Emissivity of tin-plated vs. nickel-plated copper for heatsink application - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: June 30, 2021, 08:26:32 pm
content: Hi everyone,I'm working on a small heatsink design for a surface mount MOSFET, whereby the drain tab will be directly soldered to a "through-hole heatsink".Take a look at this Infineon app note for reference:https://www.infineon.com/dgdl/Infineon-ApplicationNote_MOSFET_Lowcost_heatsinking_of_SMD_Power_MOSFETs-AN-v01_00-EN.pdf?fileId=5546d46262b31d2e0162f28389c13d12The heatsink material will (almost undoubtedly) be copper, but since copper tarnishes so easily, it needs to be plated with something or else it will have an extremely short shelf life. The most obvious options are (seemingly) tin and nickel. The whole heatsink would be plated with whatever material is selected, and the material needs to solderable.I'm having a hard time finding a definitive answer for the emissivity of tin-plated copper and and nickel-plated copper. Can anyone point me in the right direction? Also, can someone make a recommendation on the type of plating process that would give the best (highest) emissivity? (i.e. electrolytic vs electroless nickel plating.) I see some people talking about *black* electroless-nickel as a possible solution. I wonder how common of a process that is, or if it is special.Anyone have any input on this?Thanks,Tim

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2021, 10:02:01 pm
content: If emissivity were important, the oxidized copper would actually be pretty good. However, AFAIK, radiation energy is miniscule compared to even natural convection at electronic heat sink temperatures and thus not worth considering. Polished metals have a very low emissivity, but a black coating to raise emissivity to near 1.0 would likely degrade overall performance because of its insulating effect. Here's an article on heat sinks that seems useful.https://www.digikey.com/Site/Global/Layouts/DownloadPdf.ashx?pdfUrl=F51974C9A6D544F1A7D8F119514B67FF

author: TimNJ
author_rank: Super Contributor
timestamp: June 30, 2021, 10:25:45 pm
content: Thank you for the link and for the advice.This heatsink will be used in an enclosed plastic power adapter. There will certainly be some small convection currents within the enclosure. My (perhaps wrong) estimation was that the small volume of the enclosure and the limited amount of air space there within, would reduce the % of heat transfer via convection...such that radiation may become more important. Honestly, I don't have much analysis to back that claim, just a feeling really.From the link you provided:Copper polished: 0.03Copper oxidized: 0.78Nickel plate - dull: 0.11Tin plate - bright: 0.04I wish they provided difference between dull and bright plating for the same material. I can't tell if tin is actually worse as a material, or if it's mainly the surface finish. Although, as I sit here and write this, I feel surface finish (i.e. roughness)probably isthe dominant factor in most cases.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2021, 10:37:09 pm
content: Metals have very little emissivity in the IR. Even a tarnished copper surface is likely too thin to provide much improvement (the surface has to be opaque at the wavelength of interest). You can see this easily by making a few marks on a hot metal surface and viewing with a thermal cam: permanent marker goes on extremely thin and is essentially transparent in IR, while most tapes and paints should be thick enough, opaque to IR and emissive. Thick enough layers of whatever, or loosely fitting plastic film say, will see a temperature drop due to poor heat transfer through the layer versus convection cooling at its surface.Paint doesn't have much of any downside until the power density goes up, i.e. under forced convection.In still air, paint is thin, it covers a wide area, it doesn't much matter that it's a poor conductor. The improvement in radiation, at semiconductor temperatures, is small but nonzero (5-10%?).The real issue is, if that's enough to make or break your design,your design is already bad-- choose a larger heatsink, add a fan, or choose another means of heat transfer (devices in parallel, heatsinking through PCB, etc.).(Note that a fan doesn't need to be an automatic dust monster; if your enclosure is large enough, it can be a win to just improve airflow within it. You're then limited by only bearing life.)Tim

