TO-3 Transistors: Mica or Silicone Insulators?

[TomCarlos]

MEF Members -

Just curious, do you have a preference when it comes to using Mica or Silicone Insulators for TO-3 transistors?

I was wondering if or how thermal grease might impact one or the other.

What say you? Vote now!

Thanks, Tom

[dmeek]

Here's a good article on heatsinks, includes a comparison of insulators. It appears Mica with thermal grease is better

https://www.nxp.com/files/rf_if/doc/app_note/AN1040.pdf

Also I have seen a few times where a sharp edge of a transistor has cut through a silicone insulator and shorted out.

[nevetslab]

Here's a good article on heatsinks, includes a comparison of insulators. It appears Mica with thermal grease is better

https://www.nxp.com/files/rf_if/doc/app_note/AN1040.pdf

While the industry trend has been to use the low-labor imbedded silicone rubber insulators, of which there is a wide selection in the thermal loss vs cost, they are used to minimize labor cost and eliminate the mess that accompanies applying silicone grease to mica washers. The best approach is isolated heat sinks, using just thermal grease between the power semi's and the heat sink, such as found on Ashdown EVO II ABM 900 or ABM 500 bass amps. Next to that is greased mica washers on common heat sink where the heat sink is grounded. The thermal resistance goes up with every boundary between heat source and interface layer. Then, you're into the various silicone rubber insulators. Bergquist K6 and K10 weren't available when that AN1040 document was written. Those are the two best isulators from Bergquist....the K10 being the better of the two, while cost is nearly three times that of K6, which you'll find more people using.

Mica washers are the cheapest, while silicone grease IS expensive....Dow Type 340 white silicone grease is an industry standard, and, IS messy. There are others less expensive, such as Super Lube Silicone Heat Sink Compound. Not as messy, easier to get off hands and clothes. I haven't looked for white papers on comparisons of the various greases.

[TomCarlos]

Thanks Gents....

This is what I was looking for. I appreciate the replies.

Tom

[DrGonz78]

Can I throw a curve ball into the question? Ceramic insulators???

https://jimsaudio.com/ceramic-insula...t-conductiviy/

[52 Bill]

If there are silpads on the originals, I will inspect and reuse when possible. When the insulators must be replaced, I just use mica and Wakefield silicon grease.

[nevetslab]

Can I throw a curve ball into the question? Ceramic insulators???

https://jimsaudio.com/ceramic-insula...t-conductiviy/

I had looked into those years ago when the rubberized silicone washers were coming onto the market. The Ceramic washers, always used with thin layer of silicon grease on each side, work VERY WELL, though of over-torqued, can crack. Their cost is their major downfall, but, you will find them in audiophile gear and other areas where cost is NOT a constraint.

QSC used Ceramic Insulators on the power semi's of the front end of their Powerlight 6.0 and 9.0 Power Amps. PITA to service, overall, though I did find while replacing the rectifier diodes in the front end, the Ceramic insulators had cracked on a couple of the amps I serviced.

[nevetslab]

If there are silpads on the originals, I will inspect and reuse when possible. When the insulators must be replaced, I just use mica and Wakefield silicon grease.

When silpads or whoever's greased-mica substitute is installed, ALWAYS look close at those. Usually, after years of being installed, thermoset takes place which acts like an adhesive, and prying off the insulator all-too-often tears away some of the material. That has now damaged the insulation and you must replace it. I have gotten away with not replacing them, but only after close inspection....though I don't recommend that. Replace to be safe!! Yup....mica and quality silicone grease!

[Enzo]

Mica works, silicone sheets work. Other things work. One can make a case that one is "better" than another. But really, we just need it to work. SO oit isn;t a matter of which is "better", it is a matter of good enough for the job or not.

When I work on something with heat grease, I ALWAYS wind up covered in white goo. I prefer the nice clean silicone for that reason alone.