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Thermal compound for Alumina Ceramic Insulator

Electrical Engineering Asked by uk25 on December 14, 2020

My main question is there any article or standard that says that Alumina Ceramic Insulator needs any thermal compound or it could be used as it is (in particular for the Transistor/Triac/Thyristor), I mean just dry contact by tight screwing will be ok?

Second, if the first is true, what thermal compound will be the best for that type of insulator?

2 Answers

I'm not sure what construction exactly you are thinking of, but the purpose of thermal compound is to fill the gaps between the device being heat sunk (often metal, but sometimes plastic or other material) and the heatsink (usually metal). Because the surfaces are both rough on a microscopic scale the heat sink compound improves the thermal conductivity between the two surfaces so there will be less temperature drop when power is flowing across the interface.

Different thermal compounds have different performance levels. The zinc oxide type is very common (the white paste) and is inexpensive.

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There is probably some extreme with both surfaces at mirror-like polish (and either compliant or very flat) where a thin layer of thermal compound no longer improves the conductivity, but it's a function of the surface state not the materials involved. In a hybrid circuit with an alumina substrate the metalization is in intimate contact with, and bonded to, the alumina so you would simply solder the part down.

Correct answer by Spehro Pefhany on December 14, 2020

Yes there are.

Aluminium Oxide is one of the main ceramic bases used by power module manufacturers (along with AlSiC, AlN). It is always recommended to include a Thermal Interface Material (TIM) to manage the "roughness" of the modules base and the heatsink.

The article from Semikron, a module manufacturer who uses Alumina, talks about the need for TIM.

https://www.semikron.com/about-semikron/news-press/detail/miniskiip-anticipating-the-future-for-20-years.html

The benefits these components boast in terms of Rth(j-s) and output current capability are tremendous. Based on MiniSKiiP housing size 3 with six-pack topology, a typical motor drive application has been simulated. The replacement of the standard TIM with HPTP (High Performance Thermal Paste) on the existing module on an Al2O3 substrate brings about a 34%reduction in the Rth(j-s)by. In a standard motor drive application operating 4kHz with an overload of 200% for 10s, the result is a remarkable 20% higher output current at the same junction operation temperatures

There will be equivalent articles from Cree, Infineon, OnSemi ...

If you can, use a graphite sheet. This is an extremely good TIM spreading the losses in the x-y plane to maximise the heatsink utilisation

Answered by JonRB on December 14, 2020

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