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Thermal Interfaces
13
20092B/0—Sep1996
AMD-K5 Processor Thermal Considerations
Thermal Interfaces
Thermal interface between the case and heat sink is controlled
in a variety of ways using different heat conducting materials.
The interface resistance between the case and the heat sink is
dependent on three variables: p (the thermal resistance of the
interface material in units of (
o
C · inch
2
) / (watts · thickness in
inches)), t (the average material thickness in inches), and A
(the area of contact in square inches). These variables are
related in the following equation:
Equation 5
θ
cs
= (p · t ) / A
Table 2 contains typical thermal resistance values for materi-
als used in cooling solutions for semiconductors.
The thermal interface material is placed between the top of
the AMD-K5 processor case and the bottom plate of the heat
sink. It is recommended that the heat sink plate have a flatness
tolerance of 0.002
′′
to 0.003
′′
per inch. The thickness of the
thermal interface material should be minimized to obtain the
lowest possible thermal resistance. The thermal grease com-
pounds are the best materials for the thermal interface, fol-
lowed by thermal compounds, and then thermal adhesive
Table 2. Thermal Conductivity Values for Materials Used In Cooling
Solutions
Thermal Interface Materials
Thermal Conductivity
(Watts / (Meter ·
o
C)
389
200
220
240
60.5
21
0.5-1.0
0.2
0.15
0.5-1.0
0.026
Copper (pure)
Aluminum (1100 series)
Aluminum (6000 series)
Beryllia
Carbon Steel
Alumina
Anodized Finish
Silicone RTV
Polyimide
Silicone Grease
Dead Air