MPC7457 datasheet(62/71 Pages) FREESCALE

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MPC7457 RISC Microprocessor Hardware Specifications, Rev. 8

System Design Information

Freescale Semiconductor

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.

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:

0.5

1.5

1020

304050

6070

Silicone Sheet (0.006 in.)

Bare Joint

Floroether Oil Sheet (0.007 in.)

Graphite/Oil Sheet (0.005 in.)

Synthetic Grease

Contact Pressure (psi)

MPC7457 データシート(PDF) 62 Page - Freescale Semiconductor, Inc