Sil-pad/mica alternatives? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: April 12, 2023, 11:37:41 pm
content: I need to mount a diode to a steel chassis, I have the diode on a large copper sheet that contacts the steel chassis in 2 placesThe contact area to the chassis is 2 spots, 16x32mm with an M6 bolt in the middle. The steel chassis is 2mm thick mild CRS, the copper plate is 1mm thick, surface is not very level.the diode will only output around 8 watts of heat but I want the diodes to be close in temperature so the bridge stays more-or-less balanced.The voltage between the copper plate and the chassis is <20VAC, 0VDC.I was thinking maybe plain paper, maybe soaking it in BLO so it doesn't absorb a ton of water, waiting for it to cure and then installing it.maybe cooking paper, iirc it's like silicone impregnated paper so maybe that has higher than average thermal transferor many layers of saran wrap?

author: tooki
author_rank: Super Contributor
timestamp: April 12, 2023, 11:50:53 pm
content: Why would younotuse mica or silpad? It’s what they’re literally made for…

author: Siwastaja
author_rank: Super Contributor
timestamp: April 13, 2023, 11:11:12 am
content: Kapton tape is OK-ish. You want to make the surfaces and their edges 100% burr-free so that you don't accidentally cut through the tape, shorting the thing out.If you don't have Kapton tape, then I guess all you can do is use any random packaging / office tape etc. Using thin tape helps with thermal transfer, at the cost that you need to be even more careful with deburring the edges and cleaning the surfaces to avoid punch-through.Plastics usually are ten times less thermally conductive (so around 0.3 W/(mK)) than properly engineered silpad materials, so 0.1mm thick tape would correspond to a 1mm thick silpad.Kapton seems to be around 0.8 W/(mK) so surprisingly good for a plastic tape.

author: pqass
author_rank: Super Contributor
timestamp: April 13, 2023, 12:30:57 pm
content: Seehttps://sound-au.com/heatsinks.htm#s7for alternatives and their performance.

author: mikerj
author_rank: Super Contributor
timestamp: April 13, 2023, 01:13:02 pm
content: Quote from: ELS122 on April 13, 2023, 11:02:30 amNot quite sure what you are trying to say, but mica has been used to provide electrical insulation between power semiconductors and heatsinks for a very long time.

author: jonpaul
author_rank: Super Contributor
timestamp: April 13, 2023, 02:07:23 pm
content: silpad is good, but we just eliminate the problem with floated HS.j

author: Andree Henkel
author_rank: Regular Contributor
timestamp: April 13, 2023, 02:11:01 pm
content: What a stupid argument. What are actual Problems you face using Mica? Except that you don´t want to buy it?modified citation:"rubber (from tree) is a natural material, it was not actually made for making tires"

author: Andree Henkel
author_rank: Regular Contributor
timestamp: April 13, 2023, 02:13:12 pm
content: Quote from: jonpaul on April 13, 2023, 02:07:23 pmwell in my former company we had a rule: basically never let a metall part attach to a pcb be floating, you can run into a lot of trouble otherwise.

