
MPC8544E PowerQUICC III Integrated Processor Hardware Specifications, Rev. 6
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.)