DD 2.X
Preliminary
PowerPC 750FX RISC Microprocessor
5.8.6 Adhesives and Thermal Interface Materials
A thermal interface material is recommended at the package die-to-heat sink interface to minimize the
thermal contact resistance. For those applications where the heat sink is attached by a spring clip mecha-
nism, Figure 5-11 shows the thermal performance of three thin-sheet thermal-interface materials (silicon,
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 interface thermal resistance. That is, the bare joint results in a
thermal resistance approximately seven times greater than the thermal grease joint.
In this example, the heat sink is attached to the package by means of a spring clip to holes in the printed-
circuit board (see Figure 5-10 Package Exploded Cross-Sectional View with Several Heat Sink Options on
page 54). The synthetic grease offers the best thermal performance, considering the low interface pressure.
The selection of any thermal interface material depends on many factors – thermal performance require-
ments, manufacturability, service temperature, dielectric properties, cost, and so forth.
Figure 5-11. Thermal Performance of Select Thermal Interface Material
2
+
+
1.5
+
1
0.5
0
0
10
20
30
40
50
60
70
80
Contact Pressure (PSI)
Silicone Sheet (0.006 inch)
Bare Joint
+
Floroether Oil Sheet (0.007 inch)
Graphite/Oil Sheet (0.005 inch)
Synthetic Grease
Body_750FX_DS_DD2.X.fm.2.0
June 9, 2003
5. System Design Information
Page 55 of 63
IBM [ IBM ]
