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Pre-applied phase change material for bonding semiconductor modules

The thermal transfer between a power semiconductor module (e.g. an IGBT or SiC module) and a heatsink is critical to achieving high power densities and avoiding failures in the field. Using phase change material rather than thermal grease as an interface achieves much higher conductivity. Furthermore, pre-application by the manufacturer ensures a uniform layer of optimal thickness and consistency which maximises performance, ease of manufacture and repeatability.

Rear side of a Vincotech module with pre-applied phase change material 

Dissipating thermal energy

The thermal energy generated by a power semiconductor die must be dissipated to avoid exceeding the maximum junction temperature (Tj max) and consequential damage. Poor mounting of a power module also risks failures in the field. Maximising the heat dissipation by minimising the thermal resistance between the module and the heatsink is typically achieved by bonding the two using a thermal interface material.

Thermal Interface Material (TIM)

The main role of a thermal interface material is to maximise the heat transfer by filling cavities on the power module and levelling out unevenness on the heatsink. This eliminates air gaps which act as thermal insulators to achieve an effective bond between the top of the heatsink and the bottom of the power module.

TIM conductivity versus grain and layer characteristics

Conductivity is highly dependent on the size, shape and distribution of the grains in the filling material. Smaller filler grains are able to fill small surface irregularities to reduce the contact resistance and lowers the thermal resistance (Rth). However, larger particles result in a high specific thermal conductivity. The optimum thickness of the TIM layer depends on the module size and technology. The layer should be as thin as possible but as thick as necessary.

What is thermal grease?

Thermal grease, often referred to as heat paste, heatsink compound or thermal compound basically consists of two components:

• a carrier material
• a filling material

Typical carrier materials include an epoxy, silicone, urethane or an acrylate. Fillers such as aluminium nitride, aluminium oxide, boron nitride or zinc oxide are thermally conductive but electrically insulating. A thermal grease compound can be as much as 80% filler.

Phase change material (PCM)

A phase change material changes state without a change in chemical composition. In the case of a semiconductor thermal interface material, the PCM changes from solid to liquid and back to solid when crossing the phase change temperature – approximately 45°C. Above the phase change temperature the material becomes thixotropic – i.e. viscous under normal conditions but less viscous when stressed. Thus, the PCM will spread across the area in contact with the heatsink, creating an even layer between the two surfaces but not flow when heated during power up of the module. Once solidified, there is no risk of smearing and a module requires no special care during transportation, handling and application.

Advantages of PCM

• PCM is fluid during the application and dries out over time and temperature.
• Compatible with press to fit pins
• Can be used with a standard solder profile such as J-STD-001, J-STD-003
• Reduced thermal resistance
• Reduced risk of substrate cracking

Comparison of thermal interface materials

Property Std thermal grease High perf. thermal paste Si-free thermal grease Phase change material
Density at 25 °C (g/cm3) 2.1 4.2 2.1 – 2.3 2
Dielectric strength (kV/mm) 10 3 20 N/A
Thermal conductivity (W/m*K) 0.81 2.5 2.5 3.4

Benefits of pre-application of PCM

Having phase change material pre-applied means streamlined production – i.e. a reduced number of manufacturing operations. This is also relevant to the design process during which a TIM layer would need to be manually applied. A screen-printing process applies a more uniform thickness and consistent quality of TIM than can be achieved manually with a squeegee, roller, brush or putty knife. Uniformity avoids increased thermal resistance due to the layer being too thin or too thick.

Conclusion

For high voltage semiconductor modules such as IGBT and SiC modules, phase change material offers significant performance benefits over traditional thermal grease as an interface material. Furthermore, pre-applied PCM ensures optimal thickness and consistency whilst reducing the number of operational steps in manufacture.

References

  1. Thermal Management Technology for IGBT Modules – Yoshitaka Nishimu, Mitsukane Oonota, Fumihiko Momose
  2. Maximizing Precision in Thermal Interface Layers – Martin Schulz, Infineon Technologies, Article Bodo’s Power Systems 01/2013
  3. The importance of correct IGBT module mounting procedures – Stuart Europower Components, July 20, 2016
  4. Reliability Testing Of Thermal Greases  – Arun Gowda
  5. Advantages of pre-applied Thermal Interface Material for power modules – Mirko Haardt, Field Application Engineer, Vincotech
  6. Vincotech’s modules with pre-applied phase-change material – Patrick Baginski, Field Application Engineer, Vincotech GmbH
  7. Application Note THERMAL PASTE APPLICATION AN10001 – Dieter Esau, 2010/03/30

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Telephone +44 (0)1793 784389 or email: sales@ppm.co.uk