PC7447A Datasheet, PDF (13/44 Pages) ATMEL Corporation – PowerPC RISC microprocessor

PC7447A [Preliminary]
Figure 6-3. Package Exploded Cross-sectional View with Several Heat Sink Options
Heat Sink
HCTE Package
Heat Sink
Clip
Thermal Interface
Material
Printed-Circuit Board
6.4.4
Thermal Interface Materials
A thermal interface material is recommended at the package lid-to-heat sink interface to mini-
mize the thermal contact resistance. For those applications where the heat sink is attached by
spring clip mechanism, Figure 6-4 on page 14 shows the thermal performance of three thin-
sheet thermal-interface materials (silicone, graphite/oil, floroether oil), a bare joint, and a joint
with thermal grease as a function of contact pressure.
As shown, the performance of these thermal interface materials improves with increasing con-
tact pressure.
The use of thermal grease significantly reduces the interface thermal resistance. That is, the
bare joint results in a thermal resistance approximately seven times greater than the thermal
grease joint.
Often, heat sinks are attached to the package by means of a spring clip to holes in the printed-
circuit board (see Figure 6-3 on page 13). Therefore, synthetic grease offers the best thermal
performance, considering the low interface pressure, and is recommended due to the high
power dissipation of the PC7447A. Of course, the selection of any thermal interface material
depends on many factors – thermal performance requirements, manufacturability, service tem-
perature, dielectric properties, cost, and so on.
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5387B–HIREL–07/05