Heat Transfer Engineering Liquid Cooled Cold Plates for Industrial

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Liquid Cooled Cold Plates for Industrial High-Power ElectronicDevices—Thermal Design and Manufacturing ConsiderationsSatish G. Kandlikara; Clifford N. Hayner IIb

a Mechanical Engineering Department, Rochester Institute of Technology, Rochester, New York, USA b

Thermal Solutions Consultant, Rochester, New York, USA

To cite this Article Kandlikar, Satish G. and Hayner II, Clifford N.(2009) 'Liquid Cooled Cold Plates for Industrial High-Power Electronic Devices—Thermal Design and Manufacturing Considerations', Heat Transfer Engineering, 30: 12, 918— 930To link to this Article: DOI: 10.1080/01457630902837343URL: http://dx.doi.org/10.1080/01457630902837343

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Heat Transfer Engineering, 30(12):918–930, 2009Copyright C©© Taylor and Francis Group, LLCISSN: 0145-7632 print / 1521-0537 onlineDOI: 10.1080/01457630902837343

Liquid Cooled Cold Plates forIndustrial High-Power ElectronicDevices—Thermal Design andManufacturing Considerations

SATISH G. KANDLIKAR1 and CLIFFORD N. HAYNER II2

1Mechanical Engineering Department, Rochester Institute of Technology, Rochester, New York, USA2Thermal Solutions Consultant, Rochester, New York, USA

Electronics cooling research has been largely focused on high heat flux removal from computer chips in the recent years.However, the equally important field of high-power electronic devices has been experiencing a major paradigm shift fromair cooling to liquid cooling over the last decade. For example, multiple 250-W insulated-gate bipolar transistors used ina power drive for a 7000-HP motor used in pumping or in locomotive traction devices would not be sufficiently cooledwith air-cooling techniques. Another example is a “hockey puck” SCR of 63 mm diameter used to drive an electric motorthat could dissipate over 1500 W and is difficult to cool with air because of the shape of the device. Other devices includeradio-frequency generators, industrial battery chargers, printing press thermal and humidity control equipment, tractiondevices, mining devices, crude oil extraction equipment, magnetic resonance imaging, and railroad engines. This articleclassifies the cold plates into four types: formed tube cold plate, deep drilled cold plate, machined channel cold plate, andpocketed folded-fin cold plate. The article further discusses selection of cold plate type and channel configuration, and someof the relevant manufacturing issues. It is recommended that the thermal designer be involved in the early stages during theelectrical design and layout of the devices.

INTRODUCTION

Background of Cooling Technologies for High-PowerDevices

Electronic devices are at the heart of almost all major in-dustrial and military equipment. Some of these are powerdrives, insulated-gate bipolar transistor (IGBT) controllers,radio-frequency (RF) generators, magnetic resonance imag-ing (MRI) machines, traction devices for locomotives, batterychargers, UPS (uninterrupted power systems), DC-AC convert-ers, AC-DC inverters, and army tanks (using transmission fluidalready at a high temperature). The high-power, high-heat-fluxdemands on the cooling system cannot be met with air cooling,and advanced liquid cooling solutions are necessary.

The authors are grateful to Dr. William Grande of Ohmcraft, Honeoye Falls,NY, for his contribution in the electronic device characterization.

Address correspondence to Professor Satish G. Kandlikar, Mechanical Engi-neering Department, Rochester Institute of Technology, Rochester, NY 14623,USA. E-mail: [email protected]

There has been a dramatic shift in cooling high-power devicesin the industry during the past decade. Air cooling has sufficedfor many lower power electronic devices. Although it is quitedifficult to make a distinction based on total power dissipation,it seems that beyond a range of about 1500 W dissipation, thereare many physical and design constraints that may dictate a shifttoward liquid as the preferred medium.

With air cooling, a heat spreader is critical in carryingheat from the device–heat sink interface to the air-cooledsurfaces. The role of the heat spreader becomes less impor-tant at higher heat fluxes as the thermal resistance for lateralheat conduction (and the resulting temperature drop) in theheat spreader becomes unacceptably large in comparison tothe available temperature difference between the base (designcondition) and the inlet coolant temperature. For example, thetemperature drop across a 1 mm thick copper plate is approx-imately 0.25◦C at a heat flux of 10 W/cm2, while it increasesto 2.5◦C at 100 W/cm2 and 12.5◦C at 500 W/cm2. Althoughheat spreaders are quite effective at lower heat fluxes, their ef-fectiveness reduces considerably at higher heat fluxes. This is

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one of the main reasons why the air cooling option becomesunattractive, and eventually not viable for very high heat fluxconditions.

The need for liquid cooling has resulted in a major paradigmshift for many electrical system designers and manufacturers.Introducing a liquid, which is often not a dielectric, in the vicin-ity of electronic components was unthinkable until not too longago. Often electrical or electronic engineers would design anelectrically efficient layout, often without allowing an increasein cabinet size, larger fan, or enough power for a larger fan. Thisbrought the realization that liquid cooling offers a better, andoften the only feasible, solution. The compactness of the coldplates and the supply and return lines, lower power consump-tion, and reduction in noise levels are some of the attractivefeatures.

