MPC8544E datasheet(101/117 Pages) FREESCALE

101 / 117 page

MPC8544E PowerQUICC III Integrated Processor Hardware Specifications, Rev. 5

Freescale Semiconductor

101

Thermal

Figure 62 depicts the primary heat transfer path for a package with an attached heat sink mounted to a

printed-circuit board.

Figure 62. 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 and 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.

20.3.2

Thermal Interface Materials

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

contact resistance. For those applications where the heat sink is attached by spring clip mechanism,

Figure 63 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. The bare joint results in a

thermal resistance approximately six times greater than the thermal grease joint.

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.)

MPC8544E Datasheet(PDF) 101 Page - Freescale Semiconductor, Inc