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2021, 10:40:03 pm
content: Quote from: TimNJ on June 30, 2021, 10:25:45 pmI think radiation is still going to be a rounding error in that design. The convection will raise the temperature of the air inside your enclosure, which will then heat the plastic body from the inside. You may also have significant conduction through whatever parts are inside. I've seen designs where they are trying to pack 10 pounds of power supply in a 5 pound box and one common technique is to use an inner aluminum box that covers as much of the inside of the plastic body as possible, then if possible thermally bond the device heat sink to the box. Thermal conduction of aluminum will be much greater than any other factor inside the box and thus the inner box ends up at pretty much the same temperature. This maximizes the thermal conduction through the plastic because it has the same thermal gradient everywhere. I suppose it helps meet EMI standards as well. I suspect that even copper tape would have a similar effect.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2021, 10:43:23 pm
content: Quote from: TimNJ on June 30, 2021, 10:25:45 pmAhhh, this is even worse because the most common material, ABS, has awful conductivity. You need two things:1. Increase the effective surface area of the enclosure.2. Get your heat into it.1 can be done as mentioned (improve circulation e.g. with a fan), but this is probably not practical in a compact adapter (granted, you didn't mention what size it is, or the electronics within). More likely, the best plan will be to line the inside with copper foil (or pyrolytic graphite if you're really fancy).2 would be best done by placing a thermal pad under the PCB, or over the devices or heat spreader.I've taken apart some adapters which have a snap-together metal shield structure inside, tightly fitting, and often the space between is packed with white grease(!). Others, they just sit there packed in and simmering; the semiconductors aren't so bad off, but capacitor life is short, even with derated long-life parts.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: June 30, 2021, 10:48:06 pm
content: Thanks. Yes, I've applied tape to low emissivity surfaces to make a more accurate measurement with an IR gun before.Quote from: T3sl4co1l on June 30, 2021, 10:37:09 pmJust so I understand you correctly, you mean temperature drop ofwhathere? Surface temperature of the plastic film/tape? Logically, adding more layers would not be a good thing.Quote from: T3sl4co1l on June 30, 2021, 10:37:09 pmNo it is not enough to break the design...There is plenty of margin, but since I am at the point where I can make a decision on the material, plating, finish, etc...I figured I'd try to figure out what was theoretically the best option. From what I gather, in theory, a dull plate of either tin or nickel will probably give similar results overall.Thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2021, 11:10:55 pm
content: Quote from: TimNJ on June 30, 2021, 10:48:06 pmRight; drop across the paint film (or anything else on top of it), between heatsink surface (metal) and air.If you know the power density, material conductivity and thickness, you can calculate the drop.And that doesn't need to be much, even in relatively thick layers or at high power density, if the coating is, like, ceramic-loaded epoxy or something like that. Not that you'd go out of your way to use that just to improve emissivity, of course.QuoteAh, fair. Anodized aluminum (dyed black) isn't too bad; hard to solder though. (There are SMT heatsinks with tinned copper rails clinched into the extrusion, so it can be soldered.)Might also consider steel over copper/brass, for cost; conductivity stinks, but it's still a hell of a lot better than air, and that's all that matters. Tin plate is just as effective there, of course.I would avoid nickel plate, because of poor solderability. It's not a deal breaker, but it might end up with poorer yields. The finish is nice and corrosion resistant.Also for tin plate, mind whiskering, make sure it's a quality plating suitable for electronics.Could also get selective tin or even gold plating, just for the solder joint. No idea if that would end up costing more. If it's a formed sheet product, starting with tinned stock is probably enough.There's also ceramic heat sinks / spreaders, which, the stoneware looking ones don't seem to have very much conductivity at all IIRC, still better than air, sure, but maybe not even comparable to the better TIMs. Whether that's good enough for this purpose, who knows, just putting it out there for flavor.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: June 30, 2021, 11:49:09 pm
content: Quote from: T3sl4co1l on June 30, 2021, 11:10:55 pmThanks for all of the good notes. I've seen those SMT heatsinks, Ohmite makes them I think? Yeah, I wonder if copper is truly necessary, and has potential downside of being quite soft, which means it could be more easily damaged during production handling, shipping, etc...especially for a small, thin heatsink. Brass would be tougher. I gotta say, steel just doesn't feel right, but maybe for a small enough heatsink, the temperature gradient won't be so horrific.Good point about the selective plating. My assumption is that selective plating requires some amount of tooling, whereas plating the whole thing is easy/standard procedure.I attached a photo of the prototyped heatsink, with and without a hypothetical thermal pad from the heatsink to the side wall of the enclosure.Regarding getting the heat out to the plastic (polycarbonate) enclosure...I know that this is very important for external power adapters. I started a thread maybe 2 years ago in a similar vein, and the solution was to add an adhesive backed copper sheet to the inner-surface of the housing. It's a valid solution. In this case, the MOSFET is DPAK (TO-252) on the bottom-side of the board. I know heatsinking the plastic part of the package, instead of the tab, is not as effective due to the higher thermal resistance, but I do wonder if laying a sheet of copper foil on the bottom half of the housing, and then a thermal pad between MOSFET and foil might be even more effective than a top-side heatsink (as in the photo). Could just test and see!Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2021, 12:07:01 am
content: Hahah yeah that little pad isn't doing anything, it's surrounded by insulation. But add a foil lining, and it's a good start!Also, is that a heat spreader on top the transformer? Nice, a pad on top of that should do well. Still not great with the plastic enclosure over it, but it's a somewhat bigger area, and with foil lining added, bigger still. (Be sure to ground all these foils if you can, else they'll just repeat all the electric fields from these nasty components...)Hmm so a standing heatsink for a DPAK? Strange. Should work nicely for heat transfer out the side like that. But that assumes the heat has somewhere to go from there.If you can get one leg of that heatsink up against the tab, it'll conduct real nice through the solder joint. But then you have to wave solder the whole damn thing, or hand solder it. Maybe not great for production, or reliability either? Typical placement rules would require a good 200+ mils between DPAK pad and heatsink pin, not a lot of copper even if it's poured solid.The build I'd prefer, is a low-profile circuit (trimmed TH leads, nothing taller than SMA, DPAK, 1206 chips, etc. on the bottom), with lots of heat spreading (large copper pours), sandwiched against the enclosure with thermal pads. Foil lined enclosure if necessary/possible.You probably won't get too much lateral heat spreading on the board itself, unless you can swing a 4-layer design here; so the thermal pads will either be pretty localized, or doing a lot of the lateral spreading themselves.And you don't really want whole-board pads. Even the very squishy stuff, is stiffer than the board over that kind of area, and will just bow the board rather than smooshing down and conforming real nice. (Maybe feasible if you have a lot of mounting points for the board?)You want smaller strips or pads, so that there's plenty of room to smoosh out into the space between pads. Which of course means the pads aren't touching each other at all, so heat won't spread between them, and a foil lining is especially helpful in this case. (Real nice for metal / die cast cases, of course.)Tim

