MPC755 datasheet(46/56 Pages) MOTOROLA

46 / 56 page

MPC755 RISC Microprocessor Hardware Specifications, Rev. 6.1

Freescale Semiconductor

System Design Information

Figure 26. C4 Package with Heat Sink Mounted to a Printed-Circuit Board

8.8.2 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 27 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

interface thermal resistance. That is, the bare joint results in a thermal resistance approximately seven 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 25).

This spring force should not exceed 5.5 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

Radiation

Convection

Radiation

Convection

Heat Sink

Printed-Circuit Board

Thermal Interface Material

(Note the internal versus external package resistance.)

Internal Resistance

Die/Package

Package/Leads

Die Junction

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