Heatsink: Silpad sucks, Paste sucks, what else? - Page 1
Forum section: electronics / projects

author: pope
author_rank: Frequent Contributor
timestamp: April 28, 2025, 12:26:30 pm
content: OK, the title is a bit provocative, but I've been experimenting with different ways of attaching an LM317 to a heatsink.So far, I've tried using a silicon pad, thermal paste, and direct attachment (i.e., the LM317 mounted directly onto the heatsink without anything in between). As expected, the best thermal transfer was achieved with thermal paste, followed by direct attachment, and finally the silicon pad, which performed the worst. Surprisingly, attaching the LM317 directly to the heatsink provided much better thermal transfer than the silicon pad (and only a couple of degrees worse than thermal paste).Here are the detailed measurements (plastic body temperature of the LM317). I used an IR camera pointing on the 317plastic body with emissivity =0.94:62–62°C with thermal paste64–65°C with direct attachment (no paste)70–72°C with a silicon pad (0.4K/W)The LM317 is mounted on its own dedicated heatsink, so I suppose electrical insulation isn't necessary in this case.Common sense would suggest just using thermal paste and calling it a day. However, I really dislike the mess that paste creates — and more importantly, the fact that it needs to be reapplied after a few years.So, my questions are:1) Can I simply use direct attachment between the LM317 and the heatsink and not worry about inevitable air gaps?2) Is direct attachment better than dry thermal paste?3) Are there any efficient alternatives (besides thermal paste) that can fill these air gaps without needing maintenance or replacement over time?As always, your opinions are much appreciated!

author: Ian.M
author_rank: Super Contributor
timestamp: April 28, 2025, 12:44:23 pm
content: Good quality themal paste lasts for decades if undisturbed. Repeat your measurements with a thermocouple on the top surface of the mounting tab (which will be close enough to the bottom surface temperature) to get numbers you can use with the thermal parameters in the datasheet to determine Tj-max.

author: pope
author_rank: Frequent Contributor
timestamp: April 28, 2025, 12:52:37 pm
content: Thank you, Ian. I guess the first question that comes to my mind is: why should I go through all this "hassle" when the temperature difference between direct attachment and using thermal paste appears to be only a couple of degrees? Am I missing something?

author: mikerj
author_rank: Super Contributor
timestamp: April 28, 2025, 01:19:39 pm
content: There are other high performance options that avoid the mess of grease, e.g. phase change pad, graphite pad.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 28, 2025, 01:36:42 pm
content: 0.4K/W or W/mK?

author: pope
author_rank: Frequent Contributor
timestamp: April 28, 2025, 01:42:31 pm
content: Quote from: Ice-Tea on April 28, 2025, 01:36:42 pm0.4K/W

author: Ian.M
author_rank: Super Contributor
timestamp: April 28, 2025, 02:10:44 pm
content: When you run the numbers for your application using the datasheet thermal parameters, the max. possible dissipation, and the max. probable ambient temperature (at the heatsink) you will know whether or not you can get away with a Silpad, or need something better. Unless you can guarentee the unreg. rail is at max (i.e high line condition if supplied by a line frequency transformer) and the load current is at max, measured temperatures will only tell you if you already have a problem, not if it can withstand worst case conditions.

author: pope
author_rank: Frequent Contributor
timestamp: April 28, 2025, 02:26:02 pm
content: The temperatures I mentioned are at the maximum mains voltage of 253 VAC. European mains specifications are 230 VAC ±10%. At least, these are the regulations; however, I'm not sure if there are cases where the mains voltage can exceed 253 VAC.Also, I did the math, and as far as I can tell, I can get away with using a Sil-Pad.Perhaps I should rephrase the question:After theLM317 → Thermal Paste → Heatsinksetup, what would be the second-best attachment method if only thermal transfer is considered and electrical isolation is not required?LM317 → HeatsinkorLM317 → something else → Heatsink?

author: SteveThackery
author_rank: Super Contributor
timestamp: April 28, 2025, 03:27:29 pm
content: Have you considered also testing it with a mica (or modern equivalent) insulator between the IC and the heatsink? I ask only because it is a very common configuration which electrically isolates the heatsink.