The development of multichip module (MCM) conductioncooling at IBM by Oktay and Kammerer [1] and Chu et al. [2]provided major impetus to liquid cooling. Kishimoto and Ohsaki[3] presented a liquid-cooled cold plate design for VLSI chips,which employed twenty-nine 800 µm × 400 µm rectangularminichannels on an 85 mm × 105 mm alumina substrate pack-age. Each is designed to accommodate sixteen 25 W chips witha total heat load of 400 W and a volumetric heat dissipation rateof 10 kW/L, comparable to the immersion cooling techniques.Further, the packages were assembled in a stack, allowing acompact MCM cooling design employing liquid cooling withsignificantly higher cooling rates than with other liquid coolingdesigns. These authors also presented an analytical scheme toprovide uniform flow distribution in the channels. These con-cepts form the basis of today’s cold plate designs for coolinghigh-power devices.

Currently the cold plates are used extensively to provide cool-ing platforms for electronic devices. Different cooling strategieswith single-phase liquid, air, jet impingement, pool boiling, andflow boiling have been employed. An excellent survey of thesetechniques has been recently published by Anandan and Rama-lingam [4]. Sparrow et al. [5] looked into the thermal designof the coolant passages and some enhancement techniques forperiodic heat sources. Some of the earlier reviews by Satheet al. [6], Incropera [7], Yeh and Chu [8], and Bar-Cohen andKraus [9] provide basic design equations and show the pro-gression in literature toward using liquid cooling for high heatflux removal. Kandlikar [10] pointed out the effectiveness ofsmall-diameter passages in meeting the high-heat-flux removalchallenges. Zuo et al. [11] provide an overview of the cool-ing technologies at seven different levels from the chip to thecooling system. Schmidt [12] pointed out the need of movingtoward minichannel heat exchanger embedded heat sinks forcooling high-flux devices.

Chu [13] presented an in-depth review of the electronicscooling challenges faced with increasing power density. The re-liability and redundancy requirements will be critical in design-ing the liquid cooled systems for reliable operation. Williamsand Roux [14] investigated the effect of different channelinserts, including various copper fins and graphite foam, on

the thermal performance of the heat sink. The authors em-ployed computational fluid dynamics (CFD) analysis and pre-sented a rank order of various techniques. Their results showthat using smaller channel sizes yields better thermal perfor-mance. Such optimization is necessary along with a pressuredrop and overall system level analysis in deciding the channelconfiguration.

The available literature on the cold plate design is primar-ily focused on relatively low power dissipation. Although thereare a few publications that deal with CFD modeling and somematerial developments available in literature (IGBT: Romeroand Martinez [15], Romero et al. [16], Rodriguez and Fusaro[17], Lee, [18], Lasers-Liu et al. [19]; cooling and packag-ing of high-power diode lasers: Loosen [20]; cooling of irra-diated targets by conductive cooling in nuclear applications:Talbert et al. [21]), a comprehensive coverage is lacking. Al-though some of the recent publications deal with advancedcooling systems using three-dimensional (3-D) cooling of de-vices [22, 23], porous plate for high-flux cooling [24], micro-capillary pumped loop systems [25], microjets [26], diamondsubstrate windows [27], liquid metal cooling [28], double-side cooling of high-power IGBTs [29], and fin inserts andother techniques in cooling hockey-puck type semiconductordevices [30], a majority of high-power devices utilize liquid-cooled cold plates. Some recent papers focus on cooling con-figurations and local heat transfer analysis for high-power de-vices such as diode-pumped lasers [31] and solid-state lasers[32].

Iyenger and Bar-Cohen [33] present an excellent outline onthis topic for air-cooled heat exchangers used in electroniccooling considering the manufacturing issues. They consid-ered a number of fin designs and their manufacturing tech-nologies, and performed optimization to identify the maxi-mum heat transfer capability within a given design domain.They also presented useful information on different typesof fins and their manufacturing techniques for air coolingapplications.

Although some novel cooling systems are being introducedto handle the very high heat flux systems, liquid cooling is by farthe most commonly employed system for cold plates. Amongthe advanced systems being investigated are spray cooling, jetimpingement cooling, and advanced single- and two-phase mi-crochannel cooling [34–41].

This article primarily deals with cold plates for high-powerdevices, covering some of the challenges faced by liquid cool-ing, thermal management solutions, and manufacturing aspects,which are often ignored in the literature but are of great im-portance in practical implementation of these thermal designsolutions.

Thermal Characteristics of Electronic Devices

In this section, a brief introduction to the thermal charac-teristics and related issues of some of the electronic devices is

heat transfer engineering vol. 30 no. 12 2009


920 S. G. KANDLIKAR AND C. N. HAYNER II

presented [20, 42]. The discussion is to help in understandingsome of the major electronic performance issues as they relateto the operation of the cooling systems, and is by no means acomprehensive summary.

The IGBT and the silicon-controlled rectifier (SCR) are bothminority carrier devices whose ultimate high-temperature limitof operation is governed by intrinsic carrier generation. In otherwords, at any given temperature there is a steady creation ofthermally activated electrons and holes. When the density ofthese thermally generated carriers becomes comparable to theengineered doping levels, the desired characteristics of the semi-conductor layers break down. The limit of this phenomenon isthe intrinsic temperature, Ti, at which the intrinsic carrier con-centration equals the doping level of the most lightly dopedlayer. At this point, rectification of the p−n junction ceases andthe device cannot function. In many silicon devices the intrinsictemperature is about 280◦C. However, since all of the funda-mental physical parameters of semiconductors are functions oftemperature to a greater or lesser degree, the upper tempera-ture range of practical devices must be limited to well belowthe intrinsic temperature to guarantee specified performance.A common maximum temperature spec for silicon devices is125◦C, except for thyristors (including SCRs), which should bekept cooler because of the large switching pulses that are usedin triggering.

