Thermal Interface Materials

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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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.

11. 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

12. Commercially Available TIM Gwinn and Webb, pg. 219

13. TIM Test Methods Zhou, pg. 7

14. TIM Test Methods Zhou, pg. 7

15. TIM Test Methods Zhou, pg. 7

16. 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