C&H Technology PowIRtab Mounting Guidelines - Manual (Page 2)

Application Note
3.3 Clip mounting
If desired, use may be made of a clip to attach the
package to a heatsink. The recommended point for the
placement of the clip is directly over the die, ie in the
middle of the plastic body. This will give the best contact
thermal resistance. Please refer to Application Note AN-
997 ‘Mounting Guidelines for the Super-247’ for the
different types of clip available, since the type of clips
demonstrated in this note apply equally well to the
PowIRtab
TM package. Also refer to section 4.2 & 5.0 of
this application note for the optimum clip force.
3.4 Lead connection
The mounting hole in the lead of the PowIRtab TM is oval
in shape. This slotted hole allows for some movement
between the two mounting holes in an assembly, & for any
assembly tolerances. Any M4 screw, No.6-32 or 6-40
screw or nut combination may be used to secure the lead.
The use of a plane & spring washer is recommended to
allow for movement of the lead due to thermal expansion
or vibration. This will also, along with the stress relief slots,
minimise the possibility of the plastic body cracking under
tension/compression stresses. The step difference between
the back of the heatsink & the back of the lead is
nominally 3.0mm. This means that in busbar
configurations, the lead will either need forming down to
the same level as the heatsink, or the lead busbar will need
to be raised by 3.0mm to the same level as the back of the
lead. A typical busbar configuration is shown in Figure 3.
be as smooth & flat as possible to maximise the contact
area of the tab. A good flatness specification would be
0.02mm (0.0007ins) maximum per 10mm (0.393ins).
Ensure also that the heatsink mounting hole has been
deburred.
The mounting hole in the tab is designed to accept a M4
screw, No.6-32 screw or 6-40 screw. A self tapping type
screw may also be used. However, only a certain type of
screw & washer may be used to attach the tab to the
heatsink because of the proximity of the mounting hole to
the plastic body.
The recommended method of attachment is a socket
headed M4 screw, with a plain washer, as shown in the
figure 2. The washer used must be no larger than the
diameter of the socket head. If a larger washer is used, it
can bear directly on the edge of the plastic body, causing
the body to crack when the screw is tightened. . The
largest possible diameter washer that may be used is
7.2mm (0.283ins). An alternative is a suitably sized
rectangular washer.
Similarly, M4 nuts cannot be used (on the plastic side)
for the tab connection, since there is inadequate clearance
between the hole & plastic body to rotate the nut. Using
the plastic body to prevent the nut rotating will inevitably
crack the plastic & is not recommended.
Rivets may be used but the following precautions must be
noted:
The diameter of the hole in the heatsink must be of a
smaller diameter than that of the PowIRtab
TM mounting
hole, the crimping force is controlled to give a slow
pressure build-up & the rivet used must be of a soft
material. Too high a crimping speed & pressure is likely to
Small clearance
between washer &
plastic body.
M4 socket headed
bolt
damage the die inside the package & deform the header,
lifting it away from the heatsink.
Wherever possible, the use of heatsink compound is
recommended to mount the package to improve the heat
dissipation.
The recommended mounting torques, with & without
heatsink compound, may found under section 4.1 of this
application note - ‘Contact thermal resistance as a function
of torque on the mounting screw’ & also in summary form
in Section 5.0.
Figure 2
Care must be taken when tightening the fixing to prevent
distortion of the lead. The lead fixing can be typically
tightened to 3.00Nm (2.21 lbf/ft) without distortion.