Intel Pentium 4 Processor In the 423-pin Package Thermal Design Guidelines

Pentium® 4 processor in the 423-pin package Thermal Design Guidelines
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5.5 Thermal Interface Management
To optimize the heat sink design for the Pentium® 4 processor, it is important to understand the impact of factors
related to the interface between the processor and the heat sink. Specifically, the bond line thickness, interface
material area and interface material thermal conductivity should be selected to realize the most effective thermal
solution.
5.5.1 Bond Line Management
Any gap between the processor’s heat spreader and the heat sink base will degrade 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 heat sink base and the integrated heat spreader, plus the thickness of the
thermal interface material (i.e. thermal grease) used between these two surfaces and the clamping force applied by
the heat sink attach clip(s).
5.5.2 Interface Material Area
The size of the contact area between the processor and the heat sink 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. The Pentium 4 processor in the 31mm package
has an IHS surface area of 1.13 square inches (7.29 sq. cm).
5.5.3 Interface Material Performance
Two factors impact the performance of the interface material between the processor and the heat sink 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 material is at transferring heat. The thermal
resistance of the interface material has a significant impact on thermal performance. The higher the thermal
resistance, the larger the temperature drop is across the interface and the more efficient the thermal solution (i.e. heat
sink) 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 heat
sink attach clips, to spread and fill the gap between the processor and the heat sink. 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. 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.
Intel has determined through thermal characterization that it may be challenging to meet the thermal performance
targets with the use of Phase Change thermal interface materials. The use of Thermal Grease in conjunction with
high performance heat sink technologies (e.g. copper base folded fin or high aspect ratio extruded aluminum with
high performance attached fans) has been demonstrated to meet the thermal performance requirements.
The use of thermal grease is recommended. Intel’s thermal solution reference designs uses ShinEtsu* G749
Thermal Grease.