MPC7400 RISC Microprocessor Hardware Specications
39
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
System Design Information
Figure 24 depicts the primary heat transfer path for a package with an attached heat sink mounted to a
printed-circuit board.
Figure 24. C4 Package with Heat Sink Mounted to a Printed-Circuit Board
Heat generated on the active side of the chip is conducted through the silicon, then through the heat sink
attach material (or thermal interface material), and nally to the heat sink where it is removed by forced-air
convection.
Since the silicon thermal resistance is quite small, for a rst-order analysis, the temperature drop in the
silicon may be neglected. Thus, the heat sink attach material and the heat sink conduction/convective
thermal resistances are the dominant terms.
1.8.9.2 Adhesives and Thermal Interface Materials
A thermal interface material is recommended at the package lid-to-heat sink interface to minimize the
thermal contact resistance. For those applications where the heat sink is attached by spring clip mechanism,
Figure 25 shows the thermal performance of three thin-sheet thermal-interface materials (silicone, graphite/
oil, oroether 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 contact pressure. The use of
thermal grease signicantly reduces the interface thermal resistance. That is, the bare joint results in a
thermal resistance approximately 7 times greater than the thermal grease joint.
Heat sinks are attached to the package by means of a spring clip to holes in the printed-circuit board (see
Figure 24). This spring force should not exceed 10 pounds of force. Therefore, the synthetic grease offers
the best thermal performance, considering the low interface pressure. Of course, the selection of any thermal
interface material depends on many factors—thermal performance requirements, manufacturability, service
temperature, dielectric properties, cost, etc.
External Resistance
Internal Resistance
(Note the internal versus external package resistance)
Radiation
Convection
Radiation
Convection
Heat Sink
Printed-Circuit Board
Thermal Interface Material
Package/Leads
Die Junction
Die/Package
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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