MPC755EC datasheet(46/56 Pages) NXP

46 / 56 page

MPC755 RISC Microprocessor Hardware Specifications, Rev. 8

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.

Figure 27 describes the thermal performance of select thermal interface materials.

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

MPC755EC Datasheet(PDF) 46 Page - NXP Semiconductors