: Qpedia Thermal eMagazine

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 for Cooling Overclocked CPUs

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.)

Almost as long as personal computers have been around, users have been making modifications “under the hood” to make them run faster. A large segment of these users are overclockers, who make adjustments to increase the clock speeds (the speed at which processors execute instructions) of their CPUs and GPUs.

Many PC gamers get into overclocking (OC) to make their programs run faster. Gamecrate.com, a gamer site, defines overclocking as the practice of forcing a specific piece of hardware to operate at a speed above and beyond the default manufactured rating. [1]

To overclock a CPU is to set its clock multiplier higher so that the processor speeds up. For example, overclocking an Intel Core i7 CPU means to push its rated clock speed higher than the 2.80 GHz that it runs at “out of the box.” When performed correctly, overclocking can safely boost a CPU’s performance by 20 percent or more. This will let other processes on a computer run faster, too.

Cooling Overclocked CPUs

Fig. 1. An Intel Core i5-469k Processor Can Be Overclocked to Run 0.5-0.9 GHz over Its Base Frequency. Air Cooling is Provided by a Hyper D92 from Cooler Master.[2]

To serve the global overclockers market, some chip makers keep the door open to overclocking by allowing access to their multipliers. They do this with a variety of “unlocked” processors. Intel provides many unlocked versions of their processors, as denoted with a ‘k’ at the end of their model number.

For example, the Skylake Core i7-6700k and Haswell-E Core i7-5820k are made with unlocked clock multipliers. In fact, Intel targets overclockers with marketing campaigns and support services.

Fig. 2. Intel Actively Targets Overclockers with Its Unlocked Processors.[3]

Besides gaming, overclocking can improve performance for applications such as 3-D imaging or high-end video editing. For GPUs, faster speeds will achieve higher frames per second for a smoother, faster video experience. Overclocking can even save money, if a lower cost processor can be overclocked to perform like a higher end CPU.[4]

For many gamers, overclocking enhances their enjoyment by giving more control over their system and breaking the rules set by CPU manufacturers. One overclocker on Gamecrate.com said, “Primarily, I like to do it because it’s fun. On a more practical note it’s a great way to breathe some life into an old build, or to take a new build and supercharge it to the next level.”[1]

Heat Issues from Overclocking

Overclocking a processor typically means increasing voltage as well. Thus, the performance boost from overclocking usually comes with added component heat that needs to be controlled. Basically, the more voltage added to components, the more heat they are going to produce. There are many tutorials on overclocking and most of these resources stress that it’s essential to manage a component’s increased heat.[5]

Programs are available that monitor the temperature of a processor before and after overclocking it. These programs work with the DTS, digital thermal sensors that most processor manufacturers include inside their component packages. One such program is Core Temp, which can be used with both Intel and AMD cores. Some component OEMs also offer their own software to monitor temperatures in their processors.[6]

Fig. 3. The Core Temp Program Can Display Temperatures of Individual Cores in a System.[6]

Typically, an overclocker will benchmark a CPU or other component to measure how hot it runs at 100 percent. Advanced users can manually do the overclocking by changing the CPU ratio, or multiplier, for all cores to the target number. The multiplier works with the core’s BCLK frequency (usually 100) to create the final GHz number.

Tools like the freeware program Prime95 provide stability testing features, like the “Torture Test,” to see how the sped up chip performs at a higher load. These programs work with the system’s BIOS and typically use the motherboard to automatically test a range of overclocked profiles, e.g. from 4.0-4.8 GHz. From here, an overclocker may test increasing voltages, e.g. incrementally adding 0.01 – 0.1 V while monitoring chip stability.

An overclocked component’s final test is whether it remains stable over time. This ongoing stability will mainly be influenced by its excess heat. For many overclocked processors, a robust fan-cooled heat sink in place of the stock fan is essential. For others, only liquid cooling will resolve excess heat issues.

Fan Cooling

The advantage of using air coolers is no worry about leaking, which may lead to component or system damage. With the air cooled heat sinks, the bigger and faster the fan (CFM), the better, and there are a multitude of fan-sink cooling solutions that gaming PCs can accommodate.

In reality, higher performance fan-cooled sinks typically also employ liquid. It is used inside heat pipes that more efficiently convey heat from the processor into the sink’s fan cooled fin field.

Fig. 4. The Top-Rated Hyper 212 EVO CPU Air Cooler from Cooler Master Has Four Heat Pipes Transferring Heat to Aluminum Fins.[7]

Air cooled heat sinks for overclockers cost well under $50 and are available from many sources. They’re often bundled with overclock-ready processors at discounted prices.

A greater issue with air cooling can be the fan noise. A high performance fan must spin very quickly to deal with heavy system workloads. This can create an unpleasant mixture of whirs, purrs and growls. Many of the gaming desktops generate 50-80 decibels of noise at load. Though most fans are quieter, pushing out 25-80 CFM, they are louder than most standard PC processor fans.[8]

Liquid Cooling

Liquid cooling has become more common because of its enhanced thermal performance, which allows higher levels of overclocking. Prices are definitely higher than air-cooled heat sinks, but liquid systems offer enthusiasts a more intricate, quieter, and elegant thermal solution with definite eye-appeal.