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 28, 2025, 03:38:50 pm
content: Are you torquing the mounting screw to specification? If you overtighten, you can distort the mounting tab on the regulator, leading to poor thermal contact.about 0.5Nm is the sweet spot for TO-220 packages.Some references to take a look at:https://www.onsemi.com/pub/collateral/and9859-d.pdfhttps://www.aosmd.com/res/application_notes/package/AN101_TO220_Guidelines.pdfhttps://www.vishay.com/docs/72674/72674.pdfhttps://www.infineon.com/dgdl/Infineon-Infineon-AN2006_12_Mounting_Considerations_TO220_FullPAK-AN-v2.1-en.pdf-ApplicationNotes-v02_01-EN.pdf?fileId=db3a304320896aa201208a8b700d0029

author: pope
author_rank: Frequent Contributor
timestamp: April 28, 2025, 03:54:24 pm
content: That's a valid point. In fact, I don't have any torque screwdriver although I was planning to get one at some point.Perhaps thiskind of mounting snap rivetsis even better in terms of correct torque even though I don't need insulation?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 28, 2025, 04:55:49 pm
content: Random thoughts:1) There are several grades of SilPads. The common grey ones are just OK, but the more expensive ones are way better. Other than the cost, that might be the easiest and least messy solution.2) All thermal insulators are lousy. We only get away with them because we can keep the thickness low. Zero is best, but you still need to fill the inevitable air gaps. If you can lap the parts to a smooth flat surface, that helps, but you still need to have something to fill the imperfections. Semiconductor devices are invariably flat, but heat sinks vary widely. A machined surface is better than as-extruded.3) Though expensive, I like the Aavid phase change compound-https://www.digikey.com/en/products/detail/boyd-laconia-llc/100300F00000G/306960It liquifies when heated, allowing the gap to close up as much as physically possible. AFAIK, it's basically a paraffin wax loaded with 10-25% zinc oxide powder. I'm guessing you could make your own with little trouble. You don't use much, so it doesn't tend to make a mess. It also doesn't transfer like silicone grease.4) Copper heat spreaders can sometimes help.5) Don't feel you need heroics if the worst SilPad will do the job!

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 28, 2025, 04:58:18 pm
content: Those plastic rivets are about the worst thing you could use....Tightening a screw to 'about right' without a torque screwdriver is faaaar better.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 28, 2025, 05:55:53 pm
content: Bit annoying the listed thickness is 0.5-1mm but it looks like 1W/mK material. You can get 4-5x better material.

author: bdunham7
author_rank: Super Contributor
timestamp: April 28, 2025, 06:09:20 pm
content: Quote from: pope on April 28, 2025, 12:26:30 pmUse less (a lot less) paste and don't worry about reapplying it--ever. That's just something gamers do when they're overclocking their GPUs. Thermal paste on normal semiconductors will work fine for many decades, probably centuries. Also, your LM317 body temperatures are all just fine in all three cases. Using a sil-pad or mica may prevent an accidental short circuit some day.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 29, 2025, 08:29:24 am
content: Quote from: pope on April 28, 2025, 03:54:24 pmAvoid those.The best mounting method for long term stability is a clip.Faster than a screw and not operator dependent.Many heatsinks include clips.Examples of separate clipshttps://www.sameskydevices.com/sitesearch/searchbypartialmatch?partnumber=clip

author: pope
author_rank: Frequent Contributor
timestamp: April 29, 2025, 08:44:24 am
content: Thank you for you suggestions!A few thoughts/ notes:1) AFAIK change phase also dries out after some priod of time, right?2) Can you recommend some good quality and as thin as possible silicon pads?3) Can you recommend some good quality thermal paste?4) If I decide to isolate the heatsink I will also need an insulating bush so, this will help avoiding over tightening (and damaging) the 317 tab. Win-win right?5) With regards to clip Vs screw, I totally agree that a clip is a much better option as it attached the 317 body to the heatsink in a much more efficient way. However, I've spent days trying to find a big heatsink with a clip option.Maybe someone can help me? So, far I've purchased and tried these heatsinks but non of these offer a clip option. The dimensions are the same :RA-T2X-64E530002B02500G647-25ABPEAs, it was already mentioned, I'm not dealing with excessive temperatures but still, I would like to achieve the best possible thermal transfer (thermal paste excluded if possible).

