Datasheet

MPC8245 Integrated Processor Hardware Specifications, Rev. 10
Freescale Semiconductor 53
System Design
7.8.2 Adhesives and Thermal Interface Materials
A thermal interface material placed between the top of the package and the bottom of the heat sink
minimizes thermal contact resistance. For applications that attach the heat sink by a spring clip
mechanism, Figure 30 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. Thermal grease significantly reduces the interface thermal resistance. That is, the bare joint
offers a thermal resistance approximately seven times greater than the thermal grease joint.
A spring clip attaches heat sinks to holes in the printed-circuit board (see Figure 30). Therefore, synthetic
grease offers the best thermal performance, considering the low interface pressure. The selection of any
thermal interface material depends on factors such as thermal performance requirements,
manufacturability, service temperature, dielectric properties, and cost.
Figure 30. Thermal Performance of Select Thermal Interface Material
The board designer can choose between several types of thermal interfaces. Heat sink adhesive materials
are selected on the basis of high conductivity and adequate mechanical strength to meet equipment
shock/vibration requirements. Several commercially-available thermal interfaces and adhesive materials
are provided by the following vendors:
Chomerics, Inc. 781-935-4850
77 Dragon Ct.
Woburn, MA 01888-4014
Internet: www.chomerics.com
0
0.5
1
1.5
2
0 1020304050607080
Silicone Sheet (0.006 in.)
Bare Joint
Floroether Oil Sheet (0.007 in.)
Graphite/Oil Sheet (0.005 in.)
Synthetic Grease
Contact Pressure (psi)
Specific Thermal Resistance (K-in.
2
/W)