Microsemi SP4 Mounting Instructions

Mounting Instructions for SP4 Power Modules

Pierre-Laurent Doumergue

R&D Engineer

Microsemi Power Module Products

26 rue de Campilleau

33 520 Bruges, France

Introduction:

This application note gives the main
recommendations to appropriately connect the
PCB (Printed Circuit Board) to the power
module, and mount the power module onto the
heat sink. It is very important to follow the
mounting instructions to limit both the thermal
and mechanical stresses.

1. Power Module assembly onto PCB.

The PCB mounted on the SP4 power module can
be screwed onto the plastic post in order to
reduce all mechanical stress and minimize
relative movements on the pins which are
soldered onto the power module.

Plastite screw solderable pins plastite screw

SP4 POWER MODULE

plastic post plastic post

fig 1: PCB mounted on SP4 power module.

First, the PCB must be mounted onto the power
module and screwed onto the plastic posts (See
fig 1).
We recommend a self-tapering plastite screw
with a nominal diameter of 2.5 mm.
A plastite screw is a type of screw specifically
designed for use with plastic and other low
density materials. (See fig 2). The screw length
depends on the PCB thickness. With a 1.6 mm
(0.063”) thick PCB, use a plastite screw 6 mm
(0.25”) long. The maximum moun ting torque is

0.6Nm (5 lbf·in). In any case, the customer must

Application note

APT0501

December 2011

check the integrity of the plastic post after
screwing.

fig 2: example of plastite screw.

The second step consists of soldering all
electrical pins of the power module to the PCB.

Wave soldering or manual soldering processes
are recommended to solder the signal and power
terminals to the PCB. The standard wave
soldering temperature is 235°C (+/-5°C) for 5
seconds (+/- 1 second).

No-clean solder flux is required to attach the
PCB onto the module since aqueous module
cleaning is not allowed.

The PCB holes for module terminals should
have a diameter of 1.5mm

Do not reverse these two steps, because if all
pins are soldered first to the PCB, screwing the
PCB onto the plastic posts will create a
deformation of the PCB, leading to some
mechanical stress that can damage the tracks or
break the components on the PCB.

2. Power module mounting onto heat sink.

Proper mounting of the module base plate onto
the heat sink is essential to guarantee good heat
transfer. The heat sink and the power module
contact surface must be flat (recommended
flatness <50µm for 100mm continuous,
recommended roughness Rz 10) and clean (no
dirt, no corrosion, no damage) in order to avoid
mechanical stress when power module is
mounted, and to avoid an increase in thermal
resistance.

±0.1

for an easy fit.

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2.1 Thermal grease application.

To achieve the lowest case to heat sink thermal
resistance, a thin layer of thermal grease must be
applied between the power module and the heat
sink. If the grease is applied onto the module
base plate, a minimum thickness of 60µm (2.4
mils) of grease should be applied with a roller or
a spatula.

If the grease is applied onto the heat sink, it is
recommended to use screen printing technique to
ensure a uniform deposition of a minimum
thickness of 60µm (2.4 mils). In any case, the
module bottom surface must be wetted
complete ly with thermal grease.

2.2 Mounting the power module onto th e

heat sink.

Place the power module above heat sink holes,
and apply a small pressure to it. Insert the M5
screw with lock and flat washers in each
mounting hole (a #10 screw can be used instead
of M5). The screw length must be at least 12 mm
(0.5”).

First lightly tighten the two mounting screws.
Tighten alternatively the screws until their final
torque value is reached (4.7 N.m max, or 3.5
lbf·ft).

Application note

APT0501

December 2011

It is recommended to use a screwdriver with
controlled torque for this operation.
If possible, screws can be tightened again after
three hours.

The quantity of thermal grease is correct when a
small amount of grease appears around the
power module once it is bolted down onto the
heat sink with the appropriate mounting torque
(see fig 3). Figure 4 shows the thermal grease on
the SP4 module base plate when removed from
the heat sink.

Note:

Holes in the PCB are necessary to insert
and tighten the mounting screws that attach the
power module to the heat sink. These access
holes must be large enough for the screw head
and washer to pass through freely, allowing for
normal variation in PCB hole location. (See fig

5).

M5 washers

& M5 screw

Heat sink Thermal grease flows out when screws are tightened

fig 3: Proper application of thermal grease to the power module.

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M5 washers
& M5 screw

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