author: TimNJ
author_rank: Super Contributor
timestamp: July 01, 2021, 12:09:54 am
content: Quote from: bdunham7 on June 30, 2021, 10:40:03 pmIndeed, this is an approach that I'm familiar with. Unfortunately, for this particular project, there's already an existing housing that it needs to be fit to, no wiggle-room for a aluminum/copper shell ("inner box").But now that you mention it, I've started to see this approach much more commonly on high power density plastic power adapters, especially out of China. If you look at some of these disassembly photos (https://www.chongdiantou.com/wp/archives/category/teardowns/chongdantou), you'll notice plenty of designs using SMD semiconductors with limited PCB heatsinking, but big copper heatspreader shells like you mentioned.For instance:or

author: TimNJ
author_rank: Super Contributor
timestamp: July 01, 2021, 12:26:30 am
content: Quote from: T3sl4co1l on July 01, 2021, 12:07:01 amYou mean the pad in my photo? I guess I find that a little surprising as (in my experience) any thermal coupling to the housing is better than none at all.Quote from: T3sl4co1l on July 01, 2021, 12:07:01 amMaybe I was not clear on the nature of the heatsink. It is directly soldered to the drain tab. See the attached 3D of the bottom-side of the PCB. Radiated EMI pre-scan looked okay like this, not sure if it would be problematic in other circumstances. The defacto approach is usually to tie the heatsink to common.It would be wave-soldered...already got the go-ahead that it would be okay. As long as the main big tab is correctly oriented, I think this design works okay. If you rotate 90 degrees, probably not a great solder joint, though dual-wave machine may handle it fine.And 200mils why?

