MC7457RX1267LC Datasheet, PDF (61/71 Pages) Freescale Semiconductor, Inc – Microprocessor

System Design Information
Tyco Electronics
Chip Coolers™
P.O. Box 3668
Harrisburg, PA 17105-3668
Internet: www.chipcoolers.com
800-522-6752
Wakefield Engineering
33 Bridge St.
Pelham, NH 03076
Internet: www.wakefield.com
603-635-5102
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 5, 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
Internal Resistance
Printed-Circuit Board
Thermal Interface Material
Die/Package
Die Junction
Package/Leads
External Resistance
Radiation Convection
(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, 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.
MPC7457 RISC Microprocessor Hardware Specifications, Rev. 8
Freescale Semiconductor
61