Cooling Technology Options.docx
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Cooling Technology Options.docx
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CoolingTechnologyOptions
CoolingTechnologyOptions
KavehAzar,Editor-in-Chief
AdvancedThermalSolutions,Inc.
kazar@
(Editor'sNote:
Part2ofthisarticlewillappearintheNovember2003issueofElectronicsCooling.)
Introduction
Coolingelectronicsystemsisoneofthemajorfocalpointsofthedesignprocessandthekeytoasuccessfulproductlaunch.Manyoptionstoattainsuccessfuloperationareavailable,rangingfrompassivecoolingtocryogenics.Theseoptionsareobviouslypowerdissipationandheatfluxdependentwithmanypackaging,implementationandcost-of-operationissues.Fromamarketsuccessstandpoint,thesolutionwiththeleastoperationandmaintenancecostisthemostdesirableone.However,systemsoperation(devicefrequency)andpackagingconstraintsoftendictatesolutionalternativesthatmaynotfittheleastcostandmaintenancemodel.Furthermore,therearemanyso-called"thermallimits"floatingintheliteraturethatrecommendcoolingbasedonpowerdissipationratherthandevicejunctiontemperature.Therefore,itistheintentofthis2-partarticletoreviewcommerciallyavailablecoolingtechnologiesandtociteselectedexampleswiththeirsalientfeatureasbeaconsforwhathasbeenandcanbeattained.
TheHeatDissipationIssue
Beforeprovidingasynopsisofdifferentcoolingtechnologies,itisbesttodispelthenotionofheatdissipationandthecoolingtechnology.Itisoftenseenthatanengineerisdealingwithanx-kWpowerdissipationsystemory-W/cm2device.Sincethenumbersarehigh,theimmediatereactionistoconsiderahighheatfluxcoolingsolutionwithoutanyregardastowhatthefundamentalissueis.Inelectronicscooling,theprimaryandsecondarycriticalmetricsthatathermalengineermustmeetarethedevicejunctionandsoldertemperatures.Whetherthisisa0.5W/cm2devicethat,becauseofitsplacementontheboard,hasexceededitsjunctiontemperature,orahighpowerprocessor(s),whichistheengineofthatsystem;e.g.,today's1UbladeserverswithfourXenonprocessors.Irrespectiveofthesystemordevicepowerdissipation,selectionofthecoolingsolutionisdictatedbythejunctiontemperature.Therefore,intheensuingdiscussion,Ipurposefullyavoidanyreferencetotheheatremovalcapabilityofeachcoolingoption,sincethecapacityistemperatureandspacedependent.Further,makinganyreferencetoheatremovalcapacitycanbemisleading.
Whenwelookattheheattransportvehiclesavailableinthemarket,itisquiteclearthatthetechnologyiswellestablishedandextensivelydocumented.However,aseriesofquestionsaregeneratedwhenattemptingtotransferthistechnologytoelectronicssystems.Theseare:
1.Wouldtherebesufficientspacetoprovidealargesurfaceareaforheattransfer,ortoimplementthermaltransport(e.g.,heatpipe)devicestotaketheheatoutofthesystem?
2.Canweusefluidswithhigherthermalcapacitance/conductivityandsubsequentlyhigher"h"toattainthedesiredcooling?
Wouldthecostofsuchfluidsallowtheirimplementationinthemarketplace?
3.Wouldtheexistingsystempackagingallowtheuseofahighercapacitycoolingsystem?
4.Wouldthemarketbeopentoasystemthatismechanicallymuchmorecomplexandmaintenancedemandingthana"simple"airmover?
Withthesequestionsinmind,thecoolingtechnologycanbedividedintotwobroadcategories.
A.Passivecooling,wherenaturedoesthefluidmovement(e.g.,naturalconvection)orenergyistransportedbyconductionand/orradiationheattransfer.
B.Activecooling,wherethefluidmotionisassistedbyanexternalsource,afaninaforcedaircooledsystem,orpumpandfanofanimmersionorrefrigerationcooledsystem.
ConsideringthefourquestionsthatIraised,itisclearthatoptionAisthemostdesiredsolutionfromtheproductstandpointsinceithastheleastimplementation-costandrequires,effectively,nomaintenance.However,asthepowerdissipationincreasesandpackagingspacebecomeslimited(question1),optionBbeginstotakeholdandbecomesthemethodofchoicedespiteitshigherimplementationcostandconcernsfortheoperationalreliability.
CoolingSystems
Withthesetwobroadcategoriesinmind,let'slookatthecoolingoptionsthathavebeendevelopedintheindustry.
PassiveCooling
Passivelycooledsystemstakeadvantageofallmodesofheattransferforthermaltransport.Atthesystemlevel,typically,largeheatsinkswithwidefin-to-finspacingareusedforcooling.Televisionsets,set-topboxes,pole-orstrand-mountedtelecommunicationsboxesaresomeofthetypicalexamplesinthiscategory.Inpassivelycooledsystems,designerstypicallyattempttoutilizeconductionandradiationastheprimarymodesofheattransfertomaximizethethermaltransportandtoinducehigherlevelsofnaturalconvection.Inapplicationswhereconvectionislimited,e.g.,spaceshuttlecooling,radiationheattransferbecomesthesolemodeoftransportofheatfromthesourcetothesink-space,inthecaseoftheshuttle.
