The performance of a heatsink with a black surface - Page 1
Forum section: electronics / projects

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 19, 2011, 05:14:50 pm
content: According to this website ...http://robots.freehostia.com/Heatsinks/Heatsinks.htmlThe performance of a heatsink with a black surface will be 6% to 8% better than that with a plain or bright surface.But it does not say why ?I have spent lots of time of my life by exploring and researching about CPU heat-sinks for more than 12 years.My hidden hobby is that I am good at PC case modifications.Even so, in what it will help the color ?The shape does ... the material does.. the active cooling does.. but the color ?Theoretically the black color attracts heat , more than any bright color, and so this theory does no make sense to me.

author: Simon
author_rank: Unknown
timestamp: June 19, 2011, 05:22:08 pm
content: agreed, hope someone can explain

author: Unknown
author_rank: Unknown
timestamp: June 19, 2011, 05:52:10 pm
content: Black absorbs visible light (hence the black color), and may also absorb IR, but that's not a big deal if the heat sink has a higher temperature than its environment. Heat sinks mainly work by convection (and maybe some radiation), so the important issue is how well it transfers heat to the air. I remember from a conversation with a mechanical engineer that anodizing is superior to painting, don't remember if the actual color matters.

author: Unknown
author_rank: Unknown
timestamp: June 19, 2011, 05:57:35 pm
content: It has to do with blackbody radiation. A blackbody is an 'object' that absorbs all incident radiation, thus it appears black. Whatever is good at absorbing one, more, or all wavelengths (blackbody) is also good at emitting these wavelengths when excited (see sodium lamps etc).Every body (heatsink in this case) that is at a temperature above absolute 0 (0 K) will emit radiation (photons). This radiation is what say IR thermometers or thermal imagers actually image.In the case of a blackbody, the radiation energy is given by the Stefan/Boltzmann and is a function of the fourth power of the absolute temperature (Kelvin) of the body. This is the formula:E = k * T4* A [Joules*sec-1].k is a constant, in the order of 10-8and A is area in square meters.Because the constant is so small, you need a very large area or rather high (>500degC) temperature to have significant energy dissipation in the form of radiation. And by significant I mean comparable with the power levels of what you are trying to cool, in the order of Watts.So unless your heatsink is huge (football field?) or it is glowing hot, you will not dissipate a lot of heat this way. Better get a different heatsink or better interface (lower thermal resistance) between surfaces or a material with higher thermal conductivity like copper compared to aluminium.A heatsink painted black is a bad approximation of a blackbody, more like a 'grey' body where the energy radiated by the formula above is multiplied by a constant (emissivity epsilon) which is less than 1. A better approximation would be a heatsink with microholes with depth about 7x their diameter (the whole appears black inside, try it). This is used by Ersa in their IR heating plates for PCB rework.To conclude, painting heatsinks black does not contribute to their ability to dissipate heat by radiation.

author: Simon
author_rank: Unknown
timestamp: June 19, 2011, 06:06:05 pm
content: painting a heatsink black will only insulate it unless the paint is a good conductor.

author: Unknown
author_rank: Unknown
timestamp: June 19, 2011, 06:36:03 pm
content: Quote from: Simon on June 19, 2011, 06:06:05 pmIn theory the bare heatsink is already insulated by a very thin, porous layer of aluminium oxide. During anodising the 'paint' enters the porous aluminium oxide and is hard to remove. As alm said, anodising is better than painting manually in this case.However, scratches from mounting hardware (mounting screws etc) will locally remove the oxide layer (and its paint) or make it thinner. If the thickness is in the order of nm, then quantum tunneling can occur and electrons will 'go through' the oxide. What is more, if the device you are cooling has a metallic tab at relatively high voltage then the oxide will just breakdown and current will flow.So I dont think the black anodising is intended to act as an insulator. If it is, it is an extremely unreliable one.Alex