author: sandalcandal
author_rank: Unknown
timestamp: July 01, 2021, 02:45:35 am
content: Speaking of heat spreaders. I wonder if graphite film heat spreaders would be a good high performance(potentially cheaper?)option compared to copper sheets.https://neograf.com/egraf-spreadershield-heat-spreaders/Panasonic PGSEdit:Panasonic PGS, 1000 W/m*k, 180mmx115mmx0.07mm sheet,$13.62in qtyhttps://au.mouser.com/ProductDetail/Panasonic/EYG-S121807?qs=WwqriLBepZtfJIR4CZ4abQ%3D%3DCopper sheet, ~400W/m*k, 180mmx115mmx0.175mm sheet, Weight = 180x115x0.175x0.0089 = 322g, ~$4.83at ~15 $US/kg (Alibaba average for 0.175mm film)So probably not as cost effective as solid copper spreaders but weight saving could be pretty good still I think. 1.21 g/cm3 for 70um PGS film vs 8.96 g/cm3 for copper and the thermal conductivity is also more than twice as high on paper.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2021, 04:16:10 am
content: Quote from: TimNJ on July 01, 2021, 12:26:30 amYeah still better than nothing, but if you need enough cooling to require hardware (a few watts), doubtful it's enough to do the job..?QuoteAh, in the middle. Makes sense. Yeah, no problem there.200 mils or so, comes from the clearance needed for a wave soldering shield, to keep from washing away the reflow-soldered stuff on the same side. They're usually machined from composite or metal, with a conical clearance around solder joints, so you can't have stuff too tall and too close that it violates that space.So also only applicable in a reflow+wave setting, YMMV. If you can wave the whole thing for instance, the only thing that really matters I think is strain on big fat globby joints (SMT pads touching THTs)?Quote from: sandalcandal on July 01, 2021, 02:45:35 amYeah, you have to be pretty strapped for space or weight to need them, but when you do, it's a good option to have. Cellphones, say?Tim

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2021, 05:12:29 am
content: Bonjour a tous...Heat flow is a function of three modes: Conduction, radiation and convection.The heatsink design is very well documented, see numerous textbooks and app notes and papers eg thermalloy.Emissivity affects only radiation, as does the Kelvin temperature of the body. The radiation predominates at higher temps than normal electronic parts.In your case, it is conduction and convection and not radiation.You can test with natural aluminum or copper vs a black version use thermocouples and a power resistor for accurate measurements.Just rotation the part 90 deg orientation will cause the convection to changeSo I suspect the OP question is moot, and the color/coating will make no difference.Bon ChanceJon

author: Berni
author_rank: Super Contributor
timestamp: July 01, 2021, 05:27:38 am
content: Yep radiated heat is a significant factor in heatsinks without forced air movement. Its only when you introduce a fan that the conduction cooling gets so large that radiation becomes a negligibly small part of it.Indeed most metals are terrible at radiating heat as IR. As a general rule if it looks silvery shiny to visible light then it most likely is also shiny to IR light. Due to thermodynamics this also gives it low emissivity, otherwise a object that reflects IR and also emits it could cool down to below ambient temperature and this would make a perpetual motion machine possible. This is also why aluminium foil makes such a good thermal insulator, similar reason why heat shields around hot engine exhausts are shiny thin sheet metal.The typical way of dealing with this is black anodizing on aluminum. Since aluminium conducts heat almost as well as copper yet is way cheaper and lighter, so this makes it a popular choice for heatsink material. The black anodizing makes it act really close to a black body while not inhibiting the heat flow at all since the anodizing is just Aluminium Oxide (another great heat conductor) and it is in a really thin layer.If you often used a thermal camera you get to find the low emissivity of metals annoying (They look like mirrors in thermal IR). The tricks for fixing it are either coloring it with a black sharpie or sticking some kapton tape on it (surprisingly kapton has a very high emisivity, despite being mostly transparent in visible, similar to glass). So you just need to cover the surface with something that emits IR well. It can be as simple as a thin layer of black spray paint. Yes the paint does insulate it a bit but its so thin that it does not really matter.

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2021, 06:24:41 am
content: Many good references and texts on this subject.https://www.infineon.com/dgdl/an-1057.pdf?fileId=5546d462533600a401535591d3170fbdhttps://core.ac.uk/download/pdf/41809617.pdfLots of well designed equipment has a finned aluminum HS and no black coating.In the OP app, I doubt if the emissivity makes a big difference.Jon

