MPC7445 datasheet(48/60 Pages) FREESCALE

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MPC7448 RISC Microprocessor Hardware Specifications, Rev. 4

Freescale Semiconductor

System Design Information

9.7.1

Internal Package Conduction Resistance

For the exposed-die packaging technology described in Table 5, the intrinsic conduction thermal resistance

paths are as follows:

•

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

•

The die junction-to-board thermal resistance

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, 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, the temperature drop in the silicon may be neglected

for a first-order analysis. Thus, the thermal interface material and the heat sink conduction/convective

thermal resistances are the dominant terms.

9.7.2

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 25 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. 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 22). Therefore, synthetic grease offers the best thermal performance due to the low interface

pressure and is recommended due to the high power dissipation of the MPC7448. Of course, the selection

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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