Thermal grease or compound would be used any way. I am looking for available viable / commercial options to do the job of electrically insulating and thermally coupling power transistor to the heat sinks.
Been using mica for years and never had a problem with heat dissipation or shorting.
While the other options do provide better heat transfer characteristics, achieving those in practice is highly dependent on several factors like perfect polished heat sink surface, clamping force etc. In my opinion, for consumer applications such as home hifi, mica should work just fine.
I have used both mica and silicon pads whichever was available at the time.
Whenever I use mica I strip down it to the minimum thickness, haven't seen any negative effects of reduced thickness of mica till now.
My personal opinion is that silicon pads are better in heat transfer job but maybe silicon pad should of good quality for endurance with a Class A amplifier, according to me even the thermal compound will act as a electrical insulator even if the silicon pad fails.
i've used it along TX-4 thermal paste - temperature near the transistor hovers around 60 degree C and stays the same irrespective of the usage hours.
you can also see the mounting method in this pic [you can refer to Rod Eliiott's site or my thread in DIY section for more detalis]:
Hello Yogi Bear,
Mica works quite well when used with thermal grease. However, commercially available mica sheets tend to be far too thick. Try splitting them to half their width with a sharp razor-blade. I've also used Bergquist sil pads successfully. If you do use them, purchase from a reliable vendor like Mouser or EFY/Element 14. However, good sil pads tend to be expensive and, in my opinion, not worth the extra cost over a thin slice of mica. As an aside -- the most common reason for thermal failure is not the TIM. In my experience they are: (1) excessive torque on the mounting screws (2) incorrectly drilled and tapped holes (3) poor surface finish on the heat sink. Transistors with single point mounts such as TO-220, T247 and so on are particularly prone to over-tightening, which causes the contact surface to bend. This is not a problem with TO3 types. Again, incorrectly drilled holes, especially when used with self-tapping screws intended for fastening sheet-metal or plastics can cause flexion at the point of contact. The photograph posted by All For Music shows an excellent technique to side-step this problem -- use a top plate firmly to push the transistor on the heatsink. If you have access to a junked computer motherboard, you might be able to repurpose the heatsinks that cover the processor's power supply MOSFETs with the added advantage to be able to re-use heatsinks for different projects without the need ever to drill more than two holes. For more information on correct transistor mounting technique, do look at the photographs of amplifiers posted by other members, such as Shaan and Aniket. I have not used kapton tape for this purpose, but it looks quite alright to me if the adhesive doesn't dry over time. If it does, then the voids left by the dried adhesive would hamper thermal transfer.