System Design Information
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Ultimately, the final selection of an appropriate heat sink depends on many factors, such as thermal performance at
a given air velocity, spatial volume, mass, attachment method, assembly, and cost.
9.8.1 Internal Package Conduction Resistance
For the exposed-die packaging technology, shown in Table 3, the intrinsic conduction thermal resistance paths are
as follows:
•
•
The die junction-to-case (actually top-of-die since silicon die is exposed) thermal resistance
The die junction-to-ball thermal resistance
Figure 28 depicts the primary heat transfer path for a package with an attached heat sink mounted to a printed-circuit
board.
External Resistance
Radiation
Convection
Heat Sink
Thermal Interface Material
Die/Package
Die Junction
Package/Leads
Internal Resistance
Printed-Circuit Board
Radiation
Convection
External Resistance
(Note the internal versus external package resistance.)
Figure 28. 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, then 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, for a first-order analysis, the temperature drop in the silicon
may be neglected. 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 29 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
MPC7455 RISC Microprocessor Hardware Specifications, Rev. 4.1
Freescale Semiconductor
55
FREESCALE [ Freescale ]
