C&H Technology PowIRtab User Manual

PowIRtab Mounting Guidelines

by Paul W estmarland & Pamela Dugdale

AN-1010

1.0 Introduction

The PowIRtabTM package has been designed to fill the
gap in the market between the TO-247, more expensive
metal case devices & non-isolated power modules. It is
the natural replacement for metal case outlines such as
DO-4 & DO-5, but it is also suitable for new innovative
solutions, thanks to a package outline that combines low
profile, excellent die to footprint ratio & sturdy
connectivity . It utilises a large lead for high current
connection, carrying both a mounting hole & PCB
insertion pins. The body is compatible with a T0-218
outline, with an exposed heatsink & non-isolated mounting
hole.
It is anticipated that the devices would find typical
applications in busbar assemblies or finned heatsinks,
reducing component count & cost of ownership.

2.0 Scope

This application note covers the various fixment methods
that are possible with this device, & the associated thermal
properties resulting from their use:
a) Optimum mounting torque.
b) T ype of fixings.
c) Effect of torque on thermal resistance (‘wet’ & ‘dry’).
d) Effect of pressure on contact thermal resistance (‘wet’
& ‘dry’).

3.0 Mechanical Considerations

3.1 T ype of Fixings

The PowIRtabTM possesses mounting holes in the tab &
lead for electrically connecting the device to heatsinks or
busbars. The lead also carries PCB insertion pins so that
the lead end may be soldered into a board.

Tab (Header)

Stress relief slots

Lead

PCB insertion
pins

Figure 1

3.2 T ab Connection

Using the mounting hole in the tab allows a designer to
attach the PowIRtabTM to a heatsink. The tab of a
PowIRtabTM acts as one of the terminals. There is no
common additional lead, so the mounting hole contact
must be very good, with the heatsink forming part of the
circuit. For the best results the surface of the heatsink must

Application Note

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 PowIRtabTM mounting
hole, the crimping force is controlled to give a slow
pressure build-up & the rivet used must be of a soft
material. T oo 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.

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 AN­997 ‘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
PowIRtabTM 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 PowIRtabTM 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.

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.