Sorry for not replying earlier. I couldn't imagine pay $40 dollars for a silly processing fee, ours is $10
I'd like to say thanks to everyone for all their help. I really appreciate the time you guys took out, to help me source these parts. Thanks, again.
I would imagine that you could cut a simple piece of paper to shape and soak in mineral oil. Wiped with a paper towel before installing it would provide both excellent insulation and heat transfer, though I would not be certain of it's longevity. may dry out over time.
TERRIBLE advice. 😱
DO NOT follow this useless dangerous suggestion. 😡
Use greased mica or Silpads ONLY.
NO!!!
Oil and paper have terrible thermal conductivity, about 0.1W/mK for oil, compared to the 8W/mK of most thermal grease. The difference is in the filler, usually aluminum powder.
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I just cut mine myself from Bergquist silicon heat-conductive sheet that I found in a drawer. Doesn’t everyone have drawers full of such things?
I gave myself the means of testing interface materials, both thermally and electrically:
A Thermal Ohmmeter
A Low-Lethality Dielectric Strength Tester
I would advise against "normal" paper, but baking paper or siliconized back-paper of self-adhesive stickers do not perform too badly, especially when combined with thermal compound.
There are other options, like the material of oven-bags for instance.
None should be used in HV applications though: the breakdown voltage is only a few hundreds volts, sufficient for transistor, <100V circuits
Baking paper belongs in the oven protecting my "secret recipe" cupcakes 🙂
And silicone "sticker sheet" paper is so treated on *one* face but not the other, and treatment is *thin* so at best it "improves" , say, 20% of its thickness ... and leaves 80% as before.
To boot, it´s relatively thick.
Let´s avoid giving poor ideas to noobs, who are easy prey.
IF none of the good materials is available, and you are in a hurry/emergency, you can reasonably use the thinnest mylar sheet you can get.
This one is 3.6 microns thick.
XRF Thin Mylar Support Films Ultra-Thin 3in. Wide x 300 ft x 3.6 mm Thick | 01866-AB | SPI Supplies
what it doesn´t offer in high thermal conductivity, is compensated by the short path through it.
Of course, still needs grease.
And silicone "sticker sheet" paper is so treated on *one* face but not the other, and treatment is *thin* so at best it "improves" , say, 20% of its thickness ... and leaves 80% as before.
To boot, it´s relatively thick.
Let´s avoid giving poor ideas to noobs, who are easy prey.
IF none of the good materials is available, and you are in a hurry/emergency, you can reasonably use the thinnest mylar sheet you can get.
This one is 3.6 microns thick.
XRF Thin Mylar Support Films Ultra-Thin 3in. Wide x 300 ft x 3.6 mm Thick | 01866-AB | SPI Supplies
what it doesn´t offer in high thermal conductivity, is compensated by the short path through it.
Of course, still needs grease.
Oven bags are made of mylar or PA, and they tend to be thinner than the precut mylars pads, which means a lower Rth, which is a good thing, lower breakdown voltage, which is bad in theory but is not a problem in practice because of the high dielectric strength, but the real difficulty is the tolerance to surface imperfections: the slightest burr or foreign body can pierce it, requiring great care and cleanliness.
Of course, the dedicated materials (both commercial and home-made) are still preferable.
Of course, the dedicated materials (both commercial and home-made) are still preferable.
Lots of responses. I'm sticking with the mica. Ordered the parts just waiting for them to arrive.
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I agree - Aluminum Oxide insulators are all that I am interested in using any more. They transfer heat WAY better than anything else and are not nearly as fragile as mica. I did a pretty robust comparison here: https://www.diyaudio.com/forums/pas...t-thermal-interface-material.html#post5590452
You can find plenty on ebay, search on "aluminum oxide insulator"
You can find plenty on ebay, search on "aluminum oxide insulator"
Alumina is much better than organic alternatives regarding thermal conductivity, that's agreed.
However, it would be difficult to form alumina into 0.05mm sheets -and it would be even more difficult to use them-
When a reasonable thickness is taken into account, differences tend to even out .
Regular alumina pads are extremely brittle, and do not tolerate the slighest amount of flexure without breaking, even for "normal" thickness types.
If high voltages are not required, the problem is a minor one: I had to replace a number of transistors thus mounted, and the alumina had in fact fragmented, but it didn't pose any problem, because these were low-voltage circuits, and the thermal capacity wasn't too much impaired by the breakage.
That said, alumina is not the only inorganic compound useful for thermal transfer: MgO is about 3x better, and is completely safe.
It is probably less easy to sinter into ceramics, but it can be mixed with binders like silicone to make high performance (and flexible ) interface materials
.
However, it would be difficult to form alumina into 0.05mm sheets -and it would be even more difficult to use them-
When a reasonable thickness is taken into account, differences tend to even out .
Regular alumina pads are extremely brittle, and do not tolerate the slighest amount of flexure without breaking, even for "normal" thickness types.
If high voltages are not required, the problem is a minor one: I had to replace a number of transistors thus mounted, and the alumina had in fact fragmented, but it didn't pose any problem, because these were low-voltage circuits, and the thermal capacity wasn't too much impaired by the breakage.
That said, alumina is not the only inorganic compound useful for thermal transfer: MgO is about 3x better, and is completely safe.
It is probably less easy to sinter into ceramics, but it can be mixed with binders like silicone to make high performance (and flexible ) interface materials
.
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Ah - very interesting! A few searches reveals lots of MgO lab-type equipment, but I haven't found any being made as flat wafers for transistor mounting...
If it weren't so toxic BeO would be a great material, much better conductivity than many metals...
You can order diamond plates today if you want the ultimate thermal washers, but cost is decidedly problematical still: Diamond Materials - Produktubersicht
Oh, and lets not forget aluminium nitride, perhaps the most practical of the ceramics in fact.Catalog Item
You can order diamond plates today if you want the ultimate thermal washers, but cost is decidedly problematical still: Diamond Materials - Produktubersicht
Oh, and lets not forget aluminium nitride, perhaps the most practical of the ceramics in fact.Catalog Item
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OK, in the face of new evidence - I'll amend my response above: AlO is better than anything else near its price range.
Wow $500+ for one sq cm of diamond insulator. If you're building a big amp with lots of transistors, that gets pricey rather quickly: $6k on thermal pads alone 🙂
Wow $500+ for one sq cm of diamond insulator. If you're building a big amp with lots of transistors, that gets pricey rather quickly: $6k on thermal pads alone 🙂
Diamond is used for some exotic RF devices I believe, where cost is no object (BeO is commonly used for the same). Diamond heat-spreaders would probably greatly reduce secondary breakdown in power BJTs.
https://core.ac.uk/download/pdf/82038916.pdf
https://core.ac.uk/download/pdf/82038916.pdf
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