MPC7455 datasheet(55/64 Pages) FREESCALE

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MPC7455 RISC Microprocessor Hardware Specifications, Rev. 4.1

Freescale Semiconductor

System Design Information

Wakefield Engineering

603-635-5102

33 Bridge St.

Pelham, NH 03076

Internet: www.wakefield.com

Ultimately, the final selection of an appropriate heat sink depends on many factors, such as thermal performance at

a given air velocity, spatial volume, mass, attachment method, assembly, and cost.

9.8.1 Internal Package Conduction Resistance

For the exposed-die packaging technology, shown in Table 3, the intrinsic conduction thermal resistance paths are

as follows:

•

The die junction-to-case (actually top-of-die since silicon die is exposed) thermal resistance

•

The die junction-to-ball thermal resistance

Figure 28 depicts the primary heat transfer path for a package with an attached heat sink mounted to a printed-circuit

board.

Figure 28. 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 finally to the heat sink where it is removed by forced-air convection.

Because the silicon thermal resistance is quite small, for a first-order analysis, the temperature drop in the silicon

may be neglected. Thus, the thermal interface material and the heat sink conduction/convective thermal resistances

are the dominant terms.

9.8.2 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 29 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 contact pressure. The use of thermal grease significantly reduces the

External Resistance

Internal Resistance

Radiation

Convection

Radiation

Convection

Heat Sink

Printed-Circuit Board

Thermal Interface Material

Package/Leads

Die Junction

Die/Package

(Note the internal versus external package resistance.)

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