author: Berni
author_rank: Super Contributor
timestamp: July 01, 2021, 07:34:53 am
content: Quote from: jonpaul on July 01, 2021, 06:24:41 amCan you share some examples of such equipment?I find it rather rare to see bare aluminum heatsinks used without some form of forced convection. They are not always black. The color of the paint to visible light does not really matter, can be white or black or blue or whatever, the pigments usually don't have any effect at the thermal IR wavelengths, the paints base materials usually have at least somewhat decent emissivity. Be it heatsinks on the back of linear PSUs, heatsinks stuck on chips (Such as motherboard chipsets and other genetic medium heat dissipation ICs) or various forms of heat dissipating plates such as graphics card backplates, SSD heat spreaders etc... You can try it out by wrapping a laptop power brick in some aluminium foil and see how much hotter it gets. Since the aluminum is thin and well conductive it shouldn't hamper any conductive cooling, but it will stop thermal radiation dead in this tracks.Once there is a fan or some ducting that actively moves air trough a heatsink, those will indeed usually be bare aluminium. This can sometimes raise the conductive cooling potential of a heatsink up to 10x or even more, so at that point the radiated cooling is so low that could be pretty much ignored.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2021, 07:53:08 am
content: I mean there's no shortage of them from suppliers,https://www.digikey.com/short/h3btm0p9you're welcome to use them any way you like of course. But I also have plenty in my junk collection, if you stipulate things in finished products (many of which were indeed passive convection).Quote from: Berni on July 01, 2021, 07:34:53 amNot quite, though for a couple of maybe tricky reasons?The air space between enclosure and foil, will drop quite some temperature. So it will take time for the brick to heat up to its new equilibrium temperature. That's fine. At that point, thefoil surface will be at the equilibrium outer surface temperature,and will be slightly hotter as you suggest.However, if you go to measure it, by putting your hand on it, or touching it with a thermocouple say -- you will feel theinternal temperature, because the foil has almost no heat capacity and you're smooshing it into the inner surface and feeling that. For the thermocouple, it will hardly make thermal contact anyway, or if greased, its heat capacity again dominates over the thin layer of foil (even if you aren't pressing it against the enclosure in the process -- which because of the point contact, is probably okay in this case), or its conductivity just as well.And it's no good trying to measure it with a thermal cam, for obvious reasons.So, while that's technically true, it's a devil to try and actually measure, and I wouldn't suggest doing it this way (besides the small risk of cooking the poor brick like a hot potato).So, if you don't mind getting a bunch of sticky goo on your power brick -- you can do the same test but with say adhesive foil tape, or do it with plain foil but grease it the fuck up with heatsink compound so it makes good thermal contact with the enclosure. (Tape adhesive shouldn't be much worse than the enclosure already is, and it's pretty thin; I wouldn't expect it to drop much temp here.) Then you can get a good reading through either method (touch or instrument). You should find the temp is a couple degrees higher.And this is of course keeping everything constant, so, same load, same supply voltage, etc.QuoteI've seen plenty of forced-convection heatsinks in colors, too! Anodize really doesn't amount to anything (it's modestly conductive, porous aluminum hydroxide) so it's mainly just used for visual appeal (look at all the nice shiny colors in CPU heatsinks etc.) and corrosion resistance.For the cooling potential to differ by "10x or more", it would have to be compared to somereallythick paint, or very high flow rates (or fluid cooling)... not saying that's impossible, but it seems unlikely for most product designs. A much less alarming "50%" might be more in the ballpark, but really, without any qualification at all it's an utterly arbitrary number, so I guess my real point is, why bother giving any figure at all?... Anyway, I digress.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: July 01, 2021, 07:53:54 am
content: Experiment with some pyrolytic graphite sheet, see for examplehttps://www.digikey.com/en/products/detail/panasonic-electronic-components/EYG-S121807/1630954It's available with plastic film providing electrical insulation.The idea is, laterally it's many times more thermally conductive than pure copper, so you can spread the heat over a large area using a very thin sheet. This can be applied inside your plastic case. Plastic is a bad but not pathologically bad thermal conductor; you can dissipate through some 2-3 mm of plastic just fine, but the plastic itself can't spread the heat over larger area.Due to the plastic case blocking air (unless you add holes) means a metal heatsink inside becomes more like heat spreader, allowing larger surface area of that plastic to be used. But metal heatsinks are not flexible and this limits the area of the plastic case you can use for cooling; unless you get custom machined heatsinks.The graphite sheet can be basically cut in shape and the entire plastic box insides covered with it, with minimal volume reduction. If you buy the variant covered with plastic film, electrical insulation is kept, although functional isolation only, be aware of mains voltages requiring extra insulation.

author: sandalcandal
author_rank: Unknown
timestamp: July 01, 2021, 08:32:04 am
content: Quote from: Berni on July 01, 2021, 07:34:53 amCrack open any consumer grade equipment, it's almost always passively cooled bare aluminium. Look at some of Dave's videos repairing/opening dumpster TVs and monitors and you can see them. The only place I see coated heatsinks is high end stuff like "pro" audio equipment, test equipment, telcomm equipment. Computer PSU modules or any other off-the-shelf power supply module for that matter often bare aluminium WITH forced ventilation. Surprised you find them rare.Quote from: Siwastaja on July 01, 2021, 07:53:54 amGood point. I didn't consider the benefit of having the PGS conform the to internal surface of the case.

