PC7447 datasheet(16/66 Pages) ATMEL

16 / 66 page

PC7457/47 [Preliminary]

5345B–HIREL–02/04

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 5 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.

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 32 on page 55). Therefore, the synthetic grease offers

the best thermal performance, considering the low interface pressure and is recom-

mended due to the high power dissipation of the PC7457. Of course, the selection of

any thermal interface material depends on many factors – thermal performance require-

ments, manufacturability, service temperature, dielectric properties, cost, etc.

Figure 6. Thermal Performance of Select Thermal Interface Material

Heat Sink Selection Example

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

follows:

j = TI + Tr + (R

θJC + Rθint + Rθsa) × Pd

where:

j is the die-junction temperature

I is the inlet cabinet ambient temperature

r is the air temperature rise within the computer cabinet

0.5

1.5

010

Silicone Sheet (0.006 in.)

Bare Joint

Floroether Oil Sheet (0.007 in.)

Graphite/Oil Sheet (0.005 in.)

Synthetic Grease

Contact Pressure (psi)

PC7447 Datasheet(PDF) 16 Page - ATMEL Corporation