MPC8245 datasheet(53/68 Pages) FREESCALE

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MPC8245 Integrated Processor Hardware Specifications, Rev. 10

Freescale Semiconductor

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

0.5

1.5

10203040

5060

Silicone Sheet (0.006 in.)

Bare Joint

Floroether Oil Sheet (0.007 in.)

Graphite/Oil Sheet (0.005 in.)

Synthetic Grease

Contact Pressure (psi)

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