Thermal Interface Management
Thermal and Mechanical Design Guidelines 75
Appendix C Thermal Interface
Management
To optimize a heatsink design, it is important to understand the impact of factors
related to the interface between the processor and the heatsink base. Specifically, the
bond line thickness, interface material area and interface material thermal
conductivity should be managed to realize the most effective thermal solution.
C.1 Bond Line Management
Any gap between the processor integrated heat spreader (IHS) and the heatsink base
degrades thermal solution performance. The larger the gap between the two surfaces,
the greater the thermal resistance. The thickness of the gap is determined by the
flatness and roughness of both the heatsink base and the integrated heat spreader,
plus the thickness of the thermal interface material (for example thermal grease) used
between these two surfaces and the clamping force applied by the heatsink attach
clip(s).
C.2 Interface Material Area
The size of the contact area between the processor and the heatsink base will impact
the thermal resistance. There is, however, a point of diminishing returns.
Unrestrained incremental increases in thermal interface material area do not translate
to a measurable improvement in thermal performance.
C.3 Interface Material Performance
Two factors impact the performance of the interface material between the processor
and the heatsink base:
•
Thermal resistance of the material
• Wetting/filling characteristics of the material
Thermal resistance is a description of the ability of the thermal interface material to
transfer heat from one surface to another. The higher the thermal resistance, the less
efficient the interface material is at transferring heat. The thermal resistance of the
interface material has a significant impact on the thermal performance of the overall
thermal solution. The higher the thermal resistance, the larger the temperature drop
is across the interface and the more efficient the thermal solution (heatsink, fan) must
be to achieve the desired cooling.
The wetting or filling characteristic of the thermal interface material is its ability,
under the load applied by the heatsink retention mechanism, to spread and fill the gap
between the processor and the heatsink. Since air is an extremely poor thermal
conductor, the more completely the interface material fills the gaps, the lower the
temperature drops across the interface. In this case, thermal interface material area
also becomes significant; the larger the desired thermal interface material area, the
higher the force required to spread the thermal interface material.
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