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2021, 09:29:52 am
content: bonjour a Berni et tous...The TEK mainframe scopes 7104, 7904, had linear or SMPS with large vertical finned HS on back panel, as part of the PSU module.Same for many 1980s..1990s HP equipment, some without fanse, just a fijnned vertical mount convection bare Al HS.I have seldom seen the black ano on production equipment.I think the added cost is not worth the benefit with or wothout a fan.Can the OP tell us the max watts diss, ambient T and HS T .?Jon

author: Berni
author_rank: Super Contributor
timestamp: July 01, 2021, 10:21:07 am
content: Quote from: T3sl4co1l on July 01, 2021, 07:53:08 amWell yes you can buy them and you can put a fan next to any heatsink you like. It's not that uncommon to pepper small heatsinks on the chips of a PCB and then have one large ducted channel moving air along the board. It's also easy for the manufacturers to provide them since they simply skip the coating step, making them slightly cheaper too.There is also nothing stopping a designer from deciding they don't need the little bit of extra cooling efficiency, while saving 5 cents on heatsink paint, but it is worth it because they are making a production run of 1M units so they saved 50 000$ .It is a easy way of shaving off a few extra cents, just like using cheep off brand electrolytic caps.There is a noticeable amount of cooling performance to be gained from a high emissivity coating, so unless the product is being seriously cost optimized for large scale mass production it does not make sense to save a few cents with raw aluminum. In fact the OP is thinking of copper for heatsinking, this makes for a rather significant price increase for the extra performance, so it does not make sense to shave cents here by skipping a high emissivity coating.

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2021, 12:51:30 pm
content: Quote from: bdunham7 on June 30, 2021, 10:02:01 pmOxidation in the mid IR makes little difference to emissivity.Black annodised coatings on heatsinks increase performance by around 33%, according to thisdata sheet. The gain in performance will depend on the shape of the heatsink and type of coating used.

author: bdunham7
author_rank: Super Contributor
timestamp: July 01, 2021, 01:33:03 pm
content: Quote from: Zero999 on July 01, 2021, 12:51:30 pmI notice that they don't specify a temperature at which those charts are valid. Blackbody radiation varies with the 4th power of the temperature, so that should be a critical variable.Also, since the OP's device will be in an enclosed space, the fact that emissivity and absorptivity are the same might make the whole exercise pointless in the end.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2021, 02:34:34 pm
content: Yeah, useless without temperature.Y'know, so as not to go too long with utter hearsay -- how about some physics:Bastard unit chosen, just because I for some reason memorized the reference point of150 C in^2/Wfor typical convection in still air.Which crosses at merely 136°C, so radiation doesn't seem too bad after all.But that assumes emissivity = 1, and that doesn't take into account the synergy of heatsink fins and convection. Probably a heatsink at the same temperature is doing a lot better than 150 (how much better, you'll have to find some estimate of the active area and see, I guess; I don't have any measurements handy).Note that finsdon't participate radiatively, at least beyond whatever help they provide in creating an effective black body cavity (which can only increase emissivity to a maximum of 1). Consider the root between fins: the surface there is mostly surrounded by other surfaces, and very little of it sees free space, so contributes little to the total. The effective radiating surface is more or less the convex hull around the object. So, there's much less radiation area for a finned heatsink, than its actual (physical) surface. But for sheer surfaces, flat plates, enclosures, etc., the areas are pretty similar.Note also that space is radiating back upon the surface, so the ambient must be known. This has been assumed at 25°C. Rth goes to infinity as delta T goes to zero, and that asymptote is mostly cut off in these data (starting at 50°C).Note also that the effective resistance goes down considerably as temperature goes up. That is, given say a 25°C temp difference, the heat flux will besixteen times higherby merely doubling the absolute temperature (say 600K vs. 300K). Not that you gain anything from raising ambient, but it's interesting that the conductivity effectively goes down so sharply even with the high ambient. Whereas a linear material will always drop say 25°C when 1W is applied (for Rth = 25 °C/W), the radiative case only improves with rising ambient (when measured strictly in terms of temp rise).So: the contribution for plain, convex objects can be fairly high, though also such objects tend not to be very hot (e.g. enclosures), so it may still not be a dominant fraction. Certainly not for bare metal. But painted or plastic, pretty good, maybe on the order of 50%. (If my reference figure is representative at this temperature, then actually more than 50%, since the 150 C in^2/W figure is effectively the 227... resistance from the table, in parallel with an even higher resistance from convection alone.)Meanwhile, the contribution for concave shapes like heatsinks, may be significantly less. If the active-convection-surface-area to convex-hull-surface-area ratio is say 5 or 10, and the heatsink is bare metal, radiation might be merely a few percent. With good emission, 10 or 20% is very believable. Shallow fins or poor orientation (effectively reducing the active area by creating pockets of still air), even higher.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2021, 04:11:17 pm
content: Tim excellent, mille mercis,Wonder if a study or paper exists re the ratio conduction/convection/radiation for typical electronic HS at usual ambient and HS temps?Jon

