Datasheet

MPC7447A RISC Microprocessor Hardware Specifications, Rev. 5
Freescale Semiconductor 45
System Design Information
9.8.1 Internal Package Conduction Resistance
For the exposed-die packaging technology described in Table 5, the intrinsic conduction thermal resistance
paths are as follows:
• The die junction-to-case thermal resistance (the case is actually the top of the exposed silicon die)
• The die junction-to-board thermal resistance
Figure 22 depicts the primary heat transfer path for a package with an attached heat sink mounted to a
printed-circuit board.
Figure 22. 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.
9.8.2 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 23 shows the thermal performance of three thin-sheet thermal interface materials
(silicone, graphite/oil, fluoroether 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 20). Therefore, synthetic grease offers the best thermal performance, considering the low
interface pressure, and is recommended due to the high power dissipation of the MPC7447A. Of course,
External Resistance
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.)