PowerPC 750FL RISC Microprocessor
Preliminary
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 (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 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 Exploded Cross-Sectional View of Package with Several Heat Sink Options on
page 61). 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 Materials
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
System Design Information
Page 62 of 65
750flds60.fm.6.0
April 27, 2007
IBM [ IBM ]
