Intel Pentium 4 Processor with 512-KB L2 Cache on 0.13 Micron Process Thermal Design Guidelines

Thermal Specifications
R
14 Intel
®
Pentium
®
4 Processor Thermal Design Guide
3.2 Designing a Cooling Solution for the Intel
®
Pentium
®
4 Processor with 512-KB L2 Cache on
0.13 Micron Process
3.2.1 Heatsink Design Considerations
To remove the heat from the processor, three basic parameters have to be considered:
• The extension of the surface on which the heat exchange takes place. Without any
additional enhancements, this is the surface of the processor package IHS. One method used
to improve thermal performance is to increase the surface area of the IHS by attaching a
heatsink to it. Heatsinks extend the heat exchange surface through the use of fins that can be
of various shapes and are attached to a heatsink base which is then in contact with the IHS.
• The conduction path from the heat source to the heatsink fins. Providing a direct
conduction path from the heat source to the heatsink fins and selecting materials with higher
thermal conductivity typically improve heatsink performance. The length, thickness, and
conductivity of the conduction path from the heat source to the fins directly impact the
thermal performance of the heatsink. In particular, the quality of the contact between the
package IHS and the heatsink base has higher impact on the overall cooling solution
performance as processor cooling requirements become stricter. Thermal interface material
(TIM) can be used to fill any gaps between the IHS and the bottom surface of the heatsink,
thereby improving the overall performance of the stack-up (IHS-TIM-Heatsink). Although,
with extremely poor heatsink interface flatness or roughness, TIM may not adequately fill the
gap. The TIM thermal performance depends on its thermal conductivity as well as the
pressure load applied to it. Refer to Appendix A for further information regarding managing
the bond line between the IHS and the heatsink base.
• The heat transfer conditions on the surface on which heat transfer takes place.
Convective heat transfer occurs between the airflow and the surface exposed to the flow. It is
characterized by the local ambient temperature of the air, T
A
, and the local air velocity over
the surface. The higher the air velocity and turbulence over the surface, and the cooler the air,
the more efficient is the resulting cooling. In the case of a heatsink, the surface exposed to the
flow includes the fin faces and the heatsink base.
Active heatsinks typically incorporate a fan that helps manage the airflow through the heatsink.
Passive heatsink solutions require in-depth knowledge of the airflow in the chassis. In addition,
they may see lower air speeds. These heatsinks are therefore typically larger (and heavier) than
active heatsinks due to the increase in fin surface area required to match thermal performance. As
the heatsink fin density (the number of fins in a given cross-section) increases, the resistance to the
airflow increases. It is more likely that the air will travel around the heatsink instead of through it,
unless air bypass is carefully managed. Using air-ducting techniques to manage bypass are an
effective method for controlling airflow through the heatsink.