What do you call thermal pad that electrically isolates - Page 1
Forum section: electronics / projects

author: drakejest
author_rank: Regular Contributor
timestamp: September 20, 2021, 03:51:33 pm
content: I would like to buy some thermal pads that also electrically isolates them to the heatsink. What do you call themI need to cover a 75mm by 250mm area, I would like to attach a fanless powersupply to the case and since the case is sealed, hopefully lets sa case acts as a heatsink.

author: bjbb
author_rank: Contributor
timestamp: September 20, 2021, 04:11:11 pm
content: Look at products from companies such as Bergquist and Laird.All you had to do was search for something such as 'thermally conductive insulator' or go to the product search for some one like Digikey or Mouser or even Walmart.

author: drakejest
author_rank: Regular Contributor
timestamp: September 20, 2021, 07:51:51 pm
content: Quote from: bjbb on September 20, 2021, 04:11:11 pmA great idea, i found one that looks really nice but could not identify if they are electrically conductivethisgraphite based oneseems to be the best at thermal conductivity but if affraid if its electrically conductive can you check it out

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2021, 08:31:57 pm
content: The data sheet shows its electrical conductivity as 105S/cm = 107S/m, approximately that of graphite, while copper metal is approximately 6 x 107S/m

author: TimNJ
author_rank: Super Contributor
timestamp: September 20, 2021, 09:05:21 pm
content: "Sil-pad" is often the term used for a thermal pad between some TO-*** package and heatsink. These are electrically isolating.https://www.henkel-adhesives.com/us/en/products/thermal-management-materials/sil-pad-materials.html

author: pqass
author_rank: Super Contributor
timestamp: September 20, 2021, 10:05:11 pm
content: According tohttps://sound-au.com/heatsinks.htm#s7Kapton (polyimide) tape/sheet has a similar thermal resistance to Sil-Pads. Not cheap for the area you have in mind.

author: drakejest
author_rank: Regular Contributor
timestamp: September 20, 2021, 10:12:12 pm
content: Quote from: TimFox on September 20, 2021, 08:31:57 pmIm sorry i dont understand what those units means, what does S stands for?but look at it, am i right to asssume the thing is 6 time less conductive than copper? so for example if copper has a resistance of 10ohms for a given area the graphite has a 60 ohms worth of resistance?

author: TimFox
author_rank: Super Contributor
timestamp: September 21, 2021, 03:37:49 am
content: “S” means “Siemens”, the modern term for inverse ohms. So, your calculation example is correct. Back in the day, we called that unit “mhos”, ohms spelled backwards and abbreviated as an upside-down omega.

author: ajb
author_rank: Super Contributor
timestamp: September 21, 2021, 05:38:14 am
content: Quote from: pqass on September 20, 2021, 10:05:11 pmKapton's not that expensive. McMaster-Carr has 12"x12" 1 mil sheets for $12. Laird sell a polyimide film pre-coates with a ceramic phase change material which is quite convenient if the mating surfaces are flat, called Tgard K52. I think it's like $30 for an ~11x18" sheet or something. If the surfaces aren't flat enough for that to work well you can also apply your own thermal compound either side of the plain film, the conductivity of the compound doesn't matter at that point as long as it doesn't bridge over the edge of the film. But it's probably not worth using anything fancy anyway, the regular silicone stuff is probably fine since you have the polyimide film in the anyway.

author: pqass
author_rank: Super Contributor
timestamp: September 21, 2021, 06:53:47 am
content: Quote from: ajb on September 21, 2021, 05:38:14 amMaybe it's me but I can't get over the fact that a roll of 3" wide Kapton tape is over CA$100!Although, you do get a 36 yards of it which makes it cheaper per sqin than the sheet you quoted.My need just isn't that great and I haven't yet found a convenient source with the right quanity/$ for me.Last week, I just bought some TO-247 sized thermal pads for ~CA$0.50 per piece. I'm still bitter.