author: tooki
author_rank: Super Contributor
timestamp: April 13, 2023, 03:54:34 pm
content: Quote from: ELS122 on April 13, 2023, 11:02:30 amQuote from: ELS122 on April 13, 2023, 01:43:05 pmNo, but thermal insulators made of mica are intended for it.(Also, some mica products are not straight mica sheets, they're composites made from ground mica. An absolutely manmade product.)Quote from: ELS122 on April 13, 2023, 11:02:30 am[Citation needed]Quote from: ELS122 on April 13, 2023, 11:02:30 amSo... you claim mica is "clearly not that good", yet you need barely any thermal transfer. So... what's wrong with mica again?Quote from: ELS122 on April 13, 2023, 11:02:30 amPaper? A thermal insulator?Just why do you think mica got used so much? Because it's not expensive, and works well. Many alternatives can get expensive real quick.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 13, 2023, 04:38:16 pm
content: Quote from: ELS122 on April 13, 2023, 01:43:05 pmI personally only use supernatural materials, especially those from the ninth dimension.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 13, 2023, 04:44:50 pm
content: Dumbest thread in a while, but what the heck, I'll play. Electrical insulators are, with few exceptions, terrible thermal conductors compared to metals. Mica, polyimide, Sil-Pads, paper, packing tape- all terrible! "Thermal" grease- terrible, even the fancy filled stuff. The successful ones are successful because they're thin and have good dielectric strength so they don't arc or punch through, not because they're good thermal conductors. Don't ever rely on grease for insulation. I shouldn't even have to say that, but some people have. Thin goes hand in hand with having decent flat surfaces. The minute you say the surfaces aren't that good, all predictions go out the window. There are some modern thick pads that are specifically designed for non-flat surfaces and they do way better than the old stuff. Are they good thermal conductors compared to metals? Nope, terrible. Keep your interfaces thin.Now, if I were incredibly frugal (I am) I'd consider finding some discarded electronic item, say an old piece of audio equipment or appliance at the dump or on the curb, and seeing if it had some mica or sil pads inside. Odds are pretty good. If dumpster diving isn't your thing, I'd probably go with Mylar shipping tape because Mylar (polyester) is a known thing. I'd probably clean off the adhesive with solvent and use some thermal grease on both sides, unless the application is really non-critical. Then I'd just stick it on. Not obscure enough for you or not sufficiently labor intensive? Kodak made a lot of film using Mylar as the base. Find an old negative and soak it in chlorine bleach to remove the gelatin emulsion. Voila, clear Mylar.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 13, 2023, 04:58:40 pm
content: Quote from: Conrad Hoffman on April 13, 2023, 04:44:50 pmThat's true and good for first-order approximation (i.e., I agree one needs to learn what you said first). But there is a lot of variation from terrible to not-so-terrible.For example, some random paper might be 0.1 W/mK, some random plastic 0.3 W/mK, a better plastic 0.8 W/mK, a typical cheap glassfiber reinforced Sil-Pad 3.0 W/mK, and a modern well-engineered non-reinforced (but it doesn't matter if you have the physical separation) Sil-Pad at 15 W/mK! While all of them are far from the aluminum heatsink itself (200 W/mK), the better SilPads are notthatfar. If the power density isn't ridiculously high (and you can always make it lower with a copper heatspreader), these can be useful even in large thicknesses like 2-3mm (sold under names like gap filler or gap pad). Typical example would be mounting a complete PCB assembly to a heatsink, with some tiny SMD passives in the way, requiring a conforming pad. In such case, let's say a DPAK which otherwise can dissipate only 1W can now dissipate 2-3W or so just fine, a big difference.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 13, 2023, 05:03:17 pm
content: Quote from: Conrad Hoffman on April 13, 2023, 04:44:50 pmSurprisingly, most of the color or B&W negative shooting stock was and still is, AFAIK, on celluloid triacetate base. Motion picture print films were polyester (trade name Mylar) for mechanical robustness (against wear and tear when projected gazillion of times).Some negative stocks did use Mylar as you say because I remember people complaining how they curl more when drying but can't remember any specific examples now.

author: Gyro
author_rank: Super Contributor
timestamp: April 13, 2023, 05:23:02 pm
content: The average Mica washer can be cleaved 2-3 times surprisingly easily with a razor blade on the edge. I can't remember where I first read it, but for low voltage applications (and with well de-burred surfaces), this can significantly reduce thermal resistance.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 13, 2023, 05:42:54 pm
content: Attempting to readibetween the lines because the OP has not said this explicitly it seems they have chosen a custom copper heat spreader that is not insulated from the diode package. This means that off the shelf transistor insulation kits probably won't be the right size or have the mounting holes in the right place to mount the heat spreader.So one question is why? If your diode is a standard package there are probably insulating kits available for it that could attach it to the copper heat spreader. Then you don't have to worry about insulating the heat spreader from the case. If the power requirements are not very high, that's a pretty easy option.If that isn't desirable for some reason, sil-pads are a lot easier to cut to size and add holes than mica, here is a $1.32 option that could be cut to size:https://www.digikey.com/en/products/detail/3m-(tc)/35MMX35MM-8810-SQUARES/3529589Anything else will probably perform worse or cost a lot more than the sil pad, but kapton is probably the best of whats left considering cost, availability, thermal, mechanical, and electrical properties. It just requires careful deburring and mounting to avoid piercing holes in it.There is more than the interface material. You need insulating bushings for the screws, and to chamfer and deburr the metal parts where you drill the holes in order to avoid shorting out at that point. If you get a "TO220 insulating mounting kit" it will usually come with all the parts you need, but if you DIY it you will need to take care of that yourself.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 13, 2023, 06:57:10 pm
content: Quote from: Siwastaja on April 13, 2023, 05:03:17 pmMany Kodak sheet films (4x5 stuff) were advertised as having "Estar thick base", which was nothing more than their name for Mylar.

