Heat Sink Heat Sink SelectionSelection

Thermal Management of Thermal Management of ElectronicsElectronics

San JosSan Joséé State University State UniversityMechanical Engineering Mechanical Engineering

DepartmentDepartment

Heat Sink CategoriesHeat Sink Categories

Passive Heat SinksPassive Heat Sinks– Used in natural convection Used in natural convection

applications or in applications or in applications where heat applications where heat dissipation is not dissipation is not dependent on designated dependent on designated supply of air flowssupply of air flows

– Height at heat input: Height at heat input: ~10mm to large~10mm to large

– Normal load limit: 5-50 WNormal load limit: 5-50 W– Cost range at 10,000 Cost range at 10,000

pieces: $0.50-$10.00pieces: $0.50-$10.00

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Heat Sink CategoriesHeat Sink Categories Semi-Active Heat SinksSemi-Active Heat Sinks

– Leverage off of existing fans in the systemLeverage off of existing fans in the system

– Usually a passive heat sink set in front of a fan Usually a passive heat sink set in front of a fan to produce impingement or vertical flowto produce impingement or vertical flow

– Height at heat input: ~10mmHeight at heat input: ~10mm

– Normal load limit: 15-25 WNormal load limit: 15-25 W

– Cost range at 10,000 pieces: $5.00-$10.00Cost range at 10,000 pieces: $5.00-$10.00

Heat Sink CategoriesHeat Sink Categories

Active Heat SinksActive Heat Sinks– Fans are designated for Fans are designated for

its own useits own use– Reliability is dependent Reliability is dependent

on moving partson moving parts– Height at heat input: 35-Height at heat input: 35-

80mm80mm– Normal load limit: 10-Normal load limit: 10-

160W160W– Cost range at 10,000 Cost range at 10,000

pieces: $10.00-$20.00pieces: $10.00-$20.00– One big negative: if your One big negative: if your

fan dies, you need to fan dies, you need to replace the entire unitreplace the entire unit

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Heat Sink TypesHeat Sink Types StampingsStampings

– Copper or aluminum sheets stamped into desired Copper or aluminum sheets stamped into desired shapesshapes

– Low cost solution to low density thermal problemsLow cost solution to low density thermal problems CastingCasting

– High density aluminum or copper/bronze pin fins are High density aluminum or copper/bronze pin fins are produced with sand, lost core, or die casting (May or produced with sand, lost core, or die casting (May or may not be vacuum assisted)may not be vacuum assisted)

– Allows for maximum performance with impingement Allows for maximum performance with impingement coolingcooling

– More expensive but can make odd shapesMore expensive but can make odd shapes– Al alloys used have lower conductivity than Al alloys Al alloys used have lower conductivity than Al alloys

used for extrusionsused for extrusions

Heat Sink TypesHeat Sink Types

ExtrusionsExtrusions– Allows for the formation of elaborate 2-D Allows for the formation of elaborate 2-D

shapes capable of dissipating large heat shapes capable of dissipating large heat loadsloads

– Cross-cutting to produce rectangular pins Cross-cutting to produce rectangular pins may increase performance by 10-20% but may increase performance by 10-20% but the extrusion rate will be slowerthe extrusion rate will be slower

– Design limits are usually set by Design limits are usually set by Fin height-to-gap aspect ratioFin height-to-gap aspect ratio Minimum fin thickness-to-heightMinimum fin thickness-to-height Maximum base to fin thicknessMaximum base to fin thickness

Heat Sink TypesHeat Sink Types Bonded/Fabricated FinsBonded/Fabricated Fins

– Thermally conductive aluminum filled epoxy is Thermally conductive aluminum filled epoxy is used to bond planar fins on a grooved extrusion used to bond planar fins on a grooved extrusion plateplate

– Greater fin height-to-gap aspect ratio is achieved Greater fin height-to-gap aspect ratio is achieved with this method (~20-40)with this method (~20-40)

– Cooling capacity is increased without increasing Cooling capacity is increased without increasing volume requirementsvolume requirements

– But epoxy has a much lower thermal But epoxy has a much lower thermal conductivity, acting as an increased resistance.conductivity, acting as an increased resistance.

Heat Sink TypesHeat Sink Types Folded FinsFolded Fins

– Aluminum or copper sheet metal is folded into Aluminum or copper sheet metal is folded into fins and then attached to a base plate or fins and then attached to a base plate or directly to the heat surface via brazing or directly to the heat surface via brazing or epoxyingepoxying

– Due to the availability and fin efficiency folded Due to the availability and fin efficiency folded fins are not suitable for high profile heat sinksfins are not suitable for high profile heat sinks

– Allows for the fabrication of high performance Allows for the fabrication of high performance heat sinks when extrusion or bonded fins are heat sinks when extrusion or bonded fins are unacceptableunacceptable

Heat Sink GeometriesHeat Sink Geometries Rectangular fins have better performance than Rectangular fins have better performance than

square fins, whose back edges have poor air flow square fins, whose back edges have poor air flow past them. past them.

Rectangular fins also have better performance Rectangular fins also have better performance than round fins; however, pressure drop is also than round fins; however, pressure drop is also higher for rectangular fins.higher for rectangular fins.

Round fins are good if you don’t know which Round fins are good if you don’t know which direction your airflow will be from or if airflow direction your airflow will be from or if airflow may not be straight through the heat sink (it’s may not be straight through the heat sink (it’s omni-directional).omni-directional).

For natural convection/radiation, solid black For natural convection/radiation, solid black anodized fins mounted in a vertical direction tend anodized fins mounted in a vertical direction tend to work best.to work best.