In the case of IGBTs, the leakage current leads to heatingin the device. The leakage current increases dramatically withtemperature. A significant portion of total power is dissipated in-ternally within an IGBT—10% of total load is not an uncommonoccurrence.

For power applications, generally the cooler a device can bemaintained during operation the better it will perform. Higheroperating temperatures can conspire with power transients, trig-gering signals, noise, and localized heating effects to producefailures that would not occur at lower temperatures. Long-termmaterials interactions, particularly at device interfaces, are a ma-jor concern for lifetime and are often accelerated exponentiallywith temperature. A rule of thumb for silicon devices is thatfailure rates often double for every 10–15◦C rise in operatingtemperature beyond 50◦C [5].

GENERAL DESIGN ISSUES

The most effective way to deal with heat removal is toconsider the thermal requirements at the design stage of anew or upgraded product. A thermal engineer as part of thedesign group will help create the least costly mechanicaland electrical design. More often than not the electrical de-signs will be completed, and then the thermal designers workto meet the thermal requirements under original and addedconstraints resulting from decisions made by electrical engi-neers. This leads to more expensive cooling designs and moreoperational compromises, often resulting in reduced productperformance.

LIQUID TYPES—COOLANT ISSUES

The type of liquid, fluid and system pressure, fluid flow, inlettemperature, cold plate weight, type of material, and the allow-able or desired pressure drop are major factors in the coolingsystem design. Plain water is the optimum cooling choice andwill be used only in controlled environments, laboratory condi-tions, or requested solutions. Tap water may contain active ionsor other impurities, which will attack the inside of aluminumflow channels. Given time, those aluminum channels will cor-rode, causing a leak path and ultimately equipment failure. Thatis why copper in tube or channel form is the preferred solutionwith water and other liquids.

More often an ethylene glycol–water solution of a givenpercentage is specified, since it lowers the freezing point andraises the boiling point. Corrosion inhibitors must be used if anyaluminum is in the flow path, such as piping, tubes, manifolds,tanks, fittings, or cold plates. Many other fluids are available andeach has its own specific heat, viscosity, and handling character-istics. Such fluids are polyolefin, gasoline, kerosene, mineral oil,transmission fluid, JP-5, seawater, etc. Matching or optimizingthe available fluid to the target temperature of the heat sink isthe challenge.

Fluorinert is difficult to utilize since it will evaporate throughmicroscopic crevices, making proper containment a difficultmust. Its lower thermal conductivity and heat capacity comparedto water also make it unattractive as a single-phase coolant.

Distilled water or DI water is a challenge to cool with. Asuitable corrosion inhibitor must be incorporated into the systemso as not to dissolve metal from the cold plate or solderedconnections. DI water without an inhibitor will attack any stresspoints (such as tube bends) and cause a leakage path with direresults.

COLD PLATE CLASSIFICATION

The substrate and the fluid flow channels can be arranged inseveral different configurations depending on the device size andpower dissipation requirement. These cold plate configurationsare classified into four major types as described next.

Formed Tube Cold Plate (FTCP). The coolant tubes are at-tached to the cold plate substrate by soldering or using a thermalepoxy. In this design, shown in Figure 1, copper plate is generallyused, although aluminum is sometimes employed in lowpowerapplications. This is one of the simplest cold plate designs, butits performance is rather poor, limiting its use in the low-powerapplications.

Deep Drilled Cold Plate (DDCP). As the power dissipationincreases, the contact resistance of the plate and the tube wallbecome unacceptably high. In this design, shown in Figure 2,deep holes are drilled in the plane of the substrate plate, generallymade of copper. These holes are then configured with end caps(or plugs) to create coolant flow paths through the substrate. The




Figure 1 FTCP—formed tube cold plate with copper plate soldered to thecold plate substrate.

placement of the electronic devices often influences the coolantpassage layout. It is not uncommon to implement two or moreparallel paths for the coolant flow to meet the pressure drop,coolant distribution, or temperature rise considerations.

Machined Channel Cold Plates (MCCP). As the heat fluxand power increase, it becomes necessary to improve the thermalperformance of the channels. In this design, shown in Figure3, channels are machine-cut into the base plate and a coveris soldered in place to form the flow passages. Depending onthe thermal performance desired, these channels can be severalmillimeters wide or as small as 200 µm wide microchannelsfor extremely high heat flux applications (over 100 W/cm2).Cross-rib patterns, shown in Figure 3, or other enhancementfeatures may be incorporated in the channels, depending on theperformance requirements.

Pocketed Folded-Fin Cold Plates (PFCP). The local heattransfer coefficient, as well as the surface area in the coolantpassages, can be enhanced by implementing fins in the coolantpassages. In this design, shown in Figure 4, recessed pocketsare machined to accept various folded fin inserts, which aresoldered inside the passages. Similar to MCCP, a cover plate issoldered in place to form the enhanced flow channels.

Pocketed fins of various designs are available from variousmanufacturers. Figures 5a and 5b show some of the designs(courtesy Robinson Fin Machine Co., USA). Other designs in-clude straight fins with square edges, straight fins with roundededges, herringbone, ruffled, lazy ruffled, lanced, offset, lancedand offset, perforated, and triangular.

In all the designs just described, appropriate provisions aremade for coolant inlet and outlet locations. Depending on the

Figure 2 DDCP—deep drilled cold plate with single-pass or multi-passcoolant passages drilled in the copper cold plate.

