CHAPTER 6CHAPTER 6

Fundamentals of Thermal Fundamentals of Thermal ManagementManagement

6.1 WHAT IS THERMAL MANAGEMENT?

Resistance of electrical flow

Absence of cooling

Contact of Device• Cooling• roles

Steady StateSteady State• Intense Heat TransferIntense Heat Transfer

Successful Thermal PackagingSuccessful Thermal Packaging

X

6.2 6.2 WHY THERMAL MANAGEMENT?

Thermal Management of all Thermal Management of all microelectronic components is similarmicroelectronic components is similar

Prevention of Catastrophic failurePrevention of Catastrophic failureTemperature riseTemperature riseCatastrophic vulnerabilityCatastrophic vulnerability

X

6.2 Why Thermal Management 6.2 Why Thermal Management cont.cont.

Failure Rate Increases with TemperatureFailure Rate Increases with TemperatureReliabilityReliability

X

6.2 Why Thermal Management 6.2 Why Thermal Management cont.cont.

X

6.2 Why Thermal Management 6.2 Why Thermal Management cont.cont.

The main thermal transport mechanisms The main thermal transport mechanisms and the commonly used heat removal is and the commonly used heat removal is different in each packaging level.different in each packaging level.Level 1Level 1Level 2Level 2Level 3 and 4Level 3 and 4

X

6.2 Why Thermal Management 6.2 Why Thermal Management cont.cont.

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6.3 Cooling Requirements for 6.3 Cooling Requirements for MicrosystemsMicrosystems

Cooling techniquesCooling techniquesBuoyancy- induced natural circulation of airBuoyancy- induced natural circulation of airNatural convection coolingNatural convection coolingForced convectionForced convectionHeat-sink-assisted air coolingHeat-sink-assisted air cooling

6.3 Cooling Requirements for 6.3 Cooling Requirements for Microsystems cont.Microsystems cont.

6.4 Thermal Management 6.4 Thermal Management Fundamental Fundamental

Electronic cooling, there are three basic Electronic cooling, there are three basic thermal transport modethermal transport modeConduction (including contact resistance)Convection Radiation Radiation

6.4 Thermal Management 6.4 Thermal Management Fundamental cont.Fundamental cont.

One-dimensional Conduction One-dimensional Conduction

X

6.4 Thermal Management 6.4 Thermal Management Fundamental cont.Fundamental cont.

Heat flow across solid interfaceHeat flow across solid interface Perfect adhering solids Perfect adhering solids Real SurfaceReal Surface Ac = area of actual

contact

Av = fluid conduction across the open spaces.

X

6.4 Thermal Management 6.4 Thermal Management Fundamental cont.Fundamental cont.

Convection Convection Two mechanismTwo mechanism

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6.4 Thermal Management 6.4 Thermal Management Fundamental cont.Fundamental cont.

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6.4 Thermal Management 6.4 Thermal Management Fundamental cont.Fundamental cont.

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6.4 Thermal Management 6.4 Thermal Management Fundamental cont.Fundamental cont.

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6.4 Thermal Management 6.4 Thermal Management Fundamental cont.Fundamental cont.

Thermal Resistant in ParallelThermal Resistant in Parallel

X

Natural Convection air cooling of Electronic Natural Convection air cooling of Electronic equipment still very popularequipment still very popularSimplicity, reliability and low costSimplicity, reliability and low cost IC packages, PCB’s, heat sinksIC packages, PCB’s, heat sinks

Single PWBSingle PWBArray of PWB’s-array of vertical channelsArray of PWB’s-array of vertical channels

Nusselt Number: Nu=El/CNusselt Number: Nu=El/C22A, El=Elenbaas numberA, El=Elenbaas number Measures the enhancement of heat transfer from a Measures the enhancement of heat transfer from a

surface that occurs in a real situation, compared to heat surface that occurs in a real situation, compared to heat transferred if just conduction occurred. Dimensionless transferred if just conduction occurred. Dimensionless quantityquantity

6.5 Thermal Management of IC and 6.5 Thermal Management of IC and PWB Packages cont.PWB Packages cont.

Optimum SpacingOptimum Spacing Isothermal arrays the optimum spacing maximizes the total heat Isothermal arrays the optimum spacing maximizes the total heat

transfertransfer Optimum PWB spacing where max power can be dissipated in the Optimum PWB spacing where max power can be dissipated in the

