App Note: Heat sink mounting guide

This document provides
guidelines for mounting heat sinks for the
proper thermal management of power semiconductor
devices in field applications. This document
describes heat-
sink mounting methods,
considerations, contact thermal
resistance, and mounting torque for
various pack ages.
Heat Sink Mounting Considerations
Thermal Resistance and Heat Sink Mounting
The thermal performance of a package with a heat sink is
characterized by a junction

Heat Sink Mounting

Air convection is usua
lly the dominant heat transfer
mechanism in electronics. The convection heat transfer
strongly depends on the air velocity and the area of the heat-
transferring surface. Since air is a good thermal insulator, it
is important that a heat sink is used to inc
rease the overall
heat transfer area to the ambient, i.e., the overall thermal
performance

Applying the heat sink provides an air gap between the
package and
the heat sink due to the inherent surface
roughness
surfaces can only make contact at certain points or
areas, the
remaining space is filled with air. Since air is a good
insulator, the air gap significantly increases the thermal
resistance. This is called a contact thermal resistance.
The contact thermal
resistance is a function of the roughness of the contacting
surfaces and the contact pressure.
The technique of
mounting a heat sink is a key factor in mini
mizing thecontact thermal resistance.

The first way to reduce the contact
thermal resistance is to
increase the contact pressure, which is the joining force.
This maximizes the contact area between the two surfaces.
Increasing the mounting torque in the fastening screw or
using a clip with a high spring constant provides for larger
providing solid conduction heat flow paths
compared with air conduction through the air gap.
The second method is to fill the air gap with an interface
material
such as silicone grease or an insulation pad
having
better thermal conduct
ivity than air.

typical thermal resistance values of several interface

materials


Thermal information for various interface materials is
provided by leading manufactures.

Screw mounting is a traditional assembly method
accomplished with the fastening of a screw, nut,
and
washer. Although it takes long to assemble in a mass
production environment, it is easy and
quick to disassemble.
Applying the proper mounting torque is the key factor in
obtaining adequate contact pressure along the contact
surfaces of the package and the heat sink to minimize the
contact thermal resistance. With a low mounting torque, the
contact thermal resistance increases due to bad thermal
contact under insufficient contact pressure. With a high
mounting torque, the package head and mounting tab show
large deformations s
uch
that the package may be lifted away
from the heat sink,

the contact thermal resistance. Hence, an appropriate range

of mounting torque values must be applied to produce
minimal thermal
resistance. This prevents package
destruction or changes in device characteristics.
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