MPC7457 datasheet(61/71 Pages) FREESCALE

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System Design Information

MPC7457 RISC Microprocessor Hardware Specifications, Rev. 8

Freescale Semiconductor

Tyco Electronics

800-522-6752

Chip Coolers™

P.O. Box 3668

Harrisburg, PA 17105-3668

Internet: www.chipcoolers.com

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 5, 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, 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.

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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