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603 Hardware Specifications

Wakefield Engineering

617-245-5900

60 Audubon Rd.

Wakefield, MA 01880

Aavid Engineering

603-528-3400

One Kool Path

Laconia, NH 03247-0440

Ultimately, the final selection of an appropriate heat sink depends on many factors, such as thermal

performance at a given air velocity, spatial volume, mass, attachment method, assembly, and cost.

1.8.6.1 Internal Package Conduction Resistance

For this packaging technology the intrinsic thermal conduction resistance (shown in Table 3) versus the

external thermal resistance paths are shown in Figure 14 for a package with an attached heat sink mounted

to a printed-circuit board.

Figure 14. Package with Heat Sink Mounted to a Printed-Circuit Board

1.8.6.2 Adhesives and 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 15 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 7 times greater than the thermal grease joint.

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

Figure 13). 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. Of course, the selection of any thermal

interface material depends on many factors—thermal performance requirements, manufacturability, service

temperature, dielectric properties, cost, etc.

External Resistance

External Resistance

Internal Resistance

(Note the internal versus external package resistance)

Radiation

Convection

Radiation

Convection

Heat Sink

Printed-Circuit Board

Thermal Interface Material

Package/Leads

Die Junction

Die/Package

AR

CH

IVE

D B

Y F

RE

ES

CA

LE

SE

MI

CO

ND

UC

TO

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INC

.