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2011, 07:07:08 pm
content: The attached datasheet suggests black anodising an aluminium heat sink will increase its performance by around 33%I think it also depends on the shape of the heat sink and how it's used. Black anodising will probably make more of a difference for flat finned heat sinks designed for passive cooling than for complex heat sinks with forced convection cooling.

author: Simon
author_rank: Unknown
timestamp: June 19, 2011, 07:10:30 pm
content: which would be why CPU coolers don't bother with anodizing although a 33% gain is not to be sneezed at with the size of some CPU heatsinks.No I never meant that anodizing would electrically insulate and my comment about paint insulation was for heat dissipation

author: Unknown
author_rank: Unknown
timestamp: June 19, 2011, 07:16:03 pm
content: Maybe anodising reduces the heatsink-air thermal resistance drom the oxide layer?Several places online seem to back this up but i dont dare quote any of them. It should not be hard to find a good referece, most likely not from the PC modding community. Not sure, but it is definitely not due to radiated dissipation.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 19, 2011, 07:29:11 pm
content: You shouldn't (and usually can't) 'paint' heatsinks -- they are made from aluminum and paint doesn't easily stick to aluminum.Black heat sinks are normally anodized black. This is an electrochemical process that builds a very thin (several micrometers) of aluminum oxide (which is clear). The initial anodization produces a porous surface structure. The part is then dipped in black dye, and then boiling water or steam which causes the porus structure to seal up, sealing in the dye and making the surface permanently black.The primary reason to anodize is to produce a cosmetically uniform and very abrasion resistant surface. The coating is very thin but very durable and has negligible impact on the thermal conductivity. There will be a slight increase in emissivity, but it is negligible for most heat sinks. Only very hot, very large heatsinks with no forced air and small or no fins will be helped. Basically if you put any effort into improving convection it will be so much more important than radiation it doesn't matter.Another advantage of anodization is that it can produce a very thin electrically insulating layer. It is possible to use this as the insulation between a heat sink and the tab of a TO-220 transistor. This is generally not done by itself because if you do manage to scratch through the anodization layer you can get a short circuit. If you are careful with manufacturing and test for continuity after you assemble it can be fine, but if you are likely to dismount and remount the transistors it is too risky.Many aluminum parts are anodized including most heatsinks, and they are often not black. Those shiny 'metalic blue' or similar coatings are also anodization, just with a different dye, and often a thinner oxide layer. If nothing else, people do a clear anodization that leaves the natural aluminum color, but a much thicker oxide layer than the few nanometers that exists natively. This improves scratch resistance and cosmetic uniformity while maintaining the natural appearance.The reason black bodies are good radiators has to do with what is called the 'principle of detailed balance' which states that the absorptivity of a material is equal to its emissivity. If this were not true, you could put an object with a very high emissivity and low absorptivity next to an object with low emissivity and high absorptivity, and the first one would get cold and the second hot. This would violate the second law of thermodynamics. Anodized aluminum ranges from 'mirror like' to 'grey' in the mid- to far-IR region of interest for thermal radiation of materials at 0-200C.

author: Unknown
author_rank: Unknown
timestamp: June 19, 2011, 07:39:34 pm
content: Quote from: ejeffrey on June 19, 2011, 07:29:11 pmMakes sense.QuoteThanks.

author: Simon
author_rank: Unknown
timestamp: June 19, 2011, 07:42:10 pm
content: Quote from: ejeffrey on June 19, 2011, 07:29:11 pmYou better tell that to the sprayer where I work who paints endless aluminium radiators and our suppliers that powder coat aluminium radiatorsI'd not recommend letting the anodize act as insulation because when you put a hole through the heat sink it will expose bare aluminium that will touch the bolt, it is so tricky it is not even worth mentioning really

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2011, 07:49:30 pm
content: Quote from: Simon on June 19, 2011, 07:42:10 pmI'd imagine that the manufacturer would drill all the holes before anodising to avoid that.

