Insulators-Kapton, mica, silpads. - Page 1
Forum section: electronics / projects

author: GamerAndds
author_rank: Regular Contributor
timestamp: February 21, 2015, 04:52:41 pm
content: I'm looking to get some more insulators for my amplifier (The Beast-). I want to use Kapton as it comes in bulk on tape but I want to know where it falls compared to mica and silpads. Rod Elliot (http://sound.westhost.com/heatsinks.htm#s7) does a great job in explaining the thermal conductivity but I want to know about the ruggedness and just general knowledge people have with it. Problems, ease of use, manufactures to stay away from?I was looking at this tape-http://www.ebay.ca/itm/50mm-5m-x-30M-Kapton-Tape-High-Temperature-Heat-Resistant-Polyimide-F019-50-/281395195400?pt=LH_DefaultDomain_0&hash=item418475ee08#ht_1921wt_1312Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2015, 06:25:38 pm
content: Adhesive tape is not usually used for this, as far as I know...Kapton's thermal conductivity is pretty abysmal as a thermal interface material, but it has very high dielectric strength and can be made in very thin films (which are also fairly robust), so it's competitive with others.Mica sounds nice (yay, solid rock!), but it's not actually much better, and typically sold in greater thicknesses (I presume it's difficult to reliably cleave or handle it much thinner?).In both cases, you also have double layers of grease to bother with.The best solutions seem to be:- Get a device with built-in ceramic insulator (usually die-bond or DBC on AlN); some TO-220, TO-247 and similar devices offer this, while it comes standard (AFAIK) on SOT-227 and other "module" type devices. Usually costs more.- Use a heat spreader (device(s) greased to metal plate; metal plate has more area, allowing lower Rth despite mediocre materials like Sil-Pads). Costs more, especially in mechanical parts. Dramatically increases capacitance.- Give up. Just use more transistors. They're not that expensive anymore, anyway.The first two will be the preferred solution for expensive devices, but those often come with isolation already -- RF transistors and power switching modules for example -- so they don't matter. As an alternative to certain other parts (I've seen diodes and MOSFETs offered this way, occasionally), they don't seem to be too much more expensive. Do keep in mind, like Full Pack devices, they come with higher RthJC to begin with.I suppose there's always a fourth option... think smart. You should never have to just *burn* power. Audio amp? Switching (class D). Load dump? Dump it into a battery (probably with switching converter). Stuff like that. Comes with greater complexity, of course.Tim

author: GamerAndds
author_rank: Regular Contributor
timestamp: March 01, 2015, 07:28:32 am
content: Thank you for your input, and sorry for the slow reply.I dont know if you saw the amplifer in the video but I use a 1/4" peice of aluminum to mount all 20 transistors on (40 in total for two channels, NTE 59/58). This acts like a heat spreader to the case which acts like the actual heatsink. The problem I am having is with the insulators between the heatsink and power transistors, and I dont wish to buy $$300+ in output devices again to get the ceramic insulated ones (wish I thought of that earlier).I guess I should have been more specific on my initial post asking "I want the best thermally conductive electically isolated regged tough material to use for my power devices."I wanted to use more like a long (15") strip of isolation (hence why I like the tape) instead of induviduals because its easier and more 'safe' in my mind. Also since the transistors dont have much more than a quarter inch in between I think it would be alot more efficent.I dont know if this is true, but I have read that the kapton tape's adhesive is made thermally conductive- but once again I would like to know it this is true.Thank you!

author: nanofrog
author_rank: Super Contributor
timestamp: March 01, 2015, 01:08:37 pm
content: I'm not a fan of Kapton (made to take the heat of soldering processes, not as a thermal conductor for heat sink purposes), but if you do, make sure you use some thermal paste with it, just as you would with say mica or ceramic (need to fill the voids).If you're after an easy to make strip, I'd recommend Sil-Pad over Kapton (you can get it in sheet form). Relies on pressure to fill the voids instead of paste, and is low cost (makes it highly attractive for manufacturing). It can shear during installation though, so be careful (very problematic with TO-3 packages as I understand it).But I have to ask,why aren't separate ceramic pads an option?They're better thermal conductors than Sil-Pads, and if you need better than that, there is beryllium oxide (expensive).

author: digsys
author_rank: Unknown
timestamp: March 01, 2015, 01:52:46 pm
content: I've spent literally 100s hrs comparing different thermal pad types in various configurations. I can't give you my spreadsheets, but I can definitelyhelp you start. Silpads are actually pretty crappy, even though they are as good as THIN mica. Here are a few links to start -www.fujipoly.com/usa/products/sarcon-thermal-management-components/thermal-gap-filler-pads/high-performance-gap-filler/xr-e.htmlwww.frozencpu.com/cat/l3/g8/c487/s1288/list/p1/Thermal-Thermal_Pads_Tape-Thermal_Pad_-_05mm_-Page1.htmlwww.digikey.com/product-search/en/fans-thermal-management/thermal-pads-sheets/1179751I've found over 30 degrees C difference between Silpads and some of other types, in controlled identical conditions, so it's very worthwhile !Arctic silver is the ideal, but sadly it's a paste and hard to guarantee uniformity. It came in at only 3C above heatsink.Edit: If you have to use Silpad 40 (std), get the Silpad 2000 version

author: Jeroen3
author_rank: Super Contributor
timestamp: March 01, 2015, 08:56:54 pm
content: I've found Ceramic Aluminium Oxide plates to be most effective in heat transfer. But fragile in handling, depending on the thickness.You do need two layers of non-conducting thermal paste for optimal results.This stuff:http://uk.rs-online.com/web/p/thermal-gap-pads/1777773/

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 02, 2015, 05:57:32 pm
content: If you're under 400VDC, the Aavid hard anodized aluminum insulators work well. Use the best thermal paste you can- my choice is Aavid phase change compound.