From the performance standpoint, liquids (mainly water in these systems) provide better thermal conductivity than air. They can move more thermal energy from a heat source on a volume-to-volume basis.

Fig. 5. The Top-Rated Nepton 280 Liquid CPU Cooler Has a Fast Pump Flow and a Large Radiator Cooled with Dual Fans that Reach 122 CFM Airflow.[9]

A typical liquid cooling system features a water block that fits over the overclocked CPU, a large surface area, a fan-cooled heat exchanger (radiator), a pump, and a series of tubes connecting all elements. One tube carries hot fluid out from the water block, the other returns it once it is cooled by the radiator. Some liquid cooling systems can also be used on multiple processors, e.g. a CPU and a gaming chipset.

While there are more components to a liquid cooling system, there are also major advantages. For one, the water block is usually much smaller and lower-profile than an attached, high-performance air cooler. Also, the tubing set up allows the heat exchanger and pump to be installed in different locations, including outside the PC enclosure. An example is the Sub-Zero Liquid Chilled System from Digital Storm. It unlocks overclocks of Intel’s i7-980X CPU up to 4.6 GHz while idling the processor below 0°C.[10]

Fig. 6. Digital Storm’s Cryo-TEC System Places an Overclocked CPU in Direct Contact with Thermo-electric Cold Plates Dropping Core Temperatures to Below 0°C.[11]

Prices for liquid cooling systems can easily surpass $200, though newer systems can be bought for under $100.

A fan still must be attached to the radiator to help cool it, but it doesn’t have to spin as quickly as it would if it were attached to a heat sink. As a result, most liquid-cooled systems emit no more than 30 decibels.

Conclusion

Overclocking can be considered a subset of modding. This is a casual expression for modifying hardware, software or anything else to get a device to perform beyond its original intention. If you own an unlocked CPU you can get significant added performance, for free, by overclocking the processor. When modifying processor speeds, i.e. increasing them, high temperatures will occur. Higher performance cooling solutions are needed.

Fig. 7. YouTube Video of Overclocked CPU Melting Solder Before It Stops Working at 234°C.[12]

To serve the world of overclockers, a steady stream of air and liquid cooling systems are being developed. Many of them are high precision, effective, stylish and surprisingly affordable. Often they share the same technology as mass market quantity, lower performing cooling systems (basic heat sinks, heat pipes, for example), but provide much higher cooling capabilities for ever-increasing processor speeds.

References
1. Gamecrate.com, https://www.gamecrate.com/basics-overclocking/10239
2. Techreport.com, http://techreport.com/review/27543/cooler-master-hyper-d92-cpu-cooler-reviewed/3
3. Legitreviews.com, http://www.legitreviews.com/intel-devils-canyon-coming-this-month-intel-core-i7-4790k-core-i5-4690k_143234
4. Digitaltrends.com, http://www.digitaltrends.com/computing/should-you-overclock-your-pcs-processor/
5. Techradar.com, http://www.techradar.com/how-to/computing/how-to-overclock-your-cpu-1306573
6. Alcpu.com, http://www.alcpu.com/CoreTemp/
7. Coolermaster.com, http://www.coolermaster.com/cooling/cpu-air-cooler/hyper-212-evo/
8. Digitaltrends.com, http://www.digitaltrends.com/computing/heres-why-you-should-liquid-cool-your-cpu/
9. Coolermaster.com, http://www.coolermaster.com/cooling/cpu-liquid-cooler/nepton-280l/
10. Gizmodo.com, http://gizmodo.com/5696553/digital-storms-new-gaming-pcs-use-sub-zero-liquid-cooling-system-for-insane-overclocks
11. Digitalstorm.com, http://www.digitalstorm.com/cryo-tec.asp
12. Youtube.com, https://www.youtube.com/watch?v=9NEn9DHmjk0

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

100th Issue of Qpedia Published!

Qpedia100_cover

Qpedia Thermal eMagazine has just published its 100th Issue. Featured articles in this issue include:

Heat Sink Base Spreading Resistance Optimization for Achieving Better Thermal Performance

The goal of any electronic cooling solution is to lower the component junction temperature and thus maximize heat sink performance by reducing the spread resistance and fin resistance. This article will discuss an analytical model for how to select a heat sink so that maximum thermal performance can be achieved.

Rethinking Thermocouples: Creating Micro-Scale High Precision Sensors

Although thermocouples are suitable for standard thermal measurements, there can be a margin of error of 1°C. Sensors however, have an extremely fast response time to temperature changes allowing a measurement accuracy of 0.1°C. This article shows how using sensors instead of thermocouples is the optimal choice in certain thermal management situations.