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 29, 2025, 09:09:01 am
content: I think this familyhttps://www.sameskydevices.com/product/resource/hse-bx-035h.pdfworks with this cliphttps://www.sameskydevices.com/product/resource/hsc-02.pdfbut Mouser seem to have better pairing info, than the vendor

author: tooki
author_rank: Super Contributor
timestamp: April 29, 2025, 09:56:21 am
content: FYI, GamersNexus just posted a video visiting a boutique thermal paste manufacturer in Germany, showing the whole process. It is far more interesting than one might think!

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: April 29, 2025, 10:05:33 am
content: Hi pope, bom dia,5.: Have a look at Fischer-Elektronik:https://www.fischerelektronik.de/web_fischer/de_DE/K%C3%BChlk%C3%B6rper/A03/Strangk%C3%BChlk%C3%B6rper%20f%C3%BCr%20Einrast-Transistorhaltefeder/search.xhtmlGood luck

author: 2N3055
author_rank: Super Contributor
timestamp: April 29, 2025, 10:18:26 am
content: 62–62°C with thermal paste64–65°C with direct attachment (no paste)70–72°C with a silicon pad (0.4K/W)Where is the problem?

author: Thomas
author_rank: Regular Contributor
timestamp: April 29, 2025, 10:21:48 am
content: If you need electrical isolation - nothing beats Aluminium Oxide (20W/mK):https://no.rs-online.com/web/p/thermal-pads/1777795

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 29, 2025, 11:16:21 am
content: Quote from: pope on April 29, 2025, 08:44:24 amWhen I was repairing welders a while back, we would use standard white silicone thermal paste.Kemppi would supply a tube of 3M brand stuff and a small foam paint roller.We would put some paste on the heatsink and transistor/MOSFET/IGBT etc, then dab it with the flat end of the paint roller until there was a thin and even, almost translucent layer.We never had overheating problems, at hundreds and even thousands of amps.For my own use in my projects and computers when I want the best, I usually fall back to Kryonaut Extreme thermal paste made by Thermal Grizzly. It's overkill for almost everything though, but I often have some on hand for my PC etc. so it gets used here and there.Anything that isn't pushing the limits otherwise gets the white silicone paste if I find it in my piles of junk before the Kryonaut stuff.BOC had a range of welders they rebranded from EWM at that time too.The first few batches had big issues with the TO-220 vregs popping on the control board.I traced the issue to their practice of pop-riveting the vregs to the heatsink.It normally wouldn't have been a terrible solution, if the vregs had the thick style tabs (not great, not terrible). These vregs had the thin tabs though, so the force of riveting caused most of the tab to deform away from the heatsink (apparently also made worse by the springyness of the silpad between), causing the failure.A bit of back and forth had the factory using nuts and bolts and the vregs were rock solid after that.They turned out to be quite good welders after that fix, we rarely had them in for repairs.

author: artag
author_rank: Super Contributor
timestamp: April 29, 2025, 12:09:41 pm
content: If you're using a stamped heatsink it can be pretty distorted. Machined aluminium is flatter.The problem with using nothing is that the difference bnetween best-case and worst-case performance is quite wide : you can test it with a fairly decent flat heatsink but then you get a bit of swarf in the hole and it's way worse. The paste fills the gap so as well as improving the joint it also makes it far more consistent.If the no-paste solution has good enough thermal conductivity it should be fine, but beware of production variations, torque, trapped debris etc. making it much worse than usual on random samples.The spring clip attachments seem good but can be a fiddle to fit (they like to spring away when half-fitted). A custom tool such as carefully ground pliers might help.

author: 5U4GB
author_rank: Super Contributor
timestamp: April 29, 2025, 12:19:56 pm
content: Would switching to a bigger/more efficient heatsink help here? Not much point finding a slightly more efficient transfer medium if the heat has nowhere to go once it gets to the heatsink.As a data point, some time ago I had a piece of electronics on which one particular component was running quite hot. I had a piece of 20mm alu bar stock handy that I dropped on top, end-on so the not-very-flat surface contacted the chip, and it dropped the temps back to reasonable levels despite there being appallingly bad contact between the two.