Figure 3 MCCP—machined channel cold plate with channel passages ma-chined in the cold plate to match the device location and thermal dissipationrequirements.

heat flux, total heat removed, and available pressure drop, aspecific design may be selected. The cost is an important fac-tor, as it will vary significantly between these design choices.Further discussion on these aspects is presented later in thearticle.

OVERVIEW OF DESIGN APPROACH

As high-power electronic devices dissipate more power andface new constraints due to space and weight limitations, liq-uid cooling seems to offer a superior alternative for systemstraditionally equipped with air cooling. Cooling of high-powerelectronic devices poses some unique challenges that are some-what different from those in IC chip cooling. The combinationof high heat flux and high power requirements necessitates ef-ficient thermal and fluid management in the heat sink. Some ofthe overriding design considerations are:

• Temperature and heat dissipation requirements of each indi-vidual device (simultaneous peak load of relevant componentsin a group).

• Pressure drop and flow rate requirements for the cooling fluid.

In designing a cold plate, the designer works with a given setof inlet fluid temperature, mass flow rate, and pressure drop lim-its, as well as individual device power dissipations and junctiontemperature requirements, and placement of devices relative toeach other. The design of individual devices and the thermal

Figure 4 PFCP—pocketed folded-fin cold plate with folded fins inserts pro-viding enhanced heat transfer passages for coolant; a machined channel coldplate shown at the outlet.




Figure 5 Straight (a) and lazy ruffled (b) copper fin designs employed inpocketed folded-fin cold plates. Courtesy Robinson Fins, USA.

resistance at the contacting interface with the cold plate de-termine the allowable resistance for heat transfer to the fluid.These issues are covered in detail in literature, while the pri-mary focus of this article is on the thermal performance of thecold plate through the selection of cold plate type and channellayout. The coolant exit temperature depends on its mass flowrate for a given total heat load. It is also limited by the ther-mal performance requirement of the last device near the coolantexit, since it has a lower available temperature difference forheat dissipation. Devices that can sustain a higher temperatureshould therefore placed toward the exit end. Placement of thesedevices is also dictated by their functional criticality, as a lowerjunction temperature will result in a lower failure rate.

Consider the schematic arrangement shown in Figure 6awhere all devices are placed on a cold plate with the coolantserving them in a series configuration. The basic heat trans-fer/fluid flow relations take the following form:

Total coolant flow rate m may be expressed as a function ofthe total load Q on a cold plate, and coolant inlet and outlet

Figure 6 Coolant flow arrangements on a cold plate: (a) series, and (b) par-allel arrangement.

temperatures Tf,in and Tf,out:

m = m(Q, Tf,in, Tf,out) (1)

The local channel wall temperature under a device with the localcoolant temperature Tf, local heat flux q ′′, and local heat transfercoefficient h is given by:

Tw = Tw(Tf, q′′, h) (2)

Since the coolant outlet temperature is the highest near theexit, it is important to check if the maximum allowable devicetemperatures near the exit are exceeded. The local channel walltemperature under a device located near the channel exit is givenby:

Tw,exit = Tw,exit (Tf,out , q′′exit, hexit) (3)

The total pressure drop with an equivalent fluid flow resistanceof Rf,eq in the coolant passages from the inlet to exit is given by:

�p = �p(m, Rf,eq) (4)

Figure 6b illustrates a parallel arrangement that provides thelowest coolant temperature at the entrance for each device. Sincethe fluid flow rate in each parallel channel is reduced, this mayadversely affect the heat transfer coefficients in the channels ifthe flow is in the turbulent region. When using microchannels(<200 µm) or minichannels (<3 mm), the flow is generally inthe laminar region, and the heat transfer coefficient penalty may




not be significant. For enhanced channels, one needs to knowthe performance characteristics of the channels or fin inserts ifpocketed fins are employed. A combination of series and parallelconfigurations may also be employed in arriving at a satisfactorydesign arrangement.

The problem then becomes one of optimizing the heat trans-fer under the given pressure drop constraint to achieve the de-sired base temperature. The main variables available to a coldplate designer in this optimization process are the channel shapeand size.

Additional considerations are introduced in the overall de-sign of the fluid flow loop. The issues encountered here are thepressure drop in the cooling channels, flow maldistribution inparallel channels, fluid routing across different cooling zones,and overall fluid flow pathways to meet the thermal constraintof each device. Since each heat sink design is unique, some ofthe issues raised here are discussed in relation to a few specificcooling system designs.

Locating the regions of different thermal loading becomescritical to plan uniform cooling (relative to the device require-ments) and achieve satisfactory target temperatures. In caseswhere heat fluxes of the devices are uniform and the tempera-ture limits are similar, placement of the devices along the flowpath is not critical. However, if there are devices that have dif-ferent heat fluxes and different temperature requirements, theirplacement becomes quite important. Devices that can toleratehigher temperatures should be placed near the exit end, whereasdevices that have a lower temperature limit should be placednear the entrance region of the coolant channels.

In an actual design, the heat flux and total heat load havedifferent significance. In order to maintain the design junctiontemperatures, a higher heat flux in a given footprint would re-quire a lower thermal resistance of the conduction path in theheat sink and a larger hA (heat transfer coefficient times heattransfer surface area) in the coolant channels. This has a directimplication on the channel size and number of channels cover-ing the footprint of a device in an electronic component. Thetotal heat load, on the other hand, dictates the mass flow rateand the coolant temperature rise. Thus, for the case where heatload is large and heat flux is high as well, the coolant channelsneed to be smaller, with a shorter pitch between adjacent paral-lel channels. Additional factors that are available to a designerare the aspect ratio of the channels (deeper channels providinglarger heat transfer surface area) and enhanced channels, withinternal fins, such as microfins, offset strip fins, or other foldedfin configurations.

