Aug 4, 2012 - Chief Technology Officer,. Advanced ... Qpedia was launched in 2007 as a technology ..... designer to be less concerned of CTE stresses.
August 2012 | Volume VI | Issue VIII
pedia IN THIS ISSUE The Thermal Resistance of Microchannel Cold Plates Application of Thermoelectric Coolers in Thermal Management Thermal Gap Fillers Technology Review: Cold Plates, 2001 to 2003 Cooling News 4
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Advanced Thermal Solutions is a leading engineering and manufacturing company supplying complete thermal and mechanical packaging solutions from analysis and testing to final production. ATS provides a wide range of air and liquid cooling solutions, laboratory-quality thermal instrumentation, along with thermal design consulting services and training. Each article within Qpedia is meticulously researched and written by ATS’ engineering staff and contributing partners. For more information about Advanced Thermal Solutions, Inc., please visit www.qats.com or call 781-769-2800.
EDITOR
KAVEH AZAR, Ph.D. President & CEO, Advanced Thermal Solutions, Inc. MANAGING EDITOR
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BAHMAN TAVASSOLI, Ph.D.
eMagazine focused on the thermal management of
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engineering community solve the most challenging
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electronics. It is designed as a resource to help the thermal problems. The eMagazine is published monthly and distributed at no charge to over 17,000 engineers worldwide. Qpedia is also available online or for download at www.qats.com/qpedia. Qpedia’s editorial team includes ATS’ President & CEO, Kaveh Azar, Ph.D., and Bahman Tavassoli, Ph.D., the company’s chief technologist. Both Azar and Tavassoli are internationally recognized experts in the thermal management of electronics.
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AUGUST 2012 | Volume VI | Issue VIII
Features 6
The Thermal Resistance of Microchannel Cold Plates The trend towards ever higher power dissipation rates has
pushed liquid-cooled cold plate designers to look for more effective methods and structures to transfer heat from device to liquid. This paper studies how the channel size and material affects the overall thermal resistance of cold plate. When designing a cutting edge cold plate, engineers should pay a lot of attention to pressure drop, pumping power, system complexity and reliability while pursuing high performance.
Thermal
Application of Thermoelectric Coolers in Thermal Management 11
With the increase in power dissipation and the shrinkage of package sizes, air cooling is getting to its limit or soon will reach a point at which liquid cooling becomes inevitable. In order to comply with Moor’s law, higher heat transfer mechanisms are needed to achieve the goal. In some applications with limited space or lack of refrigerated liquid, liquid cooling by itself might not even be enough. In these circumstances TEC can play a vital role. In this article we will look at some work done by researchers using TEC in the liquid cooling arena.
Cooling
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Thermally efficient pads and fillers have become vital in meeting demands to increase the performance and reduce the size of electronic assemblies, such as power supplies and control units, without compromising reliability. Today’s thermal materials are typically ceramic-filled silicone elastomers, which are easy to handle. They also conform well to the shape and surface texture of heat sinks or electronic components. And, they have high thermal conductivity relative to the still air they’re designed to eliminate.
Industry
Technology
Thermal Gap Fillers
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Cold Plates, 2001 to 2003
Qpedia continues its review of technologies developed for electronics cooling applications. We are presenting selected patents that were awarded to developers around the world to address cooling challenges. After reading the series, you will be more aware of both the historic developments and the latest breakthroughs in both product design and applications. 26
News
Cooling News
Product News, Industry News, Upcoming Thermal Management Events
JULY 2012 MARCH 2012|Qpedia |Qpedia
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The Qpedia Book SerieS Advanced Thermal Solutions, Inc. (ATS) now offers the complete editorial contents of its widely-read Qpedia Thermal eMagazine in four hardbound volumes. The books provide an expert resource for design engineers,engineering managers, and others who want to learn more about the theories and applications of electronics thermal management. The four volume set contains nearly 200 in-depth articles, researched and written by veteran engineers. They address the most critical areas of electronics cooling, with a wide spectrum of topics and thorough technical explanation.
Books Available Individually, or in Book Sets at a Discount Price.