author: coppercone2
author_rank: Super Contributor
timestamp: July 01, 2021, 05:18:38 pm
content: I wonder if ferric chloride staining improves it as much as black anodization, because you can stain copper or aluminum black IIRC for much less effort

author: bdunham7
author_rank: Super Contributor
timestamp: July 01, 2021, 05:59:37 pm
content: Quote from: T3sl4co1l on July 01, 2021, 02:34:34 pmPhysics is great, thanks for constructing and posting all that. It looks like radiation is more than a rounding error in at least some circumstances. However, I'm not so quick to dismiss the 'utter hearsay' because a lot of that is based on practical experience. There are assumptions, rules-of-thumb and so on that come into play and can change the result quite a bit. Some experimental data would be helpful.The 150K/in2/W, for example, seems a very rough rule of thumb. I found this site--I haven't checked their math, but it seems well researched:https://www.heatsinkcalculator.com/free-resources/flat-plate-heat-sink-calculator.htmlI found that I could rearrange a square inch of flat plate and get very different results just by changing the ratio of length to width--which one might expect from a convective model.Also, IEC 60950 sets limits as to exterior temperatures and I think most products would not exceed these. Thus for an external case, the temperatures would need to be on the far left side of your graph. 136C would be very hot for any heat sink, enclosed or not.So the situation changes quite rapidly as the temperatures go up--a cheap, hot plastic power brick underneath a desk in completely still air might indeed lose a significant portion of its heat by radiation. I would expect a large, external finned heat sink on a power supply or audio amplifier to rely much more on convection. If I can dig up some aluminum heat sinks that come in both plain and black anodized, perhaps an experiment is in order.The heat sink calculator is a pay site, but they do also have this heat sink size calculator. You can pick some arbitrary sizes and then use different temperature and emissivity settings to see how much the calculated heat sink size varies when change the emissivity from 0.1 to 0.9, leaving all the other variables alone. I'm not sure it is accurate--it may not be taking into account the reduction in emissivity due to geometry.https://www.heatsinkcalculator.com/heat-sink-size-calculator.html

author: TimNJ
author_rank: Super Contributor
timestamp: July 01, 2021, 08:50:51 pm
content: First of all...thank you everyone for all the interesting discussion. I must admit, I am still a bit murky about my original question.Quote from: Berni on July 01, 2021, 10:21:07 amBefore I started at my present job, my ex-boss ran some experiments on one of our open-frame power supplies which ran pretty hot, given the power density and the year it was designed. He ran some experiments replacing the original brass heatsinks (why brass, I have no idea) to plain aluminum, and then to black anodized aluminum. Apparently, the black anodized aluminum outperformed the plain aluminum in a significant way...on the order of 5C reduction in semiconductor case temperature just by anodization. Of course, this was for a vented open-frame design, and not enclosed plastic adapter, so it's a different animal in some respects.But, yes, you understand my original intention for starting this thread. Basically, forgetting about "whether it's worth it or not", what would be the theoretical best coating/surface finish for a copper heatsink to maximize radiation performance?

author: bdunham7
author_rank: Super Contributor
timestamp: July 01, 2021, 09:09:28 pm
content: Quote from: TimNJ on July 01, 2021, 08:50:51 pmYay! A data point! I was looking to see if I could find any heat sinks that came in both varieties but were identical otherwise, I only found one and the plain one doesn't seem to be available.To answer your question, the answer of course is the coating with the best emissivity without any insulating effect. I suppose if it is thin enough, the insulating effect will be minimal. There's a paint called Parson's Optical or something like that that is supposedly very high emissivity, barbecue grill paint probably isn't far off the mark.EDIT: There is also a process called Ebonol C which chemically produces a layer of cupric oxide on the surface. I believe this is fairly exotic, but would be similar to anodizing in that the insulation effect would be minimal.