Design Issues for IGBTs

The manufacturers of IGBT devices have incorporated anumber of features to improve the heat transfer from the de-vices to the base plate. Some of these include incorporating ahigher thermal conductivity ceramic, such as aluminum nitride,between the power chip and the cold plate. Sometimes the base

Figure 7 Different IGBT units, counterclockwise from lower right: two-bolt,four-bolt, four-bolt, and eight-bolt units. Courtesy Powerex Corp.

plate itself is used as the heat sink for direct heat removal withbuilt-in coolant passages. Use of internal fins in the passages,microchannels, or a graphite foam mix further improves theirthermal performance.

IGBTs come in different mounting configurations. As thenumber of mounting holes increases from two to eight, as shownin Figure 7, the location and number of transistors and diodesincrease, as does the heat generated.

Two-Bolt Devices

A single cooling path under the center of this device maysuffice if the load is small or a higher temperature is allowed,around 75 to 85◦C. Since the two bolt holes are most oftencentered on the longest ends, positioning a single path alongthe length of the IGBT may not be possible (since the boltsthemselves will interfere). Further, a single path running per-pendicular to the long axis may not provide cooling to the targettemperature, and several path turns may be needed to obtain thenecessary heat removal rate.

The next choice is multiple paths in a uniform direction orcounterflow direction to reach the target temperature. Creatingmultiple channels under the device is accomplished by machin-ing coolant flow paths or by placing folded fins in a cavity.There are small copper extrusions that are employed for smallheat loads, but are more expensive and not suited for large loads,and hence not treated in this article.

Four-, Six-, and Eight-Bolt Devices

As the power dissipation increases, the size of IGBTs in-creases and the number of bolts used also goes up. Four-, six-,




and eight-bolt IGBTs share the same concerns listed earlier. Thedifference is now that the power to dissipate may range up to7000 W or higher. With the largest IGBTs measuring some 100to 150 mm, placing the cooling channels at the correct locationsis paramount. Each IGBT manufacturer uses a grid pattern tolocate the transistors and diodes. Generating this pattern willfacilitate the subsequent design of the cooling channels.

Advanced Cooling System Design Strategies

The thermal resistance of the cold plate can be estimated us-ing the standard heat transfer calculation methodology widelyavailable in heat transfer literature. However, there is little infor-mation available for modern-day transistors that are embeddedwithin silicone, potting epoxies, and plastic moldings. A morerealistic method is to determine the desired cold plate temper-ature for a particular task and then to specify the maximumallowable cold plate temperature. For any cold plate there willbe a hot spot generally located under the center of the device,or slightly off center in the direction of the fluid flow (due tofluid temperature rise resulting from heat addition). Since thishot spot is the likely location of failure of the mounted device,the design goal is to lower the hot spot temperature to the spec-ified limit. If an average cold plate temperature is used in thedesign, the device may function but may result in a reduction inexpected product life.

In order to minimize the hot spot, or hot spots if multipledevices are to be mounted, the following several questions mustbe answered.

1. Are the outside dimensions of the cold plate known? Thismay determine not only the mounting and space for drillingholes or for milling paths and manifolds, but also the typeof liquid cooling that can be applied.

2. Where are the device bolt holes and solid connections to thecold plate located? The depth of these holes may require athicker base and therefore may cost more. With multiple de-vices (up to 12), with six or eight bolt holes each, a multitudeof potential leak paths will exist.

3. Is pumping more fluid through the cold plate an option? Thisis often not possible since the planned pump has only mini-mum power allocated to it, and a more detailed optimizationat the system level (entire system served by the pump) maybe needed.

4. Is the target temperature realistic, or an aspired result thatmay not be achievable? A discussion with the submittingengineer may provide a changed answer. Once the target isknown, a first-pass solution can be made.

5. Can the fluid type be changed to provide better cooling?Again this is often dictated by the marketing factors or by acustomer’s system requirements.

6. If only a single inlet and outlet are requested, are multipleinlets or outlets possible? This could allow the coldest fluidto be directed toward the predicted hot spot(s).

7. If the inlet and outlet are shown, or desired to be on the sameedge, are there any other piping arrangements that wouldenhance the cooling performance of the cold plate? Usuallythe same edge location costs more since space is needed fora return path by milling, deep drilling, or external piping.

8. Is there a group of devices that need series cooling, or maythey be arranged for the liquid to flow under each individ-ual device through a manifold? For the series path, the lastdevice’s hot spot is the one that becomes the target for tem-perature control as described earlier. For the parallel paths, alarger cold plate is necessary for inlet and outlet manifolds,and may result in higher average temperatures for all devicessince the liquid flowing per device is less.

9. Can the devices be rotated 90 degrees to allow more surfacearea to contact the cooling paths directly? Often the electricaldesign is inflexible, but an early discussion may allow thischange.

10. Can any of the devices be moved so the hottest ones areshifted to the front, or conversely can the fluid enter from adifferent side where the hottest devices are located? Oftenthe layout is fixed, but a small shift in location to allow fordeep drilled paths is often possible.

