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Power Module
Optimized heat sink assembly method
for effective heat dissipation
Power modules are widely used in inverter and motor drive applications. Inside the module the semiconductor
devices dissipate heat from conductive loss and switching loss. Therefore, it is necessary to consider in the design
the heat dissipation mechanism. This application note shows the optimized assembly of power modules to heatsinks
and presents the effect of optimized mounting on effective dissipation of heat from the module to the heat sink.
Power module structure
As shown in Figure 1, the power module consists of chips,
a base plate, and an isolation substrate between them.
Ideally, the base plate should be flat to achieve a good
thermal contact to the heatsink. However, in reality it is
warped due to mechanical stress between the different
materials inside that remains after the manufacturing
process. A measurement of the flatness of a base plate
surface is shown in Figure 2. The height differences reach
around 40 μ m. This induces gaps at the interface of
module base plate and heat sink, deteriorating the heat
dissipation, and as a result, the chip temperatures may
exceed maximal allowed chip temperature.
About thermal grease
A thermal sheet or thermal grease is used commonly to
fill the gap between the base plate and heat sink surface.
This would be necessary, even if there were no gap at
room temperature, as the baseplate shape changes with
the temperature.
Although, in principle, it is easy to use a thermal sheet
because of its solid shape, the required torque for
installation is larger than the power modules torque
rating. In the case of thermal grease, the required torque
is smaller than power modules rating due to its liquidity.
However, it is difficult to control the thickness of grease
and the shape will be easily changed due to temperature
changes during operation (pump-out). Please pay close
attention to material selection. The assembly method and
effects of thermal grease are shown as below.
Figure 1. Power module structure
Sample:BSM300D12P2E001
Figure 2. Warped shape of power module base plate at
room temperature
Isolation
substrate
chip
solder
Base plate
wire
Silicone gel
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Optimized heat sink assembly method for effective heat dissipation
Application of Thermal greases.
The gap size between heat sink and module base plate
depends on power module. Please choose a suitable
thickness for thermal grease. Too thin or too thick thermal
grease would increase the thermal resistance. It is
recommended to control the thickness of thermal grease
in the range of 50-100̀m. Using a metal mask and a
squeegee, is strongly recommended, instead of roller or
spatula, to uniformly paint grease on module base plate
surface.
Figure 3. shows the flow of grease painting on module
surface
Figure 3. Thermal grease painting using metal mask
① Attach the power module to the mold
※ Wipe module base plate surface
② Stack metal mask on the power module
※ Wipe the metal mask
③ Paint grease on the metal mask
④ Spread the thermal grease on the base plates surface
using the squeegee.
-Hold the squeegee with a 60 angle to the surface
and move it at the speed of 2-3 cm/s. Do it several
times until thermal grease is uniformly distributed.
⑤ The Base state surface after painted with grease.
Please contact ROHM through to the following URL if you
need the metal mask schematic.
( https://www.rohm.co.jp/contactus )
The interface of the heat sink and the module (shown in
Figure 4.) should meet the following requirements:
• Flatness of heatsink of 30 ̀m or less.
• Surface roughness of heatsink of 10̀m or less
Otherwise, the isolation substrate inside the power
module can be destroyed.
Figure 4. Heat sink and module interface
The thermal grease will be spread out by the pressure
from assembly torque. Increase the torque gradually for
more uniform distribution of the grease. The sequence of
tightening the screws to fix the module to the heatsink is
shown in Figure 5. The torque in step ② should be bigger
than in step ①. Be sure that the torque applied is within
the specification range of the power module rating, to
prevent isolation damage of the substrate inside.
① 1→２→3→4 with fasten torque of 0.5 N.m
② 1→2→3→4 with fasten torque of 3.0 N.ｍ
heatsink
SiC Module
Lower than 30um
①
⑤
②
③
④