PWB’sPWB’s

Limitations-closely spaced PWB’s tend to under predict heat Limitations-closely spaced PWB’s tend to under predict heat transfertransfer

Due to between package “wall flow” and the non smooth nature of Due to between package “wall flow” and the non smooth nature of channel surfaceschannel surfaces

6.5 Thermal Management of IC and 6.5 Thermal Management of IC and PWB Packages cont.PWB Packages cont.

PWB’s in Forced ConvectionPWB’s in Forced ConvectionMost applicationsMost applications

Laminar Flow-Laminar Flow- the flow of cooling air proceeds the flow of cooling air proceeds downstream between the PWB’s in “sheet-like” downstream between the PWB’s in “sheet-like” fashion.fashion.

Forced laminar flow in long, or narrow parallel Forced laminar flow in long, or narrow parallel plate channels the heat transfer coefficient has an plate channels the heat transfer coefficient has an asymptotic value of: h=4kasymptotic value of: h=4kff/d/dee. Where d. Where dee=Hydraulic =Hydraulic

diameterdiameter

6.5 Thermal Management of IC and 6.5 Thermal Management of IC and PWB Packages cont.PWB Packages cont.

6.6 Electronic Cooling Methods6.6 Electronic Cooling MethodsHeat SinksHeat Sinks

Convective thermal resistance can be Convective thermal resistance can be reduced byreduced byIncreasing heat transfer coefficient orIncreasing heat transfer coefficient orIncreasing heat transfer areaIncreasing heat transfer area

Coefficient is function of flow conditions which Coefficient is function of flow conditions which are fixedare fixed

Most applications-increase heat transfer area Most applications-increase heat transfer area provides only means to reduce convective provides only means to reduce convective thermal resistance- by use of extended thermal resistance- by use of extended surfaces or finssurfaces or fins

Heat Sinks continued:Heat Sinks continued:The temperature of the fin is expected to The temperature of the fin is expected to

decrease from the base temperature as move decrease from the base temperature as move toward the fin tiptoward the fin tipAmount of convective heat transfer depends on the Amount of convective heat transfer depends on the

temperature difference between the fin and ambienttemperature difference between the fin and ambientHeat transfer from fin area:Heat transfer from fin area:

q=ηhAq=ηhAff(Tb-Ta)(Tb-Ta)

AAf f Base areaBase area

Η fin efficiencyΗ fin efficiency Tb base temperatureTb base temperature

Single plate fin, most thermally effective use of fin material Single plate fin, most thermally effective use of fin material achieved when efficiency is 0.63achieved when efficiency is 0.63

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Heat Sinks continued:Heat Sinks continued: ““extended” surfacesextended” surfaces

Manufacturer provides heat sink thermal Manufacturer provides heat sink thermal resistance for range of flow ratesresistance for range of flow rates

Most common are extruded heat sinksMost common are extruded heat sinks Limitation on fin height to fin gap due to structural Limitation on fin height to fin gap due to structural

strength.strength.

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods Cont.Cont.

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Thermal Vias cont.Thermal Vias cont. Large number of Vias-QLarge number of Vias-Qzzzz model to determine thermal model to determine thermal

conductivity: kconductivity: kzzzz=k=kMMaaMM + k + k11(1 – a(1 – aMM)) kkM M & k& k11 are the thermal conductivity of the metal and insulator are the thermal conductivity of the metal and insulator

and aand aM M is the fraction of cross-sectional conductivity in Z-is the fraction of cross-sectional conductivity in Z-

directiondirection

Sparse amt. of vias-QSparse amt. of vias-Qxyz xyz model:model:

““In-plane” thermal conductivity to first approximation-In-plane” thermal conductivity to first approximation-combination of vias may be neglectedcombination of vias may be neglected

1

)1(1

k

t

k

tk

M

M

Mxyz

Thermal ViasThermal ViasVIAVIA

PCB design-pad with plated hole that connects PCB design-pad with plated hole that connects copper tracks from one layer of the board to other copper tracks from one layer of the board to other layerslayers

