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The importance of correct IGBT module mounting procedures

The importance of the correct mounting procedure for IGBT modules should not be underestimated. All IGBT modules should be corectly fitted to a suitable heatsink to ensure that heat can be taken away from the module and therefore ensure the Tj(max) (Maximum junction temperature) of the IGBT is not exceeded.

Using incorrect mounting procedures can cause severe problems for IGBTs and is often the cause for field failures.

Highlighted below are some of the neccasary steps in ensuring that the Fuji IGBT module is mounted correctly.

Heatsink finish (module mounting area):

The flatness of the heat sink between the mounting holes should be less than 50μm per 100mm. The surface roughness should be less than 10μm. If the surface of the heat sink is not suitably flat, the module may have unexpected increase of the contact thermal resistance (Rth(c-f) ) between the module and the heat sink. Also, if the heat sink flatness doesn’t meet the above requirements, a high mechanical stress may be applied to the DCB on the module and it may cause insulation failure.

Thermal Paste:

Thermal grease between heat sink and module baseplate is absolutely necessary to reduce the contact thermal resistance. Screen-printing and rollers are typical methods of thermal grease pasting, however, using a stencil mask is recommended when the target grease thickness is less than 100μm. If using a roller the thermal paste should be applied to grease free, dirt free flat surface and rolled evenly on the roller prior to applying to the module.

Once the module is mounted to the heatsink you should not see excessive thermal paste seeping from the sides of the module, this is a sign that two much thermal paste has been applied. Applying to much thermal paste can result in the contact thermal resistance, between the module and the heatsink, increasing and causing the IGBT to overheat.

 Examples of thermal paste finish:

Using a roller:

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Typical pre-applied thermal paste using a stencil mask:

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Table 1 Recommended properties of thermal grease for Fuji modules:

  Items

Recommendation

 Penetration (typ.)

≥ 338 min

Thermal conductivity

≥ 0.92 W/mK

Thermal grease thickness

100μm ±30μm

 

*1) The thermal resistance between the heat sink and the module depends on the thermal grease properties and thickness. Fuji Electric strongly recommends customers to check the contact interface after mounting to see if the thermal grease spread is correct. Also Fuji Electric recommends confirmation of the thermal interface status after thermal cycling if the thermal grease has low viscosity.

*2) Electrical document of the recommended stencil mask pattern and recommended method are also available on request.

Electro Static Discharge:

If excessive static electricity is applied to the control terminals, the devices can fail. Some counter measures against static electricity is necessary, such as wearing a correctly fitted and terminated ESD wrist strap.

Mounting sequences and torque settings:

EconoPACK DUAL

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1)      Recommended tightening torques: 2.5 to 3.5 N•m (M5)

2)      Initial: 1/3 of specified torque, sequence 1 to 4

3)      Final: Full specified torque (2.5 to 3.5 N•m), sequence 1 to 4

 

PrimePACK

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1)      Recommended tightening torques: 3 to 6 N•m (M5)

2)      Initial: 1/3 of specified torque, sequence 1 to 10

3)      Final: Full specified torque (3 to 6 N•m), sequence 1 to 10

 

62x108mm

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1)      Recommended tightening torques: 2.5 to 3.5 N•m

2)      Initial: 1/3 of specified torque, sequence 1 to 4

3)      Final: Full specified torque (2.5 to 3.5 N•m), sequence 1 to 4

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