author: Unknown
author_rank: Unknown
timestamp: June 19, 2011, 07:56:45 pm
content: Yeah, he is reducing the performance of the radiators. Slap himThe oxide layer of heatsinks is also reducing their performance, and I am aware that prior to anodising the oxide is grown in thickness. I would imagine a side by side comparison (properly) would show that bare metal is ever so slightly better. There is a process where an electric arc discharge creates those microcavities that act like blackbodies on the metal surface dramatically increasing the otherwise very low emissivity of metal surfaces. But such metal blocks are used in furnaces to measure the temperature of several hundrend degrees C using IR thermometers, not for a heatsink for electronics use.QuoteI think Simon meant screw in a hole, correct me if wrong.

author: Simon
author_rank: Unknown
timestamp: June 19, 2011, 08:04:28 pm
content: Quote from: Alex on June 19, 2011, 07:56:45 pmYea that is what we say but even the military like "pretty" thingsQuoteNo i meant a drilled hole with a bolt through, eventually there will be a slip up where the bolt thread will scratch the anodize and game over

author: Unknown
author_rank: Unknown
timestamp: June 19, 2011, 08:15:14 pm
content: Ah ok.There is also thermally conductive epoxy (usually to mount heatsinks on BGAs), you should be able to use that to avoid drilling the heatsink if you have some means of mechanically securing the device. But then you cant easily replace the device etc.

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2011, 08:46:20 pm
content: Quote from: Alex on June 19, 2011, 07:56:45 pmThe data sheet I posted seems to suggest the reverse is true but so far only two people (apart from me) viewed it. Of course, I'm not saying this is always the case. It probably depends on the type of anodising, the shape and whether it relies purely on convection or forced cooling.It's probably not possible to do a general comparison as there are so many other factors to take into account.Maybe if I post a graph from the data sheet more people will view it: PDFs are a pain for some.

author: Russel
author_rank: Regular Contributor
timestamp: June 19, 2011, 09:24:42 pm
content: Heat is radiated by conduction, convection or radiation.Hero999, in the chart that you've posted, which is the method used in the test? If radiation, then the chart makes sense to me. Most heat sinks used for cooling electronics use convection cooling for dissipating most of the heat, radiation being only a small percentage of heat radiated in comparison. I wouldn't expect as much difference in performance between a black anodized heat sink and an identical one except bare metal when the primary method of heat dissipation is convection, rather than radiation.

author: Neganur
author_rank: Unknown
timestamp: June 19, 2011, 09:34:11 pm
content: It doesn't have to be black as such. The main goal is to get rid of metallic luster since it hinders the thermal radiation. (think: Mylar blankets or glass vacuum flasks for coffee etc) I don't know why they chose black though, maybe its availability was better than say, grey. (the surface of the heat sink is porous after the anodising and sated with 'paint' for protection)Quote from: Hero999 on June 19, 2011, 07:07:08 pmShape and alignment of a heat sink matter a lot but are unrelated to the benefit (around 30% surface benefit, as you say) if anodised or not.

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2011, 10:14:26 pm
content: Quote from: Russel on June 19, 2011, 09:24:42 pmI got it from a data sheet for a heat sink I found on RS Components a few years ago when a similar topic was raised on another forum. I posted the full datasheet towards the beginning of the thread.https://www.eevblog.com/forum/index.php?topic=3830.msg50872#msg50872The data sheet doesn't state the conditions under which the test was performed. I assume it's when the heat sink is mounted in the vertically where air is allowed to circulate, as is the case for most heat sinks designed for passive cooling.

author: RCMR
author_rank: Frequent Contributor
timestamp: June 19, 2011, 11:46:12 pm
content: Interestingly enough, the ceramic that is formed when aluminium is anodized is an excellent thermal insulator.I design and build jet engines and did a lot of experimentation on the thermal insulation capabilities of such a ceramic layer.In one engine we had an aluminium plate at the end of a combustion chamber that was directly exposed to combustion gases. The rest of the chamber was made from a high temperature nickel alloy but had no ceramic layer.When operating, the external temperature of the aluminium plate never exceeded 200 deg C, whereas the hi-temp nickel-alloy would glow red hot, at temperatures of around 950 deg C.The aluminum was "hard" anodized, which creates a significantly thicker (but still very thin) layer of ceramic than regular "decorative" anodizing does.In the case of heatsinks, the anodized layer is *very* thin and the thermal insulation is offset by the pigmentation (black) and the increase in surface area that the porosity that anodizing creates.However, if you want a really good thermal insulator -- hard anodizing is it!

