C&H Technology PowIRtab Mounting Guidelines - Manual (Page 4)

Application Note
4.0 Thermal Considerations
One of the major considerations when mounting all
power semiconductor packages is the dissipation of heat.
This is because the performance of the device is limited by
the junction temperature of the die and the glass transition
temperature of the plastic. Indeed there are maximum
allowable temperatures above which the device is not
functional. The way in which a device is mounted can have
a large effect on the thermal contact between the header
and the heatsink and hence on the ability of the package to
dissipate heat. This is often referred to as the contact
thermal resistance and is quoted in datasheets. A full
discussion of all of the components that make up the
thermal resistance of a power semiconductor package is
given in AN-997. In the present note we shall concentrate
on the thermal resistance between the case and the
heatsink as this is the most dependent on the mounting
technique.
The physical source of the contact resistance is a result
of the fact that surfaces are never perfectly flat. Even for
two well prepared surfaces contact only actually occurs at
several points separated by large air gaps. As air is a very
good thermal insulator this is undesirable and increases the
thermal resistance. There are two ways of reducing the
volume of air trapped between the surfaces. One is to
increase the force holding the two surfaces together and
the other is to improve the quality of the contact area by
filling in the gaps. In the case of the former this can be
done by either applying a force above the die with a clip or
by increasing the torque on the screw which mounts the
tab to the heatsink. The second technique requires the use
of a heatsinking compound. This is usually a silicone grease
loaded with electrically insulating, thermally conductive
material such as alumina. The purpose of the grease is to
fill the gaps without increasing the distance between the
two surfaces. If the layer of grease is too thick then the
thermal resistance will be increased. When using
heatsinking compound in conjunction with a Powirtab
TM it is
important to remember that electrical contact to the drain
can only be made through the mounting tab. In addition to
this, care must be taken to avoid getting any compound in
the screw threads or mounting holes as this will affect the
accuracy of the torque measurement.
4.1 Contact Thermal resistance as a Function of Torque
on the Mounting Screw.
For the purpose of this application note the contact
thermal resistance has been measured as a function of both
the torque on the mounting screw and the force above the
die. In both cases measurements have been performed
with and without heatsinking compound.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 0.5 1 1.5
Torque / Nm
Thermal Resistance C/W
Dry Mounting
With Heatsink Compound
Figure 7
Figure 7 shows the contact thermal resistance as a
function of torque with and without heatsink compound.
The package was mounted using a M4 screw in
accordance with the mounting instructions described in this
application note. It can be seen from the graph that in the
case of a dry mounted device the contact thermal
resistance can be reduced to a minimum of 1
o C/W by
increasing the torque up to an optimum value of 1.1 Nm.
Further increasing the torque is not beneficial since the
header/mounting tab becomes deformed, lifting the
package away from the heatsink and hence increasing the
thermal resistance. The use of heatsink compound reduces
the thermal resistance by a factor of 78% to 0.22
0 C/W.
The dependence on torque is also reduced. This
measurement was acheived using a device with 60W
power applied for 100secs, on an ‘infinite’ heatsink.
Recommended torque:
Without heatsink compound: 1.1 Nm (0.81lbf ft) to give a
thermal resistance, case to sink, of 1
0 C/W.
With heatsink compound: 0.8 Nm (0.59lbf ft) to give a
thermal resistance, case to sink, of 0.22
0 C/W.