A Must-Have in Every Engineer’s Library! 4
Order NOw
MARCH |Qpedia October2012 2009|Qpedia
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The Thermal Resistance of Microchannel Cold Plates The trend towards ever higher power dissipation rates has pushed liquid-cooled cold plate designers to look for more effective methods and structures to transfer heat from device to liquid. The heat dissipation level of a liquid-cooled cold plate is determined by the heat conduction in solids and the heat convection in fluids. Normally convection is the dominant factor for reducing the thermal resistance when highly conductive material is used to fabricate the heat sinks. In most cases, the single-phase flow inside microchannels is a laminar flow. For a fully developed laminar flow in a square channel, with constant wall temperature or constant wall heat flux, the Nusselt number is a constant. The heat transfer coefficient can be calculated by the following equation, h=
Nuk 1 =>h Dh Dh
The heat transfer coefficient is inversely proportional to the channel hydraulic diameter. Microchannels can be directly etched on silicon or ceramics or they can be machined on metal. Different materials have different properties. For example, the copper cold plates are widely used in personal computers to cool the CPUs. Silicon/ ceramic microchannels have potential applications in integrating on-chip cooling. This paper studies how the channel size and material affects the overall thermal resistance of cold plate. The cold plate studied is illustrated in 6
Figure 1. The cold plate has base size of 40 X 40 mm. The channel width is a and channel height is b. The cold base thickness is t. The water is chosen as the working fluid.
Figure 1. Cold Plate Configuration
Four typical materials are used to study the effect of thermal conductivity on cold plate performance. The properties of these four materials are listed in Table 1.
Table 1. Typical Cold Plate Material Properties
The overall thermal resistance of a microchannel cold plate is defined as
R=
Tcp - Tw_inlet q
Where Tcp is the cold plate base temperature, Tw_inlet is the water temperature at the cold plate inlet, and q is the heat flux dissipated by the cold plate. The overall thermal resistance of a microchannel cold plate can be calculated by the following equation,
Rconduction(oC/W)
R = Rspreading + Rconduction + Rconvection + Rcaloric Where, Rspreading is the heat spreading resistance between heat source and cold plate Rconduction is the conduction resistance of cold plate base Rconvection is the convection resistance between microchannel fins and water Rcaloric is the liquid caloric thermal resistance due to temperature rise of water To simplify the analysis, the heat source base is assumed to be the same size as the cold plate. So, the heat spreading resistance between the heat source and the cold plate is zero. For calculating the heat spreading resistance due to the size difference between the heat source and cold plate base, please refer to a previous Qpedia paper entitled “Spreading Resistance of Single and Multiple Heat Sources” in the September, 2010 issue [1]. The base conduction resistance is affect by material, base size and base thickness, Rconduction =
t kA
Where k is solid thermal conductivity, t is cold plate base thickness, and A is cold plate base area. For the studied cold plate, the base size is 40 X 40 mm, the conduction thermal resistance for different material and base thickness is shown in Table 2. The conduction thermal resistance is very small and is only a small portion of overall cold plate thermal resistance, even for a silicon-made cold plate.
Table 2. Cold Plate Conduction Thermal Resistance
The liquid caloric thermal resistance is inversely proportional to the fluid volumetric flow rate, Rcalotic =
1 • 2mCp
• Where m is the water mass flow rate, and Cp is the water specific heat. Table 3 shows the calculated liquid caloric thermal resistance at different flow rates. At an operating condition of 2 LPM (liter per minute), the resistance is around 0.0036 ºC/W.
o
C/W
Table 3. Liquid Caloric Thermal Resistance
The convection thermal resistance is dictated by the following equation, Rconvection =
1 ηhA
Where η is the fin efficiency, h is the heat transfer coefficient of the fin, and A is the total fin surface area. Three different fin configurations (CP1, CP2, and CP3) are studied. The channel aspect ratio is kept at a constant of 15. All three fin configurations have similar fin surface areas, but their hydraulic diameter Dh is different, which leads to a different fin heat transfer coefficient. The smaller the hydraulic diameter, the larger the heat transfer coefficient. The detailed information for the three different configurations is listed in Table 4.
AUGUST 2012|Qpedia |Qpedia MARCH 2012
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Table 4. Three Fin Configurations
For cold plates made of silicon, the calculated thermal resistance is shown in Table 5; all results are for 2 LPM flow and 0.5 mm base thickness. The fin efficiency of the silicon cold plate is relatively low (