Help to reduce resistance in heat flowHelp to reduce resistance in heat flowExamine thermal conductivity both analytically Examine thermal conductivity both analytically

and experimentallyand experimentally

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Thermal Vias cont.Thermal Vias cont.Trace layersTrace layers

Can help to transport heat to the edges of the boardCan help to transport heat to the edges of the boardFinite Element model simulationFinite Element model simulation

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Flotherm-3D computational fluid dynamics Flotherm-3D computational fluid dynamics softwaresoftwarePredicts airflow and heat transfer in electronic Predicts airflow and heat transfer in electronic

modelsmodelsConduction, convection and radiationConduction, convection and radiation

6.6 Electronic Cooling Methods6.6 Electronic Cooling MethodsFlowthermFlowtherm

Model used for Covidien’s ERT projectModel used for Covidien’s ERT projectSensor moduleSensor module

Completely EM shieldedCompletely EM shielded

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Heat Pipe CoolingHeat Pipe CoolingThermal transport device uses phase change Thermal transport device uses phase change

processes and vapor diffusion to transfer processes and vapor diffusion to transfer large quantities of heat over substantial large quantities of heat over substantial distances with no moving parts and constant distances with no moving parts and constant temptemp

Use is increasing especially in laptopsUse is increasing especially in laptopsHigh effective thermal conductivity of heat pipe at High effective thermal conductivity of heat pipe at

low weightlow weight

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Heat Pipe Cooling contHeat Pipe Cooling cont3 sections3 sections

Evaporator-heat absorbed and fluid vaporizedEvaporator-heat absorbed and fluid vaporizedCondenser-vapor condensed and heat rejectedCondenser-vapor condensed and heat rejectedAdiabatic-vapor and the liquid phases of the fluid Adiabatic-vapor and the liquid phases of the fluid

flow in opposite directions through the cork and flow in opposite directions through the cork and wickwick

Heat Pipe CoolingHeat Pipe CoolingMost cylindrical in shapeMost cylindrical in shape

Variety of shapes possibleVariety of shapes possible Right angle bends, S-turns, spirals…Right angle bends, S-turns, spirals… .3cm minimum thickness.3cm minimum thickness

ConcernsConcernsDegradation over timeDegradation over time

Some fail just after a few months operationSome fail just after a few months operation Contamination and trapping of air that occur during Contamination and trapping of air that occur during

fabrication processfabrication process

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Jet Impingement CoolingJet Impingement CoolingUsed when high convective heat transfer Used when high convective heat transfer

rates requiredrates requiredFor unpinned heat sink, the multiple jets yield For unpinned heat sink, the multiple jets yield

higher convective coefficients that single jet higher convective coefficients that single jet by a factor of 1.2by a factor of 1.2In presence of pins, almost no difference is seenIn presence of pins, almost no difference is seen

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Immersion CoolingImmersion CoolingDates back to 1940’sDates back to 1940’s

Mid 80’s- used in Cray 2 and ETA010 Mid 80’s- used in Cray 2 and ETA010 supercomputerssupercomputers

Well suited to cooling of advanced electronics Well suited to cooling of advanced electronics under developmentunder development

Operate in closed loopOperate in closed loop

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Immersion CoolingImmersion Cooling

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Immersion CoolingImmersion Cooling

6.6 Electronic Cooling Methods 6.6 Electronic Cooling Methods cont.cont.

Thermoelectric CoolingThermoelectric Cooling TEC-Thermal electric cooler-solid state heat pumpTEC-Thermal electric cooler-solid state heat pump

Potential placed across 2 junctions-heat absorbed into one Potential placed across 2 junctions-heat absorbed into one junction and expelled from anotherjunction and expelled from another

Most obvious in P-N junctionsMost obvious in P-N junctions e- transported from p-side to n-side, transported to higher e- transported from p-side to n-side, transported to higher

energy state and absorb heat thus cooling surrounding areaenergy state and absorb heat thus cooling surrounding area From n-side to p-side they release heatFrom n-side to p-side they release heat

Common materials- bismuth telluride, lead telluride, and Common materials- bismuth telluride, lead telluride, and silicon germaniumsilicon germanium

Selected from performance and COP (coefficient of Selected from performance and COP (coefficient of performance) curvesperformance) curves