Passivelycooleddevicestakeadvantageofaheatspreader(aconductiveplatetospreadtheheat)and/orheatsinksspecificallydesignedforsuchconditions.Similartosystems,maximizingradiationandconductionheattransferfromthedeviceiskeyforreachinghigherlevelsofpowerdissipationwhilemeetingdevicejunctiontemperatureconstraints.Inspacelimitingapplications,e.g.,laptops,heatpipesareoftenusedtoefficientlytransporttheheatfromthedevicetoalocationwherealargerspaceisavailable(Figure1).
Figure1.Heatpipeandheatspreadercombination.(CourtesyofEnertronCorp.)
ActiveCooling
Thisclassofsolutionsencompassesanarrayofcoolingtechniquesthatarediverseandextensive.Itisnottheintenttorevieweachindividualcoolingsolution,rathertohighlightwhatisavailableandtoprovidethesalientfeaturesofeachoption.Inabroadsense,thecoolingoptionscanbecategorizedasfollows:
HeatSinksandFan-SinkswithoutHeatPipes
Extendedsurfaces,so-calledheatsinks,arebyfarthemostcommonlyusedcoolingsolutionsintheelectronicsindustry.Thedesigns,manufacturingtechniques,andmaterialsusedintheirconstructionareasdiverseasthedesignerswhomakethem.Conventionallytheyareusedinsystemswithairmovers.Attimes,becauseofairflowinadequacyorspatialconstraints,afanmaybemountedontheheatsink,theso-calledfan-sink.Or,aheatpipeisalsousedalongwiththeheatsinktoimprovethermaltransport.Theinterfacialandspreadingresistancesinadditiontotheattachment,aswellasdesigningtheheatsinkfortherightlocationonaPCB,arethepointsofcontention.
FluidFlowManagementandEnhancement
Althoughthisbyitselfcannotbeconsideredasastandalonecoolingsolution,boardandsystemlevelfluidflowmanagementhasshowngreatpromises.Thetechniqueinvolveslookingattheflowdistributionintheregionofinterestandattemptingtochangethelayout,whetheraboardorsystem,toremoveflowstagnationpoints.Asaresult,whetheracomponentonaPCBoraPCBinasystem,itwillhaveasignificantlyimprovedflow-exposurethatcanoftenresultintheeliminationofheatsinksoralessercapacitycoolingsolution(Figure2).Thisisanareathatisoftenignoredsinceboardorsystemre-layoutarecostlyiffluidflowoptimizationisnotdoneapriori.
Figure2.WaterflowvisualizationofaflowoveraPCB.Ontheleft,theoriginallayoutshowsmultiplestagnationpoints.Ontheright,theflow-optimizedlayoutshowsminimalstagnationpoints.(Fromtheauthor'sshortcourse:
"ExperimentalMethodsinElectronicsCooling.")
HybridCooling
Thissimplyimpliesacombinationofliquidandaircoolingforhighpowerdissipationelectronics,whileminimizingcontactresistancethroughoutthesystem.Inthisprocess,theavionicsindustryhasmadegreatstridesandproducedcoolingsystemscapableofremovinghighheatfluxes.Figure3showsonesuchsystem[1].
Figure3a,showsarack/cardguidesystemwheretheliquidisflowingthroughthecardrack.TheheatthatisgeneratedwithinthePCBisconductionthroughthesolidcoretotherackwheretheliquidisusedasthetransportvehicletoremoveitfromthesystem.TheillustrationontherightinFigure3ashowsathru-cardschemewherethePCBcoreischamberedforfluidpassage.ThustheconductionheattransferfromthePCBtotherack,asshowninthefigureontheright,iseliminated.
Figure3a.Twohybridsystemsforanavionicsapplication:
Ontheleft,edgeliquidcooling;ontheright,thru-cardliquidcooling[1].
Figure3bshowstheextensivepackagingrequiredtomakesuchsystemshappen.Tightlysealedjointsandmechanicalcontactswithleastresistancearerequiredtoattainthelevelofthermalperformancerequiredforsuchacoolingoption.
Figure3b.Cardandracklevelpackagingrequiredforhybridcooling[1].
AlthoughIhavepurposefullyavoidedthediscussionofcoolingcapacity,theexceptiontothepathmaybemeritedheresincethesesystemsarenotcommonlyencounterednordiscussedintheopenliterature.Therefore,Figure4showstheremovalcapabilitiesofthehybridsystemsfordifferentcoolingarrangements[1].
ThedatadepictedinFigure4clearlyshowtheadvantageof,say,subcooledjetimpingementandaconventionalcooledsystem,approximatelyafactorof13!
However,onecannotoverlookthepackagingrequirementstodeliversuchacoolingsolution,asshowninFigure3b.Thecostoftheserequirementsandtheattainmentofahighreliabilitycoolingsystemmaynotmakethesesystemssuitablefortypicalcommerciallyavailableelectronics.
Figure4.Heatremovalcapaci
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