author: tecman
author_rank: Frequent Contributor
timestamp: June 20, 2011, 12:26:18 am
content: Missed here is that a heatsink surface that is sand blasted will have a greater emissivity than a shiny one. As far as anodize, there are advantages that the surface will not oxidize, which will reduce efficiency. As for black paint, be careful, since not all black paints are black in the IR spectrum. Some paints will greatly reduce emissivity.paul

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 20, 2011, 12:33:39 am
content: Quote from: tecman on June 20, 2011, 12:26:18 amYou are talking for the fins or the base ? or both ?

author: Unknown
author_rank: Unknown
timestamp: June 20, 2011, 12:58:28 am
content: The part that is outside the equipment case would be ideal. If you decide to treat the surface, electric arc sandblast or anything that makes it rought, avoid treating the area in thermal contact with the semiconductor. That area needs to be as smooth as possible to minimise thermal resistance from heatsink to the semiconductor, as the thermal paste used to fill the microscopic cavities on the surface is a worse heat conductor wrt the metal (but much better than air).

author: Neganur
author_rank: Unknown
timestamp: June 20, 2011, 01:15:47 am
content: Quote from: tecman on June 20, 2011, 12:26:18 amThis has less to do with the color than what substance the 'paint' is. It really makes no difference if it's blue or black for example, since the visible light spectrum is so narrow compared to IR.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 20, 2011, 08:28:27 am
content: Quote from: Hero999 on June 19, 2011, 08:46:20 pmThat graph is meaningless without reference to the absolute temperature - heat loss from radiation is proportional to the fourth power of absolute temperature. You won't see any difference in a fan-cooled heatsink as the airflow is the dominant factor.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 20, 2011, 09:56:46 am
content: Quote from: RCMR on June 19, 2011, 11:46:12 pmA bit of math makes this suspect. The thermal conductivity of alumina I find listed as around 20 watt / meter*K. This is about 10% the thermal conductivity of aluminum metal, which is among the most conductive metals. A really thick hard anodizing layer tops out at around 100 micron I think -- so it has the same thermal impedance of an extra mm of aluminum.High temperature alloys on the other hand, are generally rather poor thermal conductors. I just found a data sheet for "ATI 625HP" a nickel-based high temperature alloy. The thermal conductivity is listed as 10-25 W/(meter*Kelvin) over the temperature range -- about the same as alumina. For many high temperature applications you want low thermal conductivity: the hot side temperature is fixed and you want to limit heat flow across the material in order to insulate the surrounding structure. In an engine, you want to contain the heat so that you can generate power from it, not dissipate it through conductive materials.So what I suspect was happening here is a combination of two things. First, the anodized aluminum may have been in a part of the engine where the combustion gases were not as hot, or they may have been otherwise protected from the full heat load. Second, the excellent thermal conductivity of the aluminum, including the oxide layer, allows it to be cooled much more effectively than the low conductivity nickel parts. There is no way the oxide layer posed a serious thermal impedance that insulated the aluminum. The reason for the very thick hard anodization layer is for passivation. It chemically protects the underlying aluminum from attack by oxygen and other combustion gases. In a high temperature environment you need a thicker layer because gases can diffuse through the oxide more readily at higher temperature.

