What isolation between chips & heatsink are people using?

[taj]

What insulator do you use between your LM3886 or LM4780 (or whatever) and the heatsink? Is there a mica sheet that size or any of the other silicon pads? I have some SILpads for TO-247 packages but they're not really big enough for these chips. Same goes for the TO-3 micas.

I've heard Kapton tape works great, but I can't find a source for it without adhesive on one side (which I assume is not a good idea).

thanks,

..Todd

[Juergen Knoop]

I used mica, SOT93 or TO 218
don't remember it exactly...
regards

[Hartono]

usually mica don't have adhesive( I've never seen any with adhesive), put thermal paste on both side of the mica.


"analog scopes don't lie" <--------I like this

they do lie, but less than cheap digital scope

[v-bro]

I like to use silicon pads, you can get it in larger sheets so you can cut it yourself.

I get this stuff at work, but any electronics supplier should have it...

On the other hand there's nothing wrong with mica and paste....once it's mounted you won't notice the difference anymore

[Conrad Hoffman]

It always bugged me that published thermal resistances for mica, Sil-Pads, Kapton, alumina, beryllia, and anodized aluminum, are never in the same units. To this day I don't have the numeric answer, but can tell you that the heavily anodized aluminum sheets Thermalloy sold (sells?) are probably best, but not for high voltages. The beryllia ceramics are excellent, but hazardous. Mica and thermal grease should be good enough for any audio application. I use Sil-Pads for my own stuff, for convenience and lack of mess, but they may not be as good as properly installed mica and grease. I've also used alumina ceramic insulators. They're pretty good thermally, but the big advantage is that the thickness gives very low capacitance to the heat sink. You'll need a good sized piece of whatever you choose for the amp chips, and I'd go with the higher performance versions of Sil-Pads. If you can't put your finger on the chip at max temp, then get a bigger heat sink, and/or improve the thermal coupling with a better insulator. You have the advantage that all the amp chips have very good internal over-temp protection, though I don't like to test it unnecessairly.

[taj]

I apologize, I didn't phrase my question very well so the responses are not going the general direction I was hoping. I'm pretty well aware of what the material options are and how to use them. (Mica with adhesive? sheesh.) Anyway...

juergenk & V-bro, thanks for reading the between the lines and figuring out what I was talking about. I'll check into larger sil pads and SOT93/T0-218 sizes, which I'm not familiar with.

I was curious more about availability of various materials at the size of these chips, not the performance so much, or how to use mica.

Where can I buy mica sheets that large? Does anyone know a supplier for non-adhesive Kapton tape or sheets?

..Todd

[taj]

After a little research, I've found Digikey carries a size of Berquist heat sink pads for CPUs: 1.375" (~35mm) square, part no. BER135-ND, which, if cut in half, looks like it would work very nicely.

..Todd

[Juergen Knoop]

don't forget the screw insulation!
regards

[taj]

Good point. I'm planning to use a pressure clamp device that I've been playing with. If that doesn't work as expected I'll probably bolt a flat pressure plate pushing evenly against the whole chip.

The screw holes on the chip seem like a last resort option to me.

..Todd

[TwoSpoons]

Chip -> heatsink? NONE

Heatsink -> chassis ? Any old lump of plastic I can put a screw in!

In case you are wondering, my heatsink will be totally enclosed by the chassis - with a couple of lowspeed fans pushing air through the whole thing.