author: jonpaul
author_rank: Super Contributor
timestamp: April 14, 2023, 01:50:50 am
content: Rebonjur,We used many insulating tapes in magentics, Kapton is hard to cut or diecut and can be easily punctured.The thin tpaes like 3M56 same issues.Tiny burrs on the HS can eventually puncture the film.The thermal R of the interface is easy to calc , a func to area, Delta T and thermal R.See the many app notes and papers on thermal managementj

author: TimFox
author_rank: Super Contributor
timestamp: April 14, 2023, 03:49:20 am
content: Quote from: Conrad Hoffman on April 13, 2023, 06:57:10 pmMylarTMis a DuPont trademark, and EstarTMis an Eastman Kodak trademark for the same thing: polyethylene terephthalate (PET) polyester.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 14, 2023, 01:52:27 pm
content: Kapton tape handles quite high temperatures so is generally preferred, but it isn't sitting on everybody's shelf, though IMO it should be. In this case almost anything thin would appear to work.

author: coppercone2
author_rank: Super Contributor
timestamp: April 14, 2023, 02:01:43 pm
content: You can put ceramic pads, but they are even harder to engineer in, and you can get toxic dust from cutting them, and when you buy them, they always come in the wrong size for your heat sinkso when someone complains about silpads being hard, just show them ceramicI use them to keep instruments consistent on restorations,.

author: TimFox
author_rank: Super Contributor
timestamp: April 14, 2023, 02:46:14 pm
content: Your country flag isn't set.If you are in the US, seehttps://www.mcmaster.com/products/washers/material~mica/orhttps://www.mcmaster.com/products/mica/

author: TimFox
author_rank: Super Contributor
timestamp: April 14, 2023, 08:51:43 pm
content: I try to give useful information.Even though you reject mica for not-well-specified reasons, I thought you might like to see the specifications for normal mica.Note that I did not disagree with you, nor demand that you use mica, I just gave you some factual information.Perhaps someone else looking at this thread for information can make use of that site.

author: sparkydog
author_rank: Regular Contributor
timestamp: April 14, 2023, 09:41:57 pm
content: Quote from: ELS122 on April 14, 2023, 08:43:25 pmThere's a Stack Exchange for that. I see forums like this as rather a place for people to get together to discussspecific projects, as a way of gaining access to people with different knowledge that might not otherwise be available. A way, in other words, to learn by tinkering... and, yes, as a way to ask other people to check your "homework" because it didn't come out of a book that has a "right" answer and becauseyou don't have a handy teacherto tell you if you got it right.That's not to say that general questions aren't (or shouldn't be) allowed, but please don't try to turn this into another ivory tower where applied problems are off limits....my 2¢.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 15, 2023, 05:53:55 am
content: Your question was formed OK, maybe you could have elaborated a bitwhymica is not an option. "I have to do it with what I have available because I'm on a deserted island and can't mail order any mica or silpad" is understandable.However, remember this is a discussion forum, not a question - answer site. We are not here to answer your specific question, but rather using it as a starting point. Which is great because then we will see more variance in the replies and the replies could help others to learn. You can also get inspiration from some ideas you did not think about.8W through 2*16*32mm=1024mm^2 is not a problem. For example, if you use random plastic film 0.3W/(mK), with thickness of 0.2mm, you will develop 8W / ((0.001024 m^2 / 0.0002 m ) * 0.3 W/mK) = 5.2 degC temperature difference over the thermal interface material. Hardly a problem.Although, if the surface is, as you say, "not very level", if you effectively only use the thermal conductivity of 1/4 of that 1024mm^2 - then the temperature difference would be 20degC already, eating quite a bit into the margins.Which brings us to the purpose of thermal interface material - it's needed to fill the gaps, providing contact over the whole area. Plastic sheets, mica, etc. are just insulators and need additional thermal interface material like thermally conductive grease. Sil-Pad on the other hand is a complete solution.You might want to experiment with paper soaked in mineral oil (the oil would fill the gaps), but no one can give you guarantees how it works. Will the oil escape? Or get dirty and conductive?I have had excellent results using bog standard double-sided tape. The thinnest type you can get, not the thick foamy stuff! When the surfaces are relatively smooth and level, the glue of the tape itself fills any microscopical voids, and the tape carrier being just some 100µm thick, it's OK even if not engineered to high thermal conductivity. And, when the surfaces are smooth and clean, the tape is also so strong it suffices for mechanical mounting. But no one knows how well it ages.I'm a bit worried about the heatsinking capability of that steel chassis. I tried to google CRS steel as this was a new term to me, and, it can mean two totally different thing: cold rolled steel and corrosion resistance steel. This is why I hate acronyms. If you mean stainless, the thermal conductivity sucks greatly, some 15 W/mK. Normal carbon steel would be closer to 40-50 W/mK, still crap compared to aluminum, but not that bad.It's a more complex modelling question, but for a rough idea, calculate the temperature difference when the heat tries to go through the 4 sides of your 16x32mm window. The total surface area the heat has to go through is 2*2mm*16mm+2*2mm*32mm = 192mm^2. Given carbon steel, this area and 4W of power, now eachcentimeterof depth means temperature drop of 4W / ((0.000192 m^2 / 0.01 m ) * 45 W/mK) = 4.6 degC. The heat clearly can't go very far. Most of the dissipation has to happen within your coupling window area 16x32mm plus maybe 20mm in every direction, i.e., something like 50 x 70 mm. Now the silver lining is, 4W per such area is not a lot of power.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 15, 2023, 03:07:32 pm
content: I once mounted a TO-220 device to a thin steel chassis wall and was amazed at how poorly the heat spread and how how it got directly under the device. We added a spreader and it was sufficient, but just barely so.There's a crazy expensive product, Aavid Ultrastick, that's a filled paraffin. Fancy term for wax.https://media.digikey.com/pdf/Data%20Sheets/Aavid%20Thermal%20Technologies%20PDFs/ultrastick-100300F0000G.pdfThe stuff melts when the device heats up, giving you the thinnest possible layer and a liquid interface, and thus excellent heat transfer. It works very well, but it used to be way more affordable. Several other companies have similar stuff. One caution- don't use it in optical systems, especially vacuum, as it outgasses and will deposit on the optics. I suspect one could DIY something that would work as well- candle wax or a block of paraffin, plus a filler. The Ultrastick MSDS says it's 10-25% zinc oxide. That or aluminum oxide (alumina) powder would work. You could also melt some paraffin into a sheet of paper or card to good effect.

