Vishay PowerTab Mounting Guidelines Application Information

VISHAY SEMICONDUCTORS

Rectifiers

PowerTab

TM

Mounting Guidelines

Application Note

3.0 MECHANICAL CONSIDERATIONS

3.1 TYPE OF FIXINGS

The PowerTab possesses mounting holes in the tab and
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

1.0 INTRODUCTION

The PowerTabTM package has been designed to fill the gap
in the market between the TO-247, more expensive metal
case devices and non-isolated power modules. It is the
natural replacement for metal case outlines such as
DO-203AA and DO-203AB, but it is also suitable for new
innovative solutions, thanks to a package outline that
combines low profile, excellent die to footprint ratio and
sturdy connectivity. It utilises a large lead for high current
connection, carrying both a mounting hole and PCB insertion
pins. The body is compatible with a TO-218 outline, with an
exposed heatsink and non-isolated mounting hole.

It is anticipated that the devices would find typical
applications in busbar assemblies or finned heatsinks,
reducing component count and cost of ownership.

2.0 SCOPE

This application note covers the various fixment methods
that are possible with this device, and the associated thermal
properties resulting from their use:

a. Optimum mounting torque

b. Type of fixings

c. Effect of torque on thermal resistance (“wet” and “dry”)

d. Effect of pressure on contact thermal resistance (“wet”

and “dry”)

Document Number: 95179 For technical questions within your region, please contact one of the following: www.vishay.com
Revision: 11-Jun-10 [e-mail]

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3.2 TAB CONNECTION

Using the mounting hole in the tab allows a designer to
attach the PowerTab to a heatsink. The tab of a PowerTab
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 be as smooth and flat as
possible to maximise the contact area of the tab. A good
flatness specification would be 0.02 mm (0.0007") maximum
per 10 mm (0.393"). 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 and 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 7.2 mm (0.283"). An alternative is a
suitably sized rectangular washer.

PCB insertion
pins

Fig. 1

Lead

APPLICATION NOTE

Application Note

Vishay Semiconductors

PowerTabTM Mounting Guidelines

Small clearance
between washer and
plastic body

M4 socket
headed bolt

the back of the heatsink and the back of the lead is nominally

3.0 mm. 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 3.0 mm
to the same level as the back of the lead. A typical busbar
configuration is shown in Figure 3.

Care must be taken when tightening the fixing to prevent
distortion of the lead. The lead fixing can be typically
tightened to 3.00 Nm (2.21 lbf · ft) without distortion.

Fig. 2

Similarly, M4 nuts cannot be used (on the plastic side) for the
tab connection, since there is inadequate clearance between
the hole and plastic body to rotate the nut. Using the plastic
body to prevent the nut rotating will inevitably crack the
plastic and 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 PowerTab mounting hole, the
crimping force is controlled to give a slow pressure build-up
and the rivet used must be of a soft material. Too high a
crimping speed and pressure is likely to damage the die
inside the package and 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 and 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” and 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. Also
refer to section 4.2 and 5.0 of this application note for the
optimum clip force.

Sprung washer

Busbar connection to lead

Heatsink connection

washer

Plain

Fig. 3

Another solution is a laminated busbar, as supplied by the
Rogers Mektron Busbar Division. Here a single busbar is
stamped to the step height of the package and an isolating
laminate and second busbar added. The PowerTab

TM

packages can then be bolted (or rivetted) down to this strip
to form a single assembly, with two large single outputs. A
typical example is shown below in Figure 4.

3.4 LEAD CONNECTION

The mounting hole in the lead of the PowerTab is oval in
shape. This slotted hole allows for some movement between
the two mounting holes in an assembly, and for any
assembly tolerances. Any M4 screw, No. 6-32 or 6-40 screw

Fig. 4

or nut combination may be used to secure the lead. The use
of a plane and 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, minimize
the possibility of the plastic body cracking under
tension/compression stresses. The step difference between

APPLICATION NOTE

www.vishay.com For technical questions within your region, please contact one of the following: Document Number: 95179

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, [e-mail], [e-mail] Revision: 11-Jun-10