Thermal Interface Materials Thermal Management of Electronics San José State University Mechanical...

Thermal Interface Thermal Interface MaterialsMaterials

Thermal Management of Thermal Management of ElectronicsElectronics

San JosSan Joséé State University State UniversityMechanical Engineering Mechanical Engineering

DepartmentDepartment

Thermal Interface (Contact) Thermal Interface (Contact) ResistanceResistance

Thermal contact resistance is a measure of how well Thermal contact resistance is a measure of how well heat is transferred across the interface of two heat is transferred across the interface of two mating rigid surfacesmating rigid surfaces

Peaks and valleys form gaps between the two Peaks and valleys form gaps between the two materialsmaterials

Convex, concave, or wave like surfaces may result in Convex, concave, or wave like surfaces may result in large voids between surfaceslarge voids between surfaces

Gwinn and Webb, pg. 215

Unacceptable Methods of Unacceptable Methods of Reducing Interface Resistance Reducing Interface Resistance

for Electronicsfor Electronics Increase contact pressure – This will even Increase contact pressure – This will even

out peaks and valley and flatten distorted out peaks and valley and flatten distorted surfaces surfaces Pressures will usually exceed load constraint for Pressures will usually exceed load constraint for

electronic componentselectronic components

Grinding or buffing surfaces – This will Grinding or buffing surfaces – This will remove roughness from the surfaces and remove roughness from the surfaces and flatten warped surfacesflatten warped surfaces Manufacturing techniques for highly finished Manufacturing techniques for highly finished

surfaces are not cost effectivesurfaces are not cost effective

Thermal Interface MaterialsThermal Interface Materials

Thermal Interface Materials (TIM) – A Thermal Interface Materials (TIM) – A material with high thermal conductivity material with high thermal conductivity that can conform to the imperfect mating that can conform to the imperfect mating surfacessurfaces

Thermal Interface Materials (TIMs):Thermal Interface Materials (TIMs): GreasesGreases Phase change materialsPhase change materials Soft metal foilsSoft metal foils Elastomer pasteElastomer paste AdhesivesAdhesives

The Ideal TIMThe Ideal TIM High thermal conductivityHigh thermal conductivity Easily deformed by small contact pressures Easily deformed by small contact pressures Minimal thicknessMinimal thickness No leakage from the interfaceNo leakage from the interface No deterioration over timeNo deterioration over time Non-toxicNon-toxic Easy to apply/removeEasy to apply/remove


The Actual TIMThe Actual TIM Gaps will not be completely filled, Gaps will not be completely filled,

leaving some air pocketsleaving some air pockets Some leakage may occurSome leakage may occur Performance may deteriorate over timePerformance may deteriorate over time Not always manufacturing friendlyNot always manufacturing friendly


The Actual TIMThe Actual TIM


RRintint – Thermal Interface Resistance– Thermal Interface Resistance

RRcontact1contact1 – Thermal Interface – Thermal Interface Resistance between the TIM and Resistance between the TIM and CPUCPU

RRcontact2contact2 - Thermal Interface - Thermal Interface Resistance between the TIM and Resistance between the TIM and Heat SinkHeat Sink

RRcondcond – thermal conductivity of the – thermal conductivity of the TIMTIM

tt – thickness between the CPU and – thickness between the CPU and Heat SinkHeat Sink

Unsuitable TIMs for Unsuitable TIMs for ElectronicsElectronics

Soft metal foilSoft metal foil Unacceptable contact pressuresUnacceptable contact pressures

Elastomer pasteElastomer paste Only moderately effective in decreasing Only moderately effective in decreasing

thermal interface resistancethermal interface resistance

Thermally conductive adhesive Thermally conductive adhesive Slightly better than no TIM at allSlightly better than no TIM at all

Acceptable TIMSAcceptable TIMS

Thermal Grease Thermal Grease A thermally conductive filler dispersed in A thermally conductive filler dispersed in

silicone or a hydrocarbon oil to form a pastesilicone or a hydrocarbon oil to form a paste

Phase Changing Materials (PCMs) – Phase Changing Materials (PCMs) – A mixture of thermally conductive particles A mixture of thermally conductive particles

suspended in a base material like a fully suspended in a base material like a fully refined paraffin, a polymer, a co-polymer, ect.refined paraffin, a polymer, a co-polymer, ect.

Phase change does not actually occur; Phase change does not actually occur; viscosity decreases as temperature increases viscosity decreases as temperature increases so that they flow and act more like a greaseso that they flow and act more like a grease

Thermal GreasesThermal Greases

AdvantagesAdvantages Over 100 times more Over 100 times more

effective than aireffective than air With little pressure With little pressure

the surfaces to come the surfaces to come into contact with into contact with each other while each other while filling most of the air filling most of the air gapsgaps

Recommended TIM Recommended TIM for most electronicsfor most electronics

DisadvantagesDisadvantages It is messy and It is messy and

difficult to apply difficult to apply and removeand remove

Excess grease may Excess grease may leak into the leak into the surroundings and surroundings and cause a shortcause a short

Joints may dry out Joints may dry out over timeover time

Arctic Silver is a common thermal grease.

PCMsPCMs AdvantagesAdvantages

High thermal High thermal performance at performance at moderate contact moderate contact pressurespressures

Material flows to Material flows to provide contact provide contact between surfaces between surfaces and fill air gapsand fill air gaps

Viscosity prevents Viscosity prevents leakage into leakage into surroundingssurroundings

DisadvantagesDisadvantages Moderate contact Moderate contact

pressures are pressures are required to bring required to bring surfaces togethersurfaces together

Lower thermal Lower thermal conductivities than conductivities than thermal greasesthermal greases

Strong adhesive bond Strong adhesive bond between surfaces between surfaces may produce a load may produce a load that damages the that damages the electronic during electronic during shock loading or a shock loading or a dropdrop

Commercially Available TIMCommercially Available TIM


TIM Test MethodsTIM Test Methods

Zhou, pg. 7

ReferencesReferences

Cengel, Yunus A. Cengel, Yunus A. Heat Transfer: A Practical ApproachHeat Transfer: A Practical Approach. 2. 2ndnd edition. edition. New York, NY: McGraw Hill. 2004New York, NY: McGraw Hill. 2004

Gwinn, J.P. and Webb, R.L. Gwinn, J.P. and Webb, R.L. “Performance and testing of thermal “Performance and testing of thermal interface materials”.interface materials”. Microelectronics JournalMicroelectronics Journal. Issue 34. 2003. Issue 34. 2003

Zhou, Ni. Zhou, Ni. “A Thermal Interface Material Characterization Test “A Thermal Interface Material Characterization Test Apparatus”.Apparatus”. Master of Science Project. SJSU Mechanical Master of Science Project. SJSU Mechanical Engineering Department. 2005Engineering Department. 2005

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