[teknora]

I want to fix an electronic component working around 135 degree celcius by a material which must be thermally conductive and electrically insulating glue, adhesive, sealant or filling. Could you help me?

[Enthalpy]

Hi Teknora, welcome!

Your query is so usual in electronics that it's already solved and complete companies live just from providing the adequate products, so you unlikely get a better suggestion than the existing ones... Unless it's your job to develop such insulators, just stick to what exists.

Have a look at Berquist and Chomerics for instance (maybe they changed their names meanwhile).

Basically, if the stray capacitance isn't a worry, the path is to use a thin strong material. Historically mica, has been polyimide (Kapton), now it's silicone reinforced with a glass web, being soft enough to conform to both surfaces without additional grease. Some are adhesive.

If you mind stray capacitances, say at a switched mode power supply, or an RF transmitter... it gets more complicated. There you need a thick insulator, which limits the heat conductance; the material's figure of merit is the heat conductivity divided by the permittivity. Al₂O₃ is affordable and common, AlN more expensive, toxic BeO is phased out. As the plates are stiff, you need a grease to make the contacts, but this grease can conduct electricity.

The insulating pads are already cut to the component's shape, and insulating parts exists for assembly screws too.

Higher voltage (>>100V, check the datasheets) would be an additional worry.

Both surfaces must be flat enough.