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5.5.1.
Bond Line Thickness
Any gap between the processor's heat spreader and the heatsink base will impact 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 of both the heatsink base and the integrated heat
spreader, the thickness of the thermal interface material (i.e. thermal grease) used between these two
surfaces, and the clamping force applied by the heatsink retention clip(s). In comparison with FC-PGA
packaged processors, the addition of the IHS will distribute the vertical force created by the attach
clip(s). For the same applied force (e.g. the same heatsink attach clip configuration), this will result in
lower pressure applied to the thermal interface material. This will lead to an increase in thermal interface
bond line thickness and thermal resistance.
5.5.2.
Interface Material Area
The size of the contact area between the integrated heat spreader and the heatsink base will impact the
thermal resistance; increasing contact area typically decreases thermal resistance. It is recommended that
the thermal interface material should cover the entire surface of the integrated heat spreader area (31mm
x 31mm) to optimize the thermal interface performance on FC-PGA2 packaged processors.
5.5.3.
Interface Material Performance
Two factors impact the performance of the interface material between the processor and the heatsink
base:
1.
Thermal resistance of the material
2.
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 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 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 drop across the interface. Thermal interface material area-size also
becomes significant, as the larger the desired area-size, the higher the force required to spread the
thermal interface material.
5.6.
Fans
Fans are needed to move the air through the chassis and/or directly through the processor heatsink. The
airflow rate of a fan is usually directly related to the acoustic noise level of the fan and system.
Maximum acceptable noise levels may limit the fan output or the number of fans selected for a system.
The maximum noise levels may differ from country to country. Fan/heatsink assemblies are one type of
advanced solution that can be used to cool the processor. Intel has worked with fan/heatsink vendors and
computer manufacturers to make fan/heatsink cooling solutions available in the industry. Please consult
such a vendor to acquire the proper solution for your needs.
249660-001
®
®
Intel
Pentium
III Processor in the FC-PGA2 Package Thermal Design Guidelines
17
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