MC7457RX1267LC Datasheet, PDF (62/71 Pages) Freescale Semiconductor, Inc – Microprocessor

System Design Information
9.8.2 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 spring clip
mechanism, Figure 29 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.
Often, heat sinks are attached to the package by means of a spring clip to holes in the printed-circuit board
(see Figure 27). Therefore, the synthetic grease offers the best thermal performance, considering the low
interface pressure and is recommended due to the high power dissipation of the MPC7457. Of course, the
selection of any thermal interface material depends on many factors—thermal performance requirements,
manufacturability, service temperature, dielectric properties, cost, etc.
2
Silicone Sheet (0.006 in.)
Bare Joint
Floroether Oil Sheet (0.007 in.)
Graphite/Oil Sheet (0.005 in.)
Synthetic Grease
1.5
1
0.5
0
0
10
20
30
40
50
60
70
80
Contact Pressure (psi)
Figure 29. Thermal Performance of Select Thermal Interface Material
The board designer can choose between several types of thermal interface. Heat sink adhesive materials
should be selected based on high conductivity, yet adequate mechanical strength to meet equipment
shock/vibration requirements. There are several commercially available thermal interfaces and adhesive
materials provided by the following vendors:
MPC7457 RISC Microprocessor Hardware Specifications, Rev. 8
62
Freescale Semiconductor