DD 2.X
PowerPC 750FX RISC Microprocessor
Preliminary
5. System Design Information
Body_750FX_DS_DD2.X.fm.2.0
June 9, 2003
5.8.2 Internal Package Conduction
page 10, the intrinsic conduction thermal resistance paths are as follows.
Die junction-to-case thermal resistance (Primary thermal path)
Die junction-to-lead thermal resistance (Not normally a signicant thermal path)
Die junction-to-ambient thermal resistance (Largely dependent on customer-supplied heatsink)
Figure 5-9 depicts the primary heat transfer path for a package with an attached heat sink mounted to a
printed-circuit board.
Heat generated on the active side (ball) 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. Since the silicon thermal resistance is quite small, for a first-order analysis, the temperature
drop in the silicon may be neglected. Thus, the heat sink attach material and the heat sink conduc-
tion/convective thermal resistances are the dominant terms.
The heat flow path from the die, through the chip-to-substrate balls, through the substrate, through the
substrate-to-board balls, and through the board to ambient is usually too high of a resistance to offer much
cooling. In addition, various factors make the heat flow through this path very difficult to accurately determine.
Designers must not depend on cooling the 750FX using this means unless thermal modeling has been confi-
dently completed.
Figure 5-9. C4 Package with Heat Sink Mounted to a Printed-Circuit Board
External Resistance
Internal
(Note the internal versus external package resistance.)
Radiation
Convection
Radiation
Convection
Heat Sink
Die/Package
Printed-Circuit Board
Thermal Interface Material
Package/Leads
Chip Junction
Resistance