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2023, 03:30:13 pm
content: Quote from: ELS122 on April 14, 2023, 02:14:10 pmUnless I missed it you haven't given the dimensions of your copper 'spreader', but just from the discussion your description of the part you want to thermally bond to your steel chassis I would guess that simply floating the plate on standoffs is going to yield almost exactly the same result as thermally bonding it to your chassis as you describe. Of course for a more exact answer it would be helpful to know the dimensions of the chassis, the spreader and if the chassis is sealed or ventilated.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 15, 2023, 05:11:35 pm
content: Quote from: Conrad Hoffman on April 15, 2023, 03:07:32 pmYeah, thin steel has basically no lateral heat transfer. This is why we see steel handles in kettles and they work fine, as long as they are formed from thin sheet material. The 2mm thickness comes to the rescue for the OP, though.Quote from: bdunham7 on April 15, 2023, 03:30:13 pmGiven thinner steel, I would agree. With the 2mm thickness, the chassis is going to effectively at very least double the surface area available for effective cooling, if not more. (Based on my earlier napkin calculation model, from 2x512mm^2 per each spreader, to 512mm^2 + 3500mm^2 per each spreader. But take this with a grain of salt.)

author: bdunham7
author_rank: Super Contributor
timestamp: April 15, 2023, 06:24:14 pm
content: Quote from: Siwastaja on April 15, 2023, 05:11:35 pmThe OP mentioned a "large copper sheet" which I took to mean significantly larger than the proposed thermal bonding areas. I asked some questions earlier regarding details of the spreader and chassis, but unless I've missed it, they weren't answered. In case the OP wants to crib that I'm not answering the question, I actually am. My answer is that, depending on the answers to my dimensional questions, the most appropriate substitution for mica or sil-pad may turn out to be air.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 15, 2023, 06:53:54 pm
content: With the standoffs as you suggest, air has the benefit that there will be convection from both sides of the copper spreader instead of just one, so the whole chassis surface can, theoretically, be utilized for cooling. This requires large temperature difference, though.OP needs to run a practical test (long enough - reaching equilibrium could take a few hours), while monitoring Tj. Given constant diode current between runs, difference in Tj can be approximated from the Vf using the temperature coefficient listed in the datasheet.

author: rdl
author_rank: Super Contributor
timestamp: April 19, 2023, 02:06:34 pm
content: Did you make and apply the paint yourself?

author: rdl
author_rank: Super Contributor
timestamp: April 22, 2023, 01:20:39 am
content: I was just curious because there's probably not been any commercial paint made like that in over a hundred years.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 22, 2023, 01:47:14 pm
content: Never underestimate simple DIY recipes for specialized projects. Lampblack paint with high emissivity for IR temperature measurements. Put a few drops of nitro dope on a clean water surface and let it dry. Use a wire loop to lift off the film. Makes an excellent optical pellicle beam-splitter. You can add colloidal silica, carbon fiber, graphite and many other things to clear epoxy to get special properties. DIY forever!