MPC8640DEC datasheet(116/140 Pages) FREESCALE

116 / 140 page

MPC8640 and MPC8640D Integrated Host Processor Hardware Specifications, Rev. 1

116

Freescale Semiconductor

Thermal

Figure 60 depicts the primary heat transfer path for a package with an attached heat sink mounted to a

printed-circuit board.

Figure 60. 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.

19.2.2

Thermal Interface Materials

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

contact resistance. Figure 61 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.

Often, heat sinks are attached to the package by means of a spring clip to holes in the printed-circuit board

(see Figure 59). Therefore, synthetic grease offers the best thermal performance, considering the low

interface pressure, and is recommended due to the high power dissipation of the MPC8640. 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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