author: pope
author_rank: Frequent Contributor
timestamp: April 29, 2025, 01:21:42 pm
content: Unfortunately using a bigger heatsink isn't an option as there are PCB and enclosure limitation.I think I'll skip the direct attachment idea and use some high quality silicon pads.Any recommendations? I believe there are different types of silicon pads compositions (i.e silicon mixed with some other stuff)?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 29, 2025, 02:10:05 pm
content: Quote from: pope on April 29, 2025, 08:44:24 amIt's a wax so it can't really dry out. Nor have I seen candles evaporate! It seems to last a long time. I work on 50 year old audio equipment and the silicone paste is often marginal by then. I think the silicone oil in it tends to creep away, leaving dry filler.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 29, 2025, 03:10:13 pm
content: Quote from: pope on April 28, 2025, 12:52:37 pmWhat you are missing is repeatability. Direct mounting can be significantly degraded by a small machining burr in the wrong location, differing surface flatness or inadequate clamping pressure. Depending on the materials involved thermal cycling and creep can also reduce the clamping pressure over time. A good joint today might be worse in a year.Thermal grease or a sil-pad will provide a larger margin for variation.

author: pope
author_rank: Frequent Contributor
timestamp: April 30, 2025, 09:33:00 am
content: That's a very valid point that I hadn't thought of. ThanksI'm still open to some silicon pad recommendations!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 30, 2025, 11:45:04 am
content: Look on page 61 of this-https://dm.henkel-dam.com/is/content/henkel/lt-8116-brochure-thermal-interface-materials-selection-guide-1There's a big difference between the common cheap grey SilPads and the better ones.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 01, 2025, 12:21:02 am
content: Quote from: pope on April 29, 2025, 08:44:24 amThose are quite large heatsinks, I'm curious what is the Volts / Amps of your LM317 here?Do you have images of the case this all goes into?This heatsink seems compatible with standard clips, (HSC-03) at first glance. (You would need to check that)https://www.aliexpress.com/item/1005004403488820.htmlThere are also these simple clip-over-the-top 'U" clips that may be usable on a moderate height heatsink. ( Clip is 25*9*8 )https://www.aliexpress.com/item/1005001296695457.html

author: coppercone2
author_rank: Super Contributor
timestamp: May 01, 2025, 01:22:52 am
content: Are you sure you are measuring right? I find a pretty large difference after applying thermal compound.And keep in mind what the die temperature is in comparison to the tab temperature in comparison to the heat sink temperature.So if you just measure say the heat sink, a small difference in heat sink temperature is a much bigger difference in the semiconductor part of the transistor.Even though its black, I would say you want to measure the tab, which has the best thermal conduction to the metal part. And thats silver, and IR is not too useful for that.I think measuring the plastic bit of a transistor is a bit of a folly when it comes to understanding whats actually going on. Its a insulator compared to everything else in the system. And i started to wonder if its possible for there to be voids or cracks in the plastic part, which can further skew your measurements. I believe that the datasheet formula for thermal resistance would have the highest accuracy between the tab and the die. But keep in mind, its not sure fire. You can always have a bad die attach.My reasoning:1) die is attached to tab with probobly gold solder. This is a critical manufacturing step.2) plastic is glued to tab in a mold from resin. Not sure if the controls on this one in manufacturing are as strict as in #1. There are all sorts of shenanigans going on here, for instance some processes actually have the machine spit a 'gummy polymer' that is UV-gelled over the bond wires (and the die, and some splatter on the tab), and then the plastic is molded on. If you just mold it over bond wires, it can disturb them. Who knows how reliable this thermal resistance will actually be when its made like this? It's not like it needs to be controlled for thermal resistance. Its job is mainly for handling and to seal the part in from the environment. That's what they check for. If the thing needs heat sinking, they wanna do it through the tab the die is on for most things.3) unless you glue the die on instead of solder it. It still might be a higher quality bond then the molding process though4) I love ceramic and metal packages. Just solder a lid on and its good.

author: mtwieg
author_rank: Frequent Contributor
timestamp: May 01, 2025, 12:41:00 pm
content: Quote from: pope on April 28, 2025, 12:26:30 pmI'm surprised there's not more difference in these temperatures. How much power is actually being dissipated? Can you share what heatsink you're using?If those temperatures are accurate than that implies that interface between the LM317 case and heatsink just isn't contributing a lot to overall thermal impedance. Which means your heatsink may be undersized or may be getting poor convection.

author: coppercone2
author_rank: Super Contributor
timestamp: May 02, 2025, 03:43:35 am
content: or a bad die bond. It must eventually happen.Does anyone have any statistics about how rare it is?

