2/5
Application note
APT0501
December 2011
www.microsemi.com
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
completely with thermal grease.
2.2
Mounting the power module onto the
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).
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).
Heat sink Thermal grease flows out when screws are tightened
fig 3: Proper application of thermal grease to the power module.
M5 washers
& M5 screw
M5 washers
& M5 screw