author: TimNJ
author_rank: Super Contributor
timestamp: July 01, 2021, 09:33:14 pm
content: Quote from: T3sl4co1l on July 01, 2021, 02:34:34 pmI appreciate the calculations and visuals. Thanks. I understand you are showing the drop in thermal resistance, presumably from heatsink to ambient, for radiation. You noted that emissivity = 1, so you are assuming the heatsink material is a perfect black body radiator? Are the calculations you show independent of actual heatsink size, dimensions, and other factors?Can you explain what a concave and convex object is, in the context of this discussion? Obviously I am familiar with the terms in general. By concave, do you mean a radiating object which radiates back on itself due to its shape? Whereas, for an enclosure, the radiating surfaces (i.e hot surfaces facing "cool" ambient) are all pointing away from each other?Finally, the numbers you suggest 50% for convex objects, and 10-20% for concave objects...are you saying this is the fraction of the total energy released from the heatsink? (i.e. radiation + convection)Thanks!

author: TimNJ
author_rank: Super Contributor
timestamp: July 01, 2021, 09:48:24 pm
content: Quote from: bdunham7 on July 01, 2021, 09:09:28 pmI'd honestly like to run some sort of experiment like this myself, with better documented test setup...a few different scenarios representing typical use cases (i.e. the one's we've talked about, enclosed, open-air, etc.). Not that I don't believe my ex-boss's results but sometimes he had a tendency to want to prove himself rightRegarding the original question, the stipulation was that it had to be a solderable material, as most likely the whole thing would be plated/coated with it. I could not find clear data on tin vs. nickel, but suspect they are probably comparable. It seems that a dull vs bright plating finish is more important. But there are lots of different plating processes which I am just not familiar with, so there may be an ideal process (i.e. I saw "electroless black nickel" plating as possible option.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2021, 11:19:13 pm
content: Quote from: bdunham7 on July 01, 2021, 09:09:28 pmAhhhh! A half a data point!Critial information is: out of what HS and ambient temperatures?Quote from: TimNJ on July 01, 2021, 09:33:14 pmRight. The way radiation works is, everything is radiating, all the time. Maybe not 100% in all frequency bands, but always something, somewhere. An ideal dielectric is the only thing that doesn't (there are ideal conductors but not at AC, so superconductors also radiate; granted, it's not much at their low operating temperatures). And vacuum is really the only thing of that sort. Which means the temperature of a vacuum is defined by the temperature of its bounding surfaces, if that's any help. (The universe itself is bounded by the cosmic horizon, which comes to us as cosmic background radiation ~2.7K.)Radiation is a power density, so we can compare types of heat transfer in these terms, to the extent they are applicable to the other types anyway. It makes intuitive sense that bigger things can dissipate more heat. How much more, is the question; convection is very sensitive to geometry and orientation.Conductivity, is linear or very nearly so, so we can approach it in the same way. It does depend on thickness being conducted through, but if we define that -- say looking at a particular thermal pad -- we find an area thermal resistance (r_th = thickness / bulk conductivity).QuoteBasically this,https://en.wikipedia.org/wiki/Convex_hullI'm not going to try and explain the whole thing in detail (as I don't understand radiation rigorously enough myself to be able to prove something like that..), but that's the sort of idea.More specifically I think, if you look at any point on a surface, take however much solid angle of space is visible. Radiation is Lambertian, so integrate solid angle over the object. This should be better than the convex hull, as long spindly projections have excellent visibility, like a, think of whatever the star-shaped polyhedra are. Or, consider the point cloud in the article itself: if we suppose a small sphere around every point, the visibility of any given sphere (as in, solid angle not obstructed by other spheres) is very good, even in the middle of the cloud (though certainly worse there, and depending on density). We should expect such a configuration to radiate very effectively.Though I'm also not sure if such shapes radiate as well as their convex hull, which takes up the same space (if the bounded volume is what's important in your application..) but has the most unobstructed surface area in that shape(?).In any case, in terms of conventional shapes, heatsink fins, don't count towards radiation, at least very much. Broad surfaces or overall dimensions, yes. Pointy shapes, not so sure, but you rarely see, like, a pin array type where the pins are very far from the surface (and from each other), so it's at least not a question of high commercial interest.QuoteYes, breakdown between radiation and convection.Again, as mentioned, the comparison depends wholly on a reasonable figure for convection; the 150 figure I pulled out is probably from the low end of the scale, and obviously not the high end where radiation would handily account for that alone.Tim