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: May 04, 2025, 02:24:40 am
content: Two things I have not seen in years, clear silicone grease and anodized heatsinks.Jeff

author: Smokey
author_rank: Super Contributor
timestamp: May 04, 2025, 03:19:24 am
content: Ha. I've been doing a bunch of thermal characterization of interfaces recently. So many plots with thermocouples datalogging on each thermal junction with step loads. Also don't forget to note the ambient temp since what you are really interested in are differentials.Anyways. The thermal paste's only job is to take up the space between uneven surfaces that aren't perfectly flat and smooth. Thermal paste is conformal and a better conductor than air, which is about all you can say (unless it's filled with diamond chips). That's why it's possible to use "too much" paste. Metal to Metal is best and everything else is just filling gaps...From the link below (https://sound-au.com/heatsinks.htm#s7)So get as flat and smooth as possible on your heatsink and device to start with.Silpads are good and cheap and easy to assemble. Stick with that if at all possible.If you are riiiiiight on the edge of overheating and the performance of the thermal interface that requires special fancy materials is the only thing keeping your part from melting, then you might want to consider a redesign with a little more margin. If you are using a linear regulator that is getting dangerously smoking hot, that might not be the best design choice.

author: coppercone2
author_rank: Super Contributor
timestamp: May 04, 2025, 03:44:49 am
content: Someone should measure with a micrometer a TO-220 bolted to a PCB with some 'optimum quantity' of paste, and measure that in comparison to 'way too much paste'I always have a feeling with how tight that screw is, and how fluid paste is, that there is no too much and that it just means you need to do more cleaning

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 05, 2025, 05:20:48 pm
content: Have to do the test, but the maximum screw torque isn't high and at some point I doubt there's enough pressure at the lead end to get as much paste out of there as you'd like.Interesting about black anodized heat sinks. What I see today is invariably clear anodized or none at all, even though black should be at least slightly better.

author: pope
author_rank: Frequent Contributor
timestamp: May 05, 2025, 05:24:00 pm
content: Here's some good infohttps://sound-au.com/heatsinks.htm#s7

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 05, 2025, 07:57:33 pm
content: Quote from: pope on May 05, 2025, 05:24:00 pmWe used aluminium oxide insulation with clips, and that worked very well.

author: pope
author_rank: Frequent Contributor
timestamp: May 15, 2025, 09:12:27 am
content: So, I think I could do with some help and advice...I ordered the following silpads to try out. All from Bergquist1)SP1500ST-0.008-02-542)SP900S-0.009-00-583)SP900S-0.009-00-434)QII-0.006-00-54The last one (4) seems to have the best Thermal Conductivity (2.5 W/m-K) and it's the cheapest. Is there any reason to choose any of the other three and if so, why? Cost isn't really an issue since the difference is little but what is the benefit (i.e more expensive) of the other three over the last one?

author: Hydron
author_rank: Super Contributor
timestamp: May 15, 2025, 01:38:44 pm
content: Quote from: Thomas on April 29, 2025, 10:21:48 amI guess aluminium nitride doesn't "beat" aluminium oxide, it annihilates it (by an order of magnitude - gets into the ballpark of metals while remaining a good isolator) instead.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 15, 2025, 05:16:17 pm
content: Quote from: pope on May 15, 2025, 09:12:27 amRemember to take thickness into account, not just conductivity. Also, if it's the correct size for your device and whether or not you need the hole.

author: pope
author_rank: Frequent Contributor
timestamp: May 16, 2025, 08:38:43 am
content: Quote from: Conrad Hoffman on May 15, 2025, 05:16:17 pmThank you. Why the thickness is important? I guess want I'm trying to say is that a thicker pad with 2.5 W/m-K is still better than a thinner pad with 1.8 W/m-K, right?

author: Someone
author_rank: Super Contributor
timestamp: May 16, 2025, 09:51:58 am
content: Quote from: pope on May 16, 2025, 08:38:43 amk=W-m/m2-Kthickness over area simplifies to 1/(arbitrary distance)longer explanation:https://en.wikipedia.org/wiki/Thermal_insulation#Definition