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2011, 04:15:58 pm
content: Quote from: mikeselectricstuff on June 20, 2011, 08:28:27 amNormally standard conditions should be assumed, if the temperature is not specified.QuoteI agree, I already said that.

author: RCMR
author_rank: Frequent Contributor
timestamp: June 20, 2011, 09:43:02 pm
content: Quote from: ejeffrey on June 20, 2011, 09:56:46 amYes, we were also surprised at the results -- but they speak for themselves.QuoteThis is all true.An un-anodized aluminium plate however was showing signs of ablation and showed a significantly higher (300 deg C) temperature when compared to the anodized one (< 200 deg c).Another factor in explaining the temperature differences are (as you point out) the thermal conductivity of the two different materials plus the fact that the hi-temp alloys rapidly develop a dark patina which contributes to the transfer of heat from the combustion gases to the metal. The aluminium remains comparatively bright (especially the anodized surface).However, the ceramic did (in this instance) make a significant contribution to reducing operating temperatures in this part of the engine -- perhaps due to a combination of preserving reflectivity, maintaining a reduced surface area (ablation roughened the surface) and reducing the rate of thermal transfer.And, as you correctly summised, this part of the engine does run cooler than the higher-temperature regions - but we were still able to obtain a significant operating temperature reduction -- due to the combination of effects outlined. In the end we also added a second ceramic (refractory) layer and dropped the operating temperature even lower -- but the anodizing certainly had a significant and measurable effect.

author: johnwa
author_rank: Frequent Contributor
timestamp: June 21, 2011, 12:37:43 pm
content: Another factor to consider is radiation from external sources. If your heatsink is exposed to direct sunlight, a black one may actually be worse off than a shiny one. As stated above, radiation is proportional to the fourth power of the source temperature, and the sun is probably a fair bit hotter than your heatsink!Regarding electrical insulation of anodising, I am looking at using this in a design with some TO-220s, but they will be clamped rather than screwed to the heatsink. I presume a 0.02-0.03mm hard anodise would be appropriate - does anyone else have any experience with this? I guess I can always go back to silicone pads if it doesn't work out.

author: Unknown
author_rank: Unknown
timestamp: June 21, 2011, 12:41:40 pm
content: QuoteThis is a very unreliable insulation method. If the heatsink is fully enclosued in the device then it can be live. Otherwise you must use a proper insulation method or a TO220 with a plastic tab.

author: johnwa
author_rank: Frequent Contributor
timestamp: June 24, 2011, 07:33:28 am
content: OK, fair enough. This is actually an automotive application, there are no high voltages, and I am only looking for functional insulation rather than for safety. But perhaps I would be better off using an insulating washer anyway.

author: SgtRock
author_rank: Unknown
timestamp: July 24, 2011, 03:02:28 pm
content: Greeting EEVbees:--Two questions:1) Why do Bedouin Arabs wear black?2) If your were trying to cool off at night, would you wear black or white clothing?Clear Ether

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 24, 2011, 03:52:36 pm
content: Greeting EEVbees:--Two questions:1) Why do Bedouin Arabs wear black? Black paint on discount ?Quote from: SgtRock on July 24, 2011, 03:02:28 pmNone I would be naked. LOL

author: Zero999
author_rank: Super Contributor
timestamp: July 24, 2011, 06:29:02 pm
content: Quote from: SgtRock on July 24, 2011, 03:02:28 pm1) I don't know: it looks good or for religious reasons?2) Black because it improves the transfer of thermal energy from your body to the outside environment, assuming the black dye used is not reflective as mid to far infra-red wavelengths of course.Incidentally, under a dark clear sky, an infra-red absorbent surface exposed to the sky can become much colder than the air temperature due to radiative cooling which is why there can be frost on the ground despite the air temperature being above freezing. This is more common at higher altitudes because there's less of an atmosphere to reflect the infra-red radiation back to earth.

