MPC8610EC datasheet(86/96 Pages) FREESCALE

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MPC8610 Integrated Host Processor Hardware Specifications, Rev. 0

Hardware Design Considerations

Freescale Semiconductor

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.

3.14.1

Internal Package Conduction Resistance

For the exposed-die packaging technology described in Table 62, the intrinsic conduction thermal resistance paths are as

follows:

•

The die junction-to-case thermal resistance

•

The die junction-to-board thermal resistance

Figure 55 depicts the primary heat transfer path for a package with an attached heat sink mounted to a printed-circuit board.

Figure 55. C4 Package with Heat Sink Mounted to a Printed-Circuit Board

The heat sink removes most of the heat from the device. 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. The

junction-to-case thermal resistance is low enough that the heat sink attach material and heat sink thermal resistance are the

dominant terms.

3.14.2

Thermal Interface Materials

A thermal interface material is recommended at the package-to-heat sink interface to minimize the thermal contact resistance.

Figure 56 shows the thermal performance of three thin-sheet thermal-interface materials (silicone, graphite/oil, fluoroether 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 interface

thermal resistance. In contrast, 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 54).

Therefore, synthetic grease offers the best thermal performance, considering the low interface pressure, and is recommended.

Of course, the selection of any thermal interface material depends on many factors—thermal performance requirements,

manufacturability, service temperature, dielectric properties, cost, and so on.

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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