kapton film alternative

kapton film alternative
Posted by Jbates58 on 22 Oct, 2025 21:26
Hi all, I have some bitcoin asic miner boards im repairing, and originally they had a plastic insulating sheet between the bus bars and the ally heat sinks. The sinks have about 0.4mm clearance down from the top of the silicon ic package, so I don't think the insulator is strictly required.

However, I would like to replace it. I have drawn it up in cad and can cnc cut it out no problems, however I'm now trying to find a suitable material. The sheets measure 180mm x 130mm and there are 2 per pcb.

I was wondering if I can use laminate film for this? The boards reach about 85c maximum, then the unit goes into safe mode. And laminating is done at a higher temp than this, so I'm thinking they may be suitable.

So, I'm open to any ideas on what I can use instead of kapton sheet in the size I need is quite expensive. I need 50 pieces total for all the units I need to repair

Many thanks

#1 Reply
Posted by Psi on 22 Oct, 2025 21:30
Is this for thermal protection or electrical isolation?
Also, what sort of price was that kapton sheet you consider too expensive? You can get rolls on aliexpress for like $45 for 2meter x 0.5m @ 0.2mm thick. Obviously generic and not 'kapton' brand, but that seems pretty cheap to me.

One thing you can try is searching for polyimide film instead of kapton film, since kapton is just one brand of polyimide film.
But polyimide film can be made for different uses so you still want to read the properties of stuff before ordering.

You might be able to find some cheaper sheets of polyimide film made for the thermal bed of 3D printers. 180mm x 130mm sounds less than most printers. So that might be worth checking.

But there's lots of large rolls of polyimide film available on aliexpress if you dont care about getting brand name stuff.
I had a quick look at the thickest i saw was 0.25mm, i'm not sure what thickness you want. Are you trying to fill that 0.4mm gap?

#2 Reply
Posted by Jbates58 on 22 Oct, 2025 21:45
Electrical isolation. Its just to put a layer between the bus bars and heatsinks.

So any film that is ok working at up to 85c and can insulate 15v dc would do the job I think.

I did find some polyimide sheets, but the ones I found were expensive.

#3 Reply
Posted by Psi on 22 Oct, 2025 21:51
Typical film used in laser printers for overhead transparences might be able to handle 85C, you'd have to test it to be sure.
The fuser in the printer is way hotter than that but it's only exposed for a very short time.

Are you after adhesive backed film? A lot of the bigger polyimide rolls are not adhesive, you'd have to use some VHB pads or something to hold it down. Same with OHP transparences

#4 Reply
Posted by Jbates58 on 22 Oct, 2025 22:25
Haha, ivwas looking up OHT films last night but couldn't find a temp rating. But they would be perfect i would think.

Adhesive wise, it would be handy, but not required as the sheet will be held down between the ASIC chips and some thermal paste overflow. So just a floating sheet would be fine.

I will get some out films and put them in the oven at 100c and see what happens. Easy test I guess.

#5 Reply
Posted by Psi on 22 Oct, 2025 22:28
There used to be two types of OHP sheets, ones for laser printers and ones for ink jets.
The ink jet stuff couldn't always handle laser printer temps with unfortunate results if you use it. RIP printer

I think they mostly just sell the stuff for laser printers now for both laser and ink use.
People tend to get pissed off at companies who sell products that ruin their printer if used wrong. But get something that says laser printer on it, just to be sure.

#6 Reply
Posted by tHERMOL on 23 Oct, 2025 02:09
:=\I’d keep the insulator—0.4 mm is nothing once things flex. Skip laminating pouches; PET/EVA softens and creeps around 80–90 °C. Cheap, solid options: Nomex 410 (fishpaper) ~0.25–0.38 mm or thin G10/FR-4 ~0.2–0.3 mm; if you need flexible and thin, 100–200 µm polyimide sheets (A4) are affordable in bulk. Deburr the sinks and you’re set.

#7 Reply
Posted by ME on 23 Oct, 2025 02:32
What about nomex sheet?.

#8 Reply
Posted by amyk on 23 Oct, 2025 02:35
Mica is another possible alternative.

#9 Reply
Posted by Haenk on 23 Oct, 2025 06:18
Quote from: amyk on 23 Oct, 2025 02:35
Mica is another possible alternative.

Yep, should be the correct thickness, too.

#10 Reply
Posted by tooki on 23 Oct, 2025 07:16
Quote from: Psi on 22 Oct, 2025 22:28
There used to be two types of OHP sheets, ones for laser printers and ones for ink jets.
The ink jet stuff couldn't always handle laser printer temps with unfortunate results if you use it. RIP printer

I think they mostly just sell the stuff for laser printers now for both laser and ink use.
People tend to get pissed off at companies who sell products that ruin their printer if used wrong. But get something that says laser printer on it, just to be sure.
Well, it’s actually a hybrid product: laser printers require a transparency substrate which can handle fuser temperatures, and whose surface allows toner to adhere. Inkjet printers require the substrate to be coated in an absorbent coating that allows the ink to stay in place rather than beading up. A universal transparency requires a heat-resistant substrate and the absorbent coating (which must also be heat-resistant). And to really make it idiot-proof, you’d coat both sides so that it works no matter how the sheets are loaded. And finally, a universal transparency requires a paper strip across the top to allow printers that use optical feed sensors to detect it. (It’s basically a strip of post-it note, with heat-resistant adhesive of course.)

There were actually more than 2 kinds: For a long time, they sold separate transparency films for monochrome laser and color laser. Whereas with paper, there’s a clear reason for special color laser paper (it has lower dust, which color lasers are more sensitive to, and a smoother surface that works better for the higher toner load of color), it’s never been obvious to me what would need to be different in a color laser transparency. (Not that it really made sense to make color transparencies on a laser printer: the color toners are opaque, so your nice color transparency would project a black-and-white image! Dye-based inkjet is optimal for projection. Not that anyone uses overhead transparencies anymore...)

Also, the inks (and ink load) from different inkjet printers vary significantly, so an inkjet transparency from one manufacturer often did not work well in others. (For example, using Epson media in a Canon printer or vice versa.) Media designed for a low-ink-load printer would end up with ink pooling up before it could absorb and dry, producing uneven coverage as surface tension caused it to gather into little “waves”. Third-party media was usually labeled to work on all of them, but during the overhead transparency heyday, HP was the market leader, so third-party media was usually optimized for HP.

Of course, over the years, all the manufacturers have managed to develop the inkjet coatings to the point where they really do work well on most printers.