11. Is the customer committed to an aluminum product or willa copper solution be accepted? Better performing thermalproperties of copper are often needed for the higher heatloads presented.

12. Is it possible to include any design-specific items that areuncovered in discussions with the initiating designer? Milledslots, cavities, or large holes introduce thermal resistancesin the heat flow paths and must be carefully accounted for.

CFD modeling plays an important role in the thermal designof cold plates. When the thermal designer has a good handleregarding the issues just listed, it is recommended that a first so-lution pass be made with a CFD program [e.g., 18]. The targettemperature requested is compared with the predicted hottestspot, along with allowable pressure drop and the cooling fluidinlet temperature at a given flow. If any of the required pa-rameters are not met, solutions can be sought by altering manyparameters. These include possible cooler inlet temperature,higher fluid flow rate, material change, mixing cold plate ma-terials, more aggressive fin configurations, rerouting fluids, orrequesting a target temperature change. More often than not,a solution can be arrived at through in-depth discussions withthe customer (or electrical engineers). The customer will hope-fully recognize reality and make allowances to continue towarda buildable product. For example, the IGBT may not put out 700A continuously, but would have reliable deliverable 600 A overlong hours of usage.

Any time a double-sided load is presented, a number of ad-ditional issues arise. Are the heat loads to be dissipated andtheir locations the same for both sides? The exact locationsof the loads are critical since the straighter the tube runs,the less costly is the construction. Are there different target




temperatures per side? Is there a range of target temperatures oneach side based on different devices? Are there duplicate, par-allel, or counter flow paths? Are there single or multiple inletand outlets? Where are the inlet and outlet—opposite ends, at90 degrees, or on the same end or side? Each required parameterdictates the direction of a thermal solution.

Will the cold plate be placed inside of a cabinet or frame forsupport, or will the cold plate be a support structure for multipledevices? The answer will affect the weight of a part by allowingmaterial for mounting-bolt holes along the sides or corners ofthe cold plate. If used to support multiple IGBTs, stiffness inthe cold plate may be important and the plate may require moremass, to allow connected copper electrical buss work to staypositioned.

When a cold plate is to be used in any portable project—person-carried, airborne, or vehicle—total weight becomes acritical feature. Therefore the first design scenarios should startwith an all aluminum construction. Often aluminum will notachieve the target junction temperatures so a mixed metal solu-tion may be required. If the devices are mounted on a coppermachined fin set, an o-ring gasket may seal the liquid flow path,preventing dissimilar metals from touching, thereby keeping theweight to a minimum.

Manufacturing Issues

Manufacturing issues are often overlooked by the thermaldesigner, but play a critical role in practical implementation ofthe design. Some of the issues related to manufacturing consid-erations are presented in this section. This section is intended tomake the thermal designer aware of some of the manufacturingissues, not to present a comprehensive summary.

When a tube is bent or folded to make a parallel return path,the bend radius must allow for full flow and not be crimped.Tube bending mandrels are often needed to create the leastdeformation of the tube at the return bend. With a medium wallthickness of about 0.9 mm (0.035”) a 9.5 mm (3/8”) OD tube canbe formed into a return path with 25+ mm (1+”) centerline-to-centerline dimensions. Limiting deformation is most imperativewhen the top surface of a cold plate is fly cut to achieve an overallflatness for device mounting. If too much metal from a tube bendradius is removed or if it was crimped, a thin tube section andprobable leak location may occur. Once made, all cold platesmust have 100% testing to assure leak-proof functionality.

Drilling small holes for thermocouples in the cold plate underthe predicted hot spot is very costly, since they will often needto be several centimeters deep. Therefore, surface mountingthermocouples on isothermal lines next to the device will beable to check the hot-spot temperature (predicted from CFDanalysis if conducted during design stage), although these typesof measurements are subject to a number of measurement errors(e.g., contact resistance, altering local coefficients). Thermalimaging can also determine the temperatures along the edgeof the device if the view is clear, and thereby predict if the

target temperature has been achieved. The best testing is mostoften done in the customer’s laboratory, where use test or fieldconditions can be duplicated.

Manufacturing Cost Considerations

When a part must have a machined liquid flow path for ob-taining the correct target temperature, several methods of con-struction must be compared. The cost of a machined path mustbe compared with building a pocketed set of fins. The machinesetup time, with machine speeds and feeds, requires a specificamount of time to create the appropriate paths. This millingmachine has an hourly cost to arrive at a specific part cost. Onemust assume that there will be some percentage of scrap created,added to the cost. There will be set up time and machining timenecessary for pocketing a set of folded fins. This is added to thecost for the required fin set, scrap expected, for a comparisonpart cost.

For every Critical to Function or Critical for Design designa-tion on such items as hole locations, hole depths, bosses, milledpaths, pocket depths, edge features, or drawing notes, there willbe a corresponding cost associated with creating or providingthose features. Most likely there will be a need for a fixture thattightly positions the part, and an extra programming charge tocarefully drive the milling machine. In each case just describedthere will be a cover that needs to be soldered to the base, withsome additional cost.

For the pocketed folded fin set, care must be taken duringmanufacture to insure that all the flow paths are maintainedand that no solder runs down into the channels and blocks theflow paths. This is less of a problem for the machined pathsolution. There is often no clear answer for which direction ispreferable since the production quantity affects all machiningcosts. Setup time for running the entire job at one time amortizedover the total number of parts. Parts that have a series of machineruns require multiple machine set ups amortized on smallerquantities, and therefore are more costly.