author: IanB
author_rank: Super Contributor
timestamp: July 24, 2011, 07:37:01 pm
content: Going back to the beginning of this thread, radiative heat transfer at low temperatures (<100°C) can be significantunder some circumstances. Examples include the silvering of Dewar (vacuum) flasks as mentioned above, and British home heating systems. In Britain, home central heating by hot water to radiators is very common. Central heating radiators do heat the air by convection of course, but radiation is also a big component. If you sit near to an operating radiator at approximately 60°C you will soon start to feel rather warm.With heat sinks it is all about the air flow. Forced (fan driven) air flow makes ahugedifference to the heat removal in a heat sink. If the heat sink is in still air, and if it perhaps is in an unfavorable orientation, or inside an enclosure, then convective cooling will be severely restricted. Under these circumstances the proportion of cooling by radiation may be significant and a dull black surface will certainly be a better radiator than a shiny metallic surface.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 24, 2011, 08:47:27 pm
content: Quote from: SgtRock on July 24, 2011, 03:02:28 pmI am not certain, but I think from the physics point of view (rather than cultural or whatever) that the black cloth is less transparent than white, so the sun heats your clothing more than you. This would only work if the clothes were very loose and flowing. If you have fitted clothes, you want white because it reflects more of the sunlight.QuoteWouldn't matter. The emissivity of black and white fabric is essentially the same in the far infra-red that is relevant for black-body radiation at human body temperature. The color only makes a difference in sunlight because a lot of the sun's power is in the visible spectrum and most of the mid- to far-IR is blocked by the atmosphere i.e., by greenhouse gases.

author: Time
author_rank: Frequent Contributor
timestamp: July 25, 2011, 06:18:02 pm
content: Analytically, the coating reduces the thermal resistance between the air and heatsink. If you look at a good heatsink company (like aavid thermalloy) you can see specs on their products. Different extrusions with different heatsink/air thermal resistances based on geometries and coatings. They even offer a proprietary coating with better than normal coating heat transfer characteristics.

author: IanB
author_rank: Super Contributor
timestamp: July 25, 2011, 07:54:42 pm
content: Quote from: Time on July 25, 2011, 06:18:02 pmThe Aavid website seems to be a good resource for technical information. Here is a note for example that explains why anodizing of heat sink surfaces can be useful:http://www.aavidthermalloy.com/product-group/extrusions-na/anodizeAs to reducing the heatsink/air thermal resistance, that really depends only on surface roughness and the turbulence of the air flow. Greater roughness and greater air flows both will increase the film coefficient at the surface. But with low air flows and natural convection, the radiation effects are much more important than the roughness effects.

author: Frangible
author_rank: Regular Contributor
timestamp: July 27, 2011, 05:46:57 pm
content: Quote from: SgtRock on July 24, 2011, 03:02:28 pmThat was explained in a Scientific American article years ago. Evidently the dark coloring does absorb more heat, but since the clothes are loose and flowing, it creates a natural convection of the air next to the wearer's skin, thus creating a sort of "micro climate". The circulating air dries the sweat on the wearer's skin, cooling it and the air right next to it.With heat sinks, many are painted or coated in some way in order to increase its surface area, as would sandblasting. This is especially the case with non-reflective or flat finishes.

author: IanB
author_rank: Super Contributor
timestamp: July 27, 2011, 07:21:32 pm
content: Quote from: Frangible on July 27, 2011, 05:46:57 pmIndeed, but it is not actually the increase in surface area that is responsible for the improvement, it is the increase in surface roughness. The primary barrier to heat transfer from the surface is the so called "film coefficient", the resistance of a stagnant boundary layer of air touching the surface that the heat has to get through (remember that air is an insulator). Increasing the roughness of the surface will increase heat transfer by inducing air turbulence and reducing the thickness of the stagnant film. Of course to be effective all heat sinks must have good air flow, so increasing the surface roughness without having good air flow will not do much for you.

author: SgtRock
author_rank: Unknown
timestamp: August 17, 2011, 10:52:09 pm
content: To All:--Thanks for your informative answers concerning Bedouin Arabs and white vs black Clothing. I think I have a handle on it now.--As I understand it the mirrored glass, vacuum sealed Dewar Flask tends to keep cold liquids cold and hot liquids hot. My question is: How does it know?Clear Ether

author: Unknown
author_rank: Unknown
timestamp: August 17, 2011, 11:09:42 pm
content: Quote from: SgtRock on August 17, 2011, 10:52:09 pmIt done by memory, unfortunately Dewar Flask memory is particularly volatile, with each flask forgetting either hot or cold within a matter of hours.