Technical Note: Effect of Vacuum and Fill Ratio on the Performance of Heat Pipes

In this new section, Qpedia reviews fundamental thermal engineering principles, calculations and equations needed for the successful cooling of electronics. In this issue we discuss a heat pipe’s performance as a function of a liquid fill ratio and vacuum.

Industry Developments: Thermal Imaging Cameras

Though invisible to the eye, thermal radiation can be detected by thermal imaging cameras, also called thermographic or infrared cameras. In the engineering industry, these cameras allow one to view, pinpoint and analyze differing thermal patterns, including heat transfer and location of hot spots on a PCB, chip or any electronic device. This article reviews the latest developments and types of thermal imaging products available on the market.

Technology Review: Liquid Cooling Devices

Qpedia reviews innovative technologies developed for electronics cooling applications. This section presents selected patents that were awarded to developers around the world to address cooling challenges.In this issue our spotlight is on liquid cooling devices.

Cooling News

The latest technology, products, and news from around the electronics cooling industry.

Also included in this special issue is an editorial from Qpedia’s founder Dr. Kaveh Azar. Download the issue.

Qpedia Issue 98 Just Released

Qpedia, Issue 98, has just been released! Download the issue.

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Featured articles in this issue include:

COOLING IGBTS USING VAPORIZABLE DIELECTRIC FLUIDS

Dramatic increases in both power density and total power dissipation for power electronic systems, especially vehicle and airborne systems, together with demands for increased performance and speed of response, limit the application of a traditional forced convection air-cooled or a single phase liquid system. This article discusses how the newly developed Vaporizable Dielectric Fluid (VDF) cooling system, uses common dielectric refrigerants to effectively cool IGBT devices.

OPTIMAL HEAT SINK DESIGN FOR NATURAL CONVECTION IN VERTICAL ORIENTATION

Many tough-to-cool electronics applications don’t allow the use of fans, therefore heat sink solutions need to be optimized through natural convection. This article explores and compares the optimal design of traditional pin, straight fin and nontraditional heat sink designs in natural convection.

FUNDAMENTALS: HEAT PIPE HEAT TRANSPORT CAPABILITY

In this new section, Qpedia reviews fundamental thermal engineering principles, calculations and equations needed for the successful cooling of electronics. In this issue we discuss the heat transport capabilities and limitations of heat pipes.

INDUSTRY DEVELOPMENTS: MILITARY ELECTRONICS COOLING

Modern militaries around the world are equipped with very hot-running electronics that must reliably provide high levels of performance in critical situations. In addition, thermal management for military electronics can be extremely challenging, due to the strict size, weight and power (SWaP) requirements, rugged use, harsh environments, and cost restraints. This article presents recent cooling developments for military electronics.

TECHNOLOGY REVIEW: LED THERMAL MANAGEMENT

Qpedia reviews innovative technologies developed for electronics cooling applications. This section presents selected patents that were awarded to developers around the world to address cooling challenges. In this issue our spotlight is on thermal management systems for LED lighting.

COOLING NEWS

The latest products & technology from around the thermal management industry.

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Qpedia Issue 96 Just Published

Qpedia, Issue 96 has just been released and can be downloaded at: http://www.qats.com/Qpedia-Thermal-eMagazine/Back-Issues

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Featured articles include:

Using Integrated Planar Thermosyphon PCBs to Enhance Cooling of High Brightness LEDs

Because high brightness LEDs have a heat flux that reaches an excess of 80 W/cm2, thermal management is critical to ensure the LED properly performs. This article discusses the promising idea of improving the heat dissipation from the LED package to the PCB using thermosyphons. While the concept of a planar thermosyphon to enhance heat transfer by itself is not new, integrating it into a printed circuit board is innovative and appears to be a promising technology.

Nanofluids in Heat Pipes

The thermophysical properties of the working fluid have a direct impact on a heat pipe’s performance. Water is the most common and effective heat transfer fluid in the electronics cooling industry, but its properties can be enhanced by adding nano-particles. This article discusses what nanofluids are available and their benefits on a heat pipe’s performance.

Fundamentals: Effective Thermal Conductivity of a Heat Pipe

In this new section, Qpedia reviews fundamental thermal engineering principles, calculations and equations needed for the successful cooling of electronics. This issue we discuss how to identify the thermal conductivity of a heat pipe.

Industry Developments: Cooling COB LEDs

Chip-on-board (COB) LED’s are one of the most widely used types of LED packaging. However, the tight spacing between the LEDs and small light emitting surface that make COB LEDs attractive, also present high heat fluxes that must be addressed. Because COBs have such a small surface area, addressing power density is the primary challenge. This article reviews the recent methods that LED manufacturers are using to cool COB LEDs.

Technology Review: Phase Change Materials

Qpedia reviews innovative technologies developed for electronics cooling applications. This section presents selected patents that were awarded to developers around the world to address cooling challenges. In this issue our spotlight is on phase change materials.

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