Standard machine tools can be purchased for nominal pricesand are used for creating the machined flow paths, pockets, andtube attachments. If, however, a modified or nonstandard toolsize is required or specified, the machine tool costs could goup by a factor of six and require a larger number of these toolspurchased to allow for any breakage. Sometimes a wider flowpath can be created by running a smaller tool in two passes tocreate the correct width. However, this affects the cost by addingmachine time.

There are several sets of fixtures needed for any machiningof production parts. First, the material must be held for precisereplication. Second, fixtures for side features (if any) are alsonecessary. All features such as holes, cavities, bosses, slots ortrenches, and dowel pin holes require exactness of location.Some holes may be necessary for T-bolts to hold the fixture to themilling machine base. The fixturing will consist of locating pins,blocks, multiple clamps, and other necessary machining. There




may be fixtures created to hold various components together forfinal assembly. They will include locating pins, blocks, multipleclamps, supports, or braces to allow uniform heating and preventwarping when heated.

Flatness of parts is a critical issue as well. Generally a flatnessratio of 0.001”/” (inches per inch length, or mm/mm length) isan acceptable standard for machined cold plates. This is accept-able for most electrical devices, since there will be an interfacematerial of some thickness, between the cold plate and the de-vice. Usually about 100 to 150 µm (4 to 6 mils) thick, thesematerials allow for close contact between surfaces. For a tighterrequirement, .0005”/”, there will be a corresponding cost in-crease. As the requirement gets closer to dead flat, more precisemanufacturing processes are needed. Blanchard grinding is thefinal most expensive step.

Folded Fin Sets

If machined path fins are not able to achieve the target tem-peratures due to the large number of fins (surface area) required,the alternative is folded fin sets. These will be set into a pocketor cavity with a manifold needed to avoid maldistribution. Thetype of folded fin sets are straight fin, herringbone fin, wavyfin, square fin, lanced fin, offset fin, triangle fin, perforated fin,ruffled fin, and rounded fin. The thickness of the desired fins dic-tates the fin choices. All of these types are available with shortfin heights, short flow lengths, and very thin material. Since re-moving significant amounts of heat requires rather large surfaceareas and greater thickness, fin type selections are limited.

For example, if 1 cm tall fins of 0.6 mm (0.25”) thicknessare specified at 8 fins/cm (20 fins per inch) and are 380 mm(15”) long, the manufacture of these fins may not be possible,even if the CFD program created this answer. Therefore anothersolution needs to be found to achieve the target temperature.This is where compromises get made, such as higher pressure,shorter fins, or a different fin set. An entirely new design maybe needed to meet the required target temperature.

The most common choice is straight folded fins of a givenheight, pitch, thickness, length, and cutoff or overall width. Ifthese fins are to be pocketed between the base and cover, anaddition of two fin thickness to the height will reduce pressuredrop across the fin inlet and outlet.

A second common choice is a ruffled or lazy ruffled fin set.Again the height, pitch, material thickness, length, and cutoffare specified. The same pocketing applies. However, these finsets have a smaller manufacturing tolerance, which makes coldplate construction easier. However, it is recognized that thisfin will create a higher pressure drop. The additional surfacearea gained by the ruffling makes them better for higher heattransfer duties. Often a compromise with the thermal engineerwill enable ruffled fins to be used with the attendant higherpressure requirement.

All of the items just listed may become part of the cost fora particular solution. The optimum thermal solution is based

on the heat to be dissipated, number of devices to be cooled,their location on the cold plate, material, fluid type, fluid flowrate, inlet temperature, and pressure drop, weight, and platetemperature targets. That is why no two cooling solutions arequite alike.

SAMPLE CASE STUDIES

A few sample case studies are presented here to show theselection of the cold plate type and placement of the channelsin different applications.

Power Drives

Conventional three-phase motors powering fluctuating loadsoperate at a given voltage and amperage. Whenever they operateat lower or reduced loads they become inefficient by consumingmore energy than necessary for the load presented. The applica-tion of IGBTs to motor drives offers a more efficient operatingmethod. Circuit control of the IGBTs will only draw as muchamperage or energy at a given voltage as is required for theload presented. Even so, the IGBT is not 100% efficient, soany excess heat generated to create the step wave form must beremoved for steady operation.

In Figure 8 there are four 8-bolt 4800-W IGBT devicesmounted on a cold plate. This is one of three cold plates used inthe system. There are also three 400 W devices centered in themiddle of the cold plate. This design required cooling to 75◦Con the surface of the second device in series. This could onlybe accomplished by mounting ruffled folded fin sets pocketedin the base and by creating manifold like flow paths in and outof each fin set. Fluid flow was also channeled below the centerdevices to remove 400 W each, above a soldered cover.

In a clamshell design, two small copper plates are machinedwith diagonal grooves, as shown in Figure 3, allowing for aninternal manifold spreading and collecting the fluid. The coolingfluid flows up and down repeatedly as it crosses the cold plate,so this design could easily have a double-sided load of IGBTs.Leak testing is required to ensure long-term performance.

Figure 9 depicts another power drive cooling channel config-uration. Ten parallel paths are milled in the base for 10 flat tubesto be pressed in and fly cut to the required flatness. The totaldissipated heat load was 8900 W. The external inlet manifold

Figure 8 Cooling channel configuration for a cold plate serving four 8-boltIGBTs.




Figure 9 Flat tube manifold pressed into aluminum base plate.

provided the coolest fluid to each IGBT. The dies were locatedso that the hottest spots could be targeted with the flat tubes.

