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

Appendix A: Thermal Interface Management

Intel

Pentium

4 Processor Thermal Design Guide 59

Appendix A: 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.

Bond Line Management

Any gap between the processor integrated heat spreader (IHS) and the heatsink base will impact

thermal solution performance. The larger the gap between the two surfaces, the greater the thermal

characterization parameter. 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).

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.

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 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.