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Wakefield Engineering
60 Audubon Rd.
Wakefield, MA 01880
Aavid Engineering
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.
External Resistance
Internal Resistance
External Resistance
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.
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Radiation
Heat Sink
Printed-Circuit Board
Radiation
(Note the internal versus external package resistance)
617-245-5900
603-528-3400
Convection
Thermal Interface Material
Die/Package
Die Junction
Package/Leads
Convection
603 Hardware Specifications
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