Heat Sink Materials and Heat Sink Materials and FinishesFinishes

Aluminum (6063 or 6061) is most Aluminum (6063 or 6061) is most common, followed by copper (which is common, followed by copper (which is 4-6x more expensive, 3x as heavy, by 4-6x more expensive, 3x as heavy, by has 2x the conductivity)has 2x the conductivity)

It is difficult to alter the surface of It is difficult to alter the surface of copper to improve radiation.copper to improve radiation.

External finish of aluminum is usually External finish of aluminum is usually anodize or chromate of various colors.anodize or chromate of various colors.

Design ConsiderationsDesign Considerations

The following cases always increase The following cases always increase thermal performance - True or False?thermal performance - True or False?

– Longer fin heightsLonger fin heights

– Longer heat sinks in the direction of air Longer heat sinks in the direction of air flowflow

– Increased number of finsIncreased number of fins

Design considerationsDesign considerations

All of the cases are falseAll of the cases are false

– Longer fin heights mean increased surface Longer fin heights mean increased surface area but with a fixed volumetric flow rate area but with a fixed volumetric flow rate performance may actually decrease with fin performance may actually decrease with fin heightheight

– Longer heat sinks in the direction of flow and Longer heat sinks in the direction of flow and more fins both mean increased surface area more fins both mean increased surface area but both have adverse affects on pressure but both have adverse affects on pressure drops and flow bypass, and the average heat drops and flow bypass, and the average heat transfer coefficient goes downtransfer coefficient goes down

Flow BypassFlow Bypass

Flow bypass occurs when the flow Flow bypass occurs when the flow duct is larger than the cross sectional duct is larger than the cross sectional dimensions of the heat sinkdimensions of the heat sink

Velocity of the approaching fluid may Velocity of the approaching fluid may be much greater than the velocity be much greater than the velocity through the channelsthrough the channels

Most commonly occurs with semi-Most commonly occurs with semi-active heat sinksactive heat sinks

Flow BypassFlow Bypass

The amount of The amount of flow bypass will flow bypass will be greatly be greatly dependent on the dependent on the cross sectional cross sectional geometry and the geometry and the pressure drop pressure drop across the heat across the heat sinksink

Lee, S. Lee, S. “Optimum Design and Selection of “Optimum Design and Selection of Heat Sinks”Heat Sinks”. Pg. 815. Pg. 815

Radiation in Heat SinksRadiation in Heat Sinks Radiation in heat sinks Radiation in heat sinks

is usually accounted for is usually accounted for with an open-sided U-with an open-sided U-shaped channelshaped channel

3 surfaces are 3 surfaces are considered opaque and considered opaque and the other 3 are the other 3 are transparenttransparent

The 3 opaque surfaces The 3 opaque surfaces should be analyzed for should be analyzed for the amount of energy the amount of energy that is emitted, that is emitted, absorbed, and reflected absorbed, and reflected

Kraus, A. Pg. Kraus, A. Pg. 292292

Thermal Performance Thermal Performance GraphsGraphs

Provided to show the thermal Provided to show the thermal performance of a heat sink under performance of a heat sink under ideal natural and force convection ideal natural and force convection conditionsconditions

Ideal conditions mean:Ideal conditions mean:– Mounted to a heat source the same size Mounted to a heat source the same size

of the heat sinkof the heat sink– At sea levelAt sea level– Mounted with the fins in the direction of Mounted with the fins in the direction of

air flowair flow– The heat sink is painted or anodized blackThe heat sink is painted or anodized black– Zero Flow BypassZero Flow Bypass

Thermal Performance Thermal Performance GraphsGraphs

Natural Convection Natural Convection CurveCurve– Extends from the lower Extends from the lower

left to the upper rightleft to the upper right– Temperature rise vs. QTemperature rise vs. Q

Forced Convection Forced Convection CurveCurve– Extends fro them upper Extends fro them upper

left to the lower rightleft to the lower right– Thermal resistance Thermal resistance

versus air velocityversus air velocityLee, S. Lee, S. How to Select a Heat SinkHow to Select a Heat Sink. . Pg. 5Pg. 5

Attachment MeansAttachment Means

Clip to deviceClip to device Clip to PCBClip to PCB Snap-on stampingsSnap-on stampings Double-sided tapesDouble-sided tapes Solder or adhesivesSolder or adhesives

Thermal interface resistance minimized Thermal interface resistance minimized using grease or pad and by making using grease or pad and by making surfaces as flat as possiblesurfaces as flat as possible

ReferencesReferences Lee, Serri. Lee, Serri. “Optimum Design and Selection of “Optimum Design and Selection of

Heat Sinks”. IEEE Transactions on Components, Heat Sinks”. IEEE Transactions on Components, Packaging, and ManufacturingPackaging, and Manufacturing. Part A, Vol. 18, . Part A, Vol. 18, No. 4. December 1995.No. 4. December 1995.

Lee, Serri. Lee, Serri. How to Select a Heat SinkHow to Select a Heat Sink. Aavid . Aavid Thermal Technologies, Inc. Laconia, New Thermal Technologies, Inc. Laconia, New Hampshire.Hampshire.

Kraus, A. Kraus, A. The Heat Sink Design ProcedureThe Heat Sink Design Procedure..

Markstein, H., “Optimizing Heat Sink Markstein, H., “Optimizing Heat Sink Performance”, Electronic Packaging and Performance”, Electronic Packaging and Production, v 35, n 10, Sept 1995, p. 38.Production, v 35, n 10, Sept 1995, p. 38.