PowerPC 740 and PowerPC 750 Embedded Microprocessor
IBM CMOS 0.20 um Copper Technology EMPPC740L and EMPPC750L
Figure 21. C4 Package with Heat Sink Mounted to a Printed-Circuit Board
External Resistance
Radiation
Convection
Heat Sink
Thermal Interface Material
Die/Package
Chip Junction
Internal Resistance
Package/Leads
Printed-Circuit Board
Radiation
Convection
External Resistance
(Note the internal versus external package resistance.)
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 conduction/con-
vective thermal resistances are the dominant terms.
Adhesives and 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 a spring clip mechanism,
Figure 22 shows the thermal performance of three thin-sheet thermal-interface materials (silicon, graphite/oil,
flouroether 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 ther-
mal grease significantly reduces the interface thermal resistance. That is, the bare joint results in a thermal
resistance approximately 7 times greater than the thermal grease joint.
Heat sinks are attached to the package by means of a spring clip to holes in the printed-circuit board (see
Figure 20). Therefore the synthetic grease offers the best thermal performance, considering the low interface
pressure. Of course, the selection of any thermal interface material depends on many factors – thermal per-
formance requirements, manufacturability, service temperature, dielectric properties, cost, etc.
5/20/99
Version 1.51
PowerPC 740 and PowerPC 750 Datasheet
Page 41
IBM [ IBM ]
