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Thermal Interface Materials. Thermal Management of Electronics San Jos é 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

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thermal interface materials

Thermal Interface Materials

Thermal Management of Electronics

San José State University

Mechanical Engineering Department

thermal interface contact resistance
Thermal Interface (Contact) Resistance
  • 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
  • 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
Unacceptable Methods of Reducing Interface Resistance for Electronics
  • Increase contact pressure – This will even out peaks and valley and flatten distorted surfaces
    • Pressures will usually exceed load constraint for electronic components
  • 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
thermal interface materials4
Thermal Interface Materials
  • Thermal Interface Materials (TIM) – A material with high thermal conductivity that can conform to the imperfect mating surfaces
  • Thermal Interface Materials (TIMs):
    • Greases
    • Phase change materials
    • Soft metal foils
    • Elastomer paste
    • Adhesives
the ideal tim
The Ideal TIM
  • High thermal conductivity
  • Easily deformed by small contact pressures
  • Minimal thickness
  • No leakage from the interface
  • No deterioration over time
  • Non-toxic
  • Easy to apply/remove

Gwinn and Webb, pg. 217

the actual tim
The Actual TIM
  • Gaps will not be completely filled, leaving some air pockets
  • Some leakage may occur
  • Performance may deteriorate over time
  • Not always manufacturing friendly

Gwinn and Webb, pg. 217

the actual tim7
The Actual TIM

Rint– Thermal Interface Resistance

Rcontact1 – Thermal Interface Resistance between the TIM and CPU

Rcontact2 - Thermal Interface Resistance between the TIM and Heat Sink

Rcond – thermal conductivity of the TIM

t – thickness between the CPU and Heat Sink

Gwinn and Webb, pg. 217

unsuitable tims for electronics
Unsuitable TIMs for Electronics
  • Soft metal foil
    • Unacceptable contact pressures
  • Elastomer paste
    • Only moderately effective in decreasing thermal interface resistance
  • Thermally conductive adhesive
    • Slightly better than no TIM at all
acceptable tims
Acceptable TIMS
  • Thermal Grease
    • A thermally conductive filler dispersed in silicone or a hydrocarbon oil to form a paste
  • 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.
    • Phase change does not actually occur; viscosity decreases as temperature increases so that they flow and act more like a grease
thermal greases
Advantages

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

Recommended TIM for most electronics

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

Thermal Greases

Arctic Silver is a common thermal grease.

slide11
Advantages

High thermal performance at moderate contact pressures

Material flows to provide contact between surfaces and fill air gaps

Viscosity prevents leakage into surroundings

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

PCMs
commercially available tim
Commercially Available TIM

Gwinn and Webb, pg. 219

tim test methods
TIM Test Methods

Zhou, pg. 7

tim test methods14
TIM Test Methods

Zhou, pg. 7

tim test methods15
TIM Test Methods

Zhou, pg. 7

references
References
  • Cengel, Yunus A. Heat Transfer: A Practical Approach. 2nd edition. New York, NY: McGraw Hill. 2004
  • Gwinn, J.P. and Webb, R.L. “Performance and testing of thermal interface materials”.Microelectronics Journal. Issue 34. 2003
  • Zhou, Ni. “A Thermal Interface Material Characterization Test Apparatus”. Master of Science Project. SJSU Mechanical Engineering Department. 2005