Transcript Thermal Interface Materials

Thermal Interface
Materials
Thermal Management of Electronics
San José State University
Mechanical Engineering Department
Thermal Interface (Contact)
Resistance
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Thermal contact resistance is a measure of how well heat
is transferred across the interface of two mating rigid
surfaces
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Peaks and valleys form gaps between the two materials
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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
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Increase contact pressure – This will even out
peaks and valley and flatten distorted surfaces
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Pressures will usually exceed load constraint for
electronic components
Grinding or buffing surfaces – This will remove
roughness from the surfaces and flatten warped
surfaces
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Manufacturing techniques for highly finished surfaces
are not cost effective
Thermal Interface Materials
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Thermal Interface Materials (TIM) – A material
with high thermal conductivity that can conform
to the imperfect mating surfaces
Thermal Interface Materials (TIMs):
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Greases
Phase change materials
Soft metal foils
Elastomer paste
Adhesives
The Ideal TIM
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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
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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 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
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Soft metal foil
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Elastomer paste
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Unacceptable contact pressures
Only moderately effective in decreasing thermal
interface resistance
Thermally conductive adhesive
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Slightly better than no TIM at all
Acceptable TIMS
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Thermal Grease
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A thermally conductive filler dispersed in silicone or a
hydrocarbon oil to form a paste
Phase Changing Materials (PCMs) –
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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
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Advantages
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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
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Disadvantages
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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
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Advantages
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High thermal
performance at
moderate contact
pressures
Material flows to
provide contact
between surfaces and
fill air gaps
Viscosity prevents
leakage into
surroundings
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Disadvantages
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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
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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