Thermal Interface Materials Thermal Management of Electronics San José State University Mechanical Engineering Department

Thermal Interface (Contact) Resistance Thermal contact resistance is a measure of how well heat is transferred across the interface of two mating rigid surfaces Thermal contact resistance is a measure of how well heat is transferred across the interface of two mating rigid surfaces Peaks and valleys form gaps between the two materials Peaks and valleys form gaps between the two materials Convex, concave, or wave like surfaces may result in large voids between surfaces Convex, concave, or wave like surfaces may result in large voids between surfaces Gwinn and Webb, pg. 215

Unacceptable Methods of Reducing Interface Resistance for Electronics Increase contact pressure – This will even out peaks and valley and flatten distorted surfaces Increase contact pressure – This will even out peaks and valley and flatten distorted surfaces Pressures will usually exceed load constraint for electronic components Pressures will usually exceed load constraint for electronic components Grinding or buffing surfaces – This will remove roughness from the surfaces and flatten warped surfaces Grinding or buffing surfaces – This will remove roughness from the surfaces and flatten warped surfaces Manufacturing techniques for highly finished surfaces are not cost effective Manufacturing techniques for highly finished surfaces are not cost effective

Thermal Interface Materials Thermal Interface Materials (TIM) – A material with high thermal conductivity that can conform to the imperfect mating surfaces Thermal Interface Materials (TIM) – A material with high thermal conductivity that can conform to the imperfect mating surfaces Thermal Interface Materials (TIMs): Thermal Interface Materials (TIMs): Greases Greases Phase change materials Phase change materials Soft metal foils Soft metal foils Elastomer paste Elastomer paste Adhesives Adhesives

The Ideal TIM High thermal conductivity High thermal conductivity Easily deformed by small contact pressures Easily deformed by small contact pressures Minimal thickness Minimal thickness No leakage from the interface No leakage from the interface No deterioration over time No deterioration over time Non-toxic Non-toxic Easy to apply/remove Easy to apply/remove Gwinn and Webb, pg. 217

The Actual TIM Gaps will not be completely filled, leaving some air pockets Gaps will not be completely filled, leaving some air pockets Some leakage may occur Some leakage may occur Performance may deteriorate over time Performance may deteriorate over time Not always manufacturing friendly Not always manufacturing friendly Gwinn and Webb, pg. 217

The Actual TIM Gwinn and Webb, pg. 217 R int – Thermal Interface Resistance R contact1 – Thermal Interface Resistance between the TIM and CPU R contact2 - Thermal Interface Resistance between the TIM and Heat Sink R cond – thermal conductivity of the TIM t – thickness between the CPU and Heat Sink

Unsuitable TIMs for Electronics Soft metal foil Soft metal foil Unacceptable contact pressures Unacceptable contact pressures Elastomer paste Elastomer paste Only moderately effective in decreasing thermal interface resistance Only moderately effective in decreasing thermal interface resistance Thermally conductive adhesive Thermally conductive adhesive Slightly better than no TIM at all Slightly better than no TIM at all

Acceptable TIMS Thermal Grease Thermal Grease A thermally conductive filler dispersed in silicone or a hydrocarbon oil to form a paste A thermally conductive filler dispersed in silicone or a hydrocarbon oil to form a paste Phase Changing Materials (PCMs) – Phase Changing Materials (PCMs) – A mixture of thermally conductive particles suspended in a base material like a fully refined paraffin, a polymer, a co-polymer, ect. A mixture of thermally conductive particles suspended in a base material like a fully refined paraffin, a polymer, a co-polymer, ect. Phase change does not actually occur; viscosity decreases as temperature increases so that they flow and act more like a grease Phase change does not actually occur; viscosity decreases as temperature increases so that they flow and act more like a grease

Thermal Greases Advantages Advantages Over 100 times more effective than air Over 100 times more effective than air With little pressure the surfaces to come into contact with each other while filling most of the air gaps With little pressure the surfaces to come into contact with each other while filling most of the air gaps Recommended TIM for most electronics Recommended TIM for most electronics Disadvantages Disadvantages It is messy and difficult to apply and remove Excess grease may leak into the surroundings and cause a short Joints may dry out over time Arctic Silver is a common thermal grease.

PCMs Advantages Advantages High thermal performance at moderate contact pressures High thermal performance at moderate contact pressures Material flows to provide contact between surfaces and fill air gaps Material flows to provide contact between surfaces and fill air gaps Viscosity prevents leakage into surroundings Viscosity prevents leakage into surroundings Disadvantages Disadvantages Moderate contact pressures are required to bring surfaces together Lower thermal conductivities than thermal greases Strong adhesive bond between surfaces may produce a load that damages the electronic during shock loading or a drop

Commercially Available TIM Gwinn and Webb, pg. 219

TIM Test Methods Zhou, pg. 7

TIM Test Methods Zhou, pg. 7

TIM Test Methods Zhou, pg. 7

References Cengel, Yunus A. Heat Transfer: A Practical Approach. 2 nd edition. New York, NY: McGraw Hill Cengel, Yunus A. Heat Transfer: A Practical Approach. 2 nd edition. New York, NY: McGraw Hill Gwinn, J.P. and Webb, R.L. “Performance and testing of thermal interface materials”. Microelectronics Journal. Issue Gwinn, J.P. and Webb, R.L. “Performance and testing of thermal interface materials”. Microelectronics Journal. Issue Zhou, Ni. “A Thermal Interface Material Characterization Test Apparatus”. Master of Science Project. SJSU Mechanical Engineering Department Zhou, Ni. “A Thermal Interface Material Characterization Test Apparatus”. Master of Science Project. SJSU Mechanical Engineering Department. 2005