PC755 datasheet(20/50 Pages) ATMEL

20 / 50 page

PC755/745

2138D–HIREL–06/03

Adhesives and Thermal

Interface Materials

Figure 8. Thermal Performance of Select Thermal Interface Material

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 8 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 perfor-

mance 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 7 times greater than the ther-

mal grease joint.

Heat sinks are attached to the package by means of a spring clip to holes in the printed-

circuit board (see Figure 7). This spring force should not exceed 5.5 pounds of force.

Therefore, the synthetic grease offers the best thermal performance, considering the

low interface pressure.

The board designer can choose between several types of thermal interface. Heat sink

adhesive materials should be selected based upon high conductivity, yet adequate

mechanical strength to meet equipment shock/vibration requirements.

Heat Sink Selection Example

For preliminary heat sink sizing, the die-junction temperature can be expressed as

follows:

j = Ta + Tr + (θjc + θint + θsa) * Pd

Where:

j is the die-junction temperature

a is the inlet cabinet ambient temperature

r is the air temperature rise within the computer cabinet

0.5

1.5

Silicone Sheet (0.006 inch)

Bare Joint

Floroether Oil Sheet (0.007 inch)

Graphite/Oil Sheet (0.005 inch)

Synthetic Grease

Contact Pressure (psi)

10203040

5060

7080

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