SCRs are also widely employed in high-power electronicsystems. They are very sensitive to high temperature, as shownin Figure 10. SCR efficiency drops drastically beyond 125◦C.When cooling SCRs, the design must provide adequate coolingto limit the hot spot temperatures, which usually occur near thecenter of the device. Most electrical designs using SCRs willhave multiple devices in banks or racks. Therefore the lowesttemperature coolant must be supplied to each device present foruniform performance.

Conveyor Systems

In mining operations moving ore to a processing plant isoften done by 5000+ HP motors. Efficient motor drives thatfollow this type of loading, unloading, and varying loads arepowered by SCRs. Cooling of these drives has been done withself contained liquid systems coupled to an air cooled radiator,since water is often scarce.

Figure 10 SCR voltage–temperature performance [20].

RF Generators

Typically RF generators have a large number of devicesmounted on a cold plate. A deep drilled design shown in Figure 2is appropriate for this case. Many small mounting holes are lo-cated very close to the 224 mm long paths. This cold plate canremove 3600 W. Since the cold plate is only 12 mm (0.46”)thick, many of these plates are stacked in a cabinet, maximiz-ing the space available for the RF devices. Proper coolant flowto each cold plate is accomplished through external manifolds.Typically these cold plates are the mounting structure for theelectronics, so the weight of the cold plate is not a concern.

FLOW MALDISTRIBUTION

Flow maldistribution in the parallel fluid flow passages is amajor concern, as it may degrade the thermal performance ofthe cold plate below the acceptable limit. Lu and Wang [43] andLiu et al. [44] present a detailed analysis on the effect of inletand outlet locations on the flow maldistribution in a cold plate.As an illustration, a CFD analysis of flow maldistribution wascarried out and is shown in Figure 11 for the flow arrangementshown in Figure 9. There are in total 10 parallel channels. Eachchannel is 9 mm × 33 mm and 241 mm long, and the header is18 mm in diameter and 812 mm long. For the inlet and outletson the opposite sides, CFD simulation was carried out for atotal water flow rate of 0.085 kg/s. Figure 12 shows a similaranalysis for the case shown in Figure 9, but with the outlet onthe same side as the inlet. Both these figures show that the flowis significantly skewed. Water exiting from the lower flow ratechannels will be at significantly higher temperatures, resulting

Figure 11 Normalized flow rates in individual channels for cold plate shownin Figure 9 with inlet and outlet on the opposite sides.




Figure 12 Normalized flow rates in individual channels for cold plate inFigure 9 with inlet and outlet on the same side.

in temperature overshoot of the devices mounted directly abovethem.

Flow maldistribution has the potential of causing a catas-trophic failure of a cold plate. It is highly recommended thateach design be analyzed using a CFD program [e.g., 18]. Sinceflow maldistribution depends on the actual operating conditionssuch as temperature and flow rate, such simulation under differ-ent operating scenarios is needed for a reliable design throughoutthe operating range.

CONCLUSIONS

A brief overview of liquid cooled cold plates is presented.Based on the current practice, a classification scheme is intro-duced to identify cold plate types. Liquid cooling of high-powerelectronic devices requires unique solutions, a few of whichhave been presented in this article. Each task has its own targettemperatures and design constraints, which provide challengesfor the thermal designers. Some of the thermal design issues,manufacturing constraints, and cost considerations have beenpresented. A decision tree of design choices is outlined in theprocess for arriving at an optimum design. The inclusion of athermal engineer at the outset of a new or upgraded project willresult in the best design at the least cost.

NOMENCLATURE

h heat transfer coefficient, W/m2Km coolant flow rate, kg/s�p pressure drop in the cold plate

q ′′ heat flux at the exit, W/m2

Q total heat dissipation, WR flow resistance in the cold plateT fluid temperature

Subscripts

eq equivalentexit at the exit sectionf fluidin channel inletout channel outletw wall

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Satish Kandlikar is the Gleason Professor of Me-chanical Engineering at Rochester Institute of Tech-nology (RIT). He received his Ph.D. degree from theIndian Institute of Technology in Bombay in 1975and was a faculty member there before coming toRIT in 1980. His current work focuses on the heattransfer and fluid flow phenomena in microchannelsand minichannels. He is involved in advanced single-phase and two-phase heat exchangers incorporatingsmooth, rough, and enhanced microchannels. He has

published over 180 journal and conference papers. He is a Fellow of the ASME,associate editor of a number of journals including ASME Journal of Heat Trans-fer, and executive editor of Heat Exchanger Design Handbook, published byBegell House. He received the RIT Eisenhart Outstanding Teaching Award in

1997 and the Trustees Outstanding Scholarship Award in 2006. Currently heis working on a DOE-sponsored project on fuel cell water management underfreezing conditions.

Clifford Hayner earned his B.S. in mechanicalengineering from the University of Rochester andjoined the U.S. Navy. After the Navy, he workedfor Rochester Gas & Electric Corporation for over30 years. He primarily helped in solving the energyneeds of major industrial customers. Throughout hiscareer at ERM Thermal Technologies Inc. (now VetteCorp.) he has designed over 800 successful ther-mal solutions. He is now retired from the industry andworks as a thermal solutions consultant in Rochester,

New York. He was the president of the local chapter of Toastmasters Interna-tional, president of the Irondequoit Chamber of Commerce, and is a member ofthe Otetiana Council and on the Board of Directors of the Rochester Chapter ofASME.



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