MT6M15962a
3. Mounting instruction of the IPM
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3.1 Layout of IPMs on a heat sink
The thermal resistance varies depending on the IPM mounting position. Please note the following :
• When one IPM is mounted to a heat sink, it is recommended to place the IPM on the center of the
heat sink to minimize the thermal resistance.
• When multiple IPMs are mounted to a single heat sink, the IPMs location and layout should be
designed in consideration of the generated losses and the spread and flow of heat on the heat sink.
Allocate the largest area to the IPM which generates the largest loss.
3.2 Application of thermal grease
To reduce the contact thermal resistance, apply thermal grease between the IPM and the heat sink
mounting surface.
The general methods of applying thermal grease is applying using a stencil mask and applying with a
roller.
The purpose of the thermal grease is to promote heat transmission to the heat sink, but the grease
has a limited thermal capacity. Therefore, if the thermal grease thickness is larger than the appropriate
thickness, the grease layer prevents the heat transfer from the IPM to the heat sink and the junction
temperature will be increased. On the other hand, if the thermal grease thickness is less than the
appropriate thickness, non contact part such as a void area, a gap or space may be generated and
cause an increase of the contact thermal resistance. Therefore thermal grease should be applied in
appropriate thickness.
If the thermal grease thickness is inadequate, the thermal dissipation will be poor, and in the worst
case, there is a possibility of IPM breakdown due to the junction temperature exceeding 175
°
C.
Application of thermal grease using a stencil mask is recommended so that uniform thickness can be
achieved. Figure 5-2 shows an example of thermal grease application using a stencil mask. The basis
of this method is applying specified weight of thermal grease to the base plate of the IPM using a
stencil mask.
It is possible to achieve a uniform thermal grease thickness by mounting the IPM with thermal grease
to the heat sink and tightening screws with the recommended torque for each product.
Fuji Electric can provide recommended stencil mask designs upon
customer’s request.
5-3
