System Design Information
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 MPC7455. Of course, the selection of any
thermal interface material depends on many factors—thermal performance requirements, manufacturability, service
temperature, dielectric properties, cost, etc.
Silicone Sheet (0.006 in.)
Bare Joint
2
Floroether Oil Sheet (0.007 in.)
Graphite/Oil Sheet (0.005 in.)
Synthetic Grease
1.5
1
0.5
0
0
10
20
30
Contact Pressure (psi)
Figure 29. Thermal Performance of Select Thermal Interface Material
40
50
60
70
80
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:
The Bergquist Company
18930 West 78 St.
800-347-4572
th
Chanhassen, MN 55317
Internet: www.bergquistcompany.com
Chomerics, Inc.
781-935-4850
77 Dragon Ct.
Woburn, MA 01888-4014
Internet: www.chomerics.com
MPC7455 RISC Microprocessor Hardware Specifications, Rev. 4.1
56
Freescale Semiconductor
FREESCALE [ Freescale ]