author: IanB
author_rank: Super Contributor
timestamp: August 18, 2011, 02:35:23 am
content: Quote from: SgtRock on August 17, 2011, 10:52:09 pmWell, you see, what it actually does is keep the outside at the same temperature regardless of what is going on on the inside. So it doesn'tneedto know about the inside...

author: SgtRock
author_rank: Unknown
timestamp: August 18, 2011, 05:46:19 am
content: Dear Uncle Vernon and IanB:--Alas, once again I am hoist by my own petard. I am heartened to know that my humble remarks were accepted in the same spirit as they were given.--From "The Complete Idiot's Guide to Wilderness Survival":High Mountain Survival Tip #7 "Don't eat yellow snow"Best RegardsClear Ether

author: EEVblog
author_rank: Unknown
timestamp: August 18, 2011, 06:32:06 am
content: Quote from: Alex on June 19, 2011, 05:57:35 pmBlack anodising of heatsinksdoesimprove the thermal dissipation performance. This is a known and measured fact.Dave.

author: SgtRock
author_rank: Unknown
timestamp: August 18, 2011, 07:06:31 am
content: Here, here! Well said Sir. And just what would be expected from the Stefan–Boltzmann Law for "Black Body" radiators.Clear Ether

author: amspire
author_rank: Super Contributor
timestamp: August 18, 2011, 07:45:34 am
content: Painting or anodizing definitely improves radiation dissipation, but it doesn't have to be black. The colour in the visible spectrum has absolutely no correlation with the colour in the IR heat spectrum of a heatsink. So you can get any colour you want, as long as it is "black" in the IR frequencies. Most standard paint of any colour is fine to use so fluorescent green is as good as black. You don't even have to paint or anodize it. Stick masking tape or electricians tape to the raw aluminium heatsink and you also get radiation cooling close to a black anodised one.This is not the same as choosing the paint colour for a car as there is a lot of energy from the Sun at much higher frequencies then heatsink IR emission, and definitely a black car gets hotter then a white one. I think the Sun must be a bit hotter then a 100 deg C heatsink.Unpainted aluminum - shine or rough - has lousy radiation and also doesn't absorb radiation from other heats sources.This fact is really useful. If you want to have fan cooled heatsinks in a case, you want them unpainted, so they aren't radiating heat inside the case. Also if you paint the outside of a heatsink but not the inside, it will only radiate significant heat to the outside. Nice!On the other hand external convection cooled only work with a height up to about 100mm. A 200mm high heatsink only gets about 50% more convection cooling as a 100mm high one and over 200mm, you are just wasting expensive aluminium for no good reason. So if you need a very tall heatsink, forget about the fins. Make it a flat sheet, paint the outside surface any colour you like, and get rid of almost all the heat by radiation.

author: N TYPE
author_rank: Regular Contributor
timestamp: June 27, 2013, 11:51:19 am
content: Dragging up an old topic here but..Say I have a Heatsink which is painted or powdercoated on all sides, and I want to mount my TO220 reg onto the heatsink. Sure I'm gonna use thermal paste + mica pad and a nut & bolt to fasten it but is it necessary/beneficial to scrape the paint from the contact surface of the heatsink to transfer the heat better?

author: SnakeBite
author_rank: Regular Contributor
timestamp: June 27, 2013, 01:20:22 pm
content: Quote from: N TYPE on June 27, 2013, 11:51:19 ami don't think anodising and powercoating is the same thing.. in powercoating you apply paint on the surface of the heatsink that it's also used as insolator and anodising locks the color inside the metal itself that gives you the better heat transfer.BUT i'm not sure

author: digsys
author_rank: Unknown
timestamp: June 27, 2013, 01:36:09 pm
content: Anodizing and Alodining have excellent heat transfer properties. Powder coating is a bit trickier. What you lose in heat transfer, you gainby not having Al oxide as an insulator. In general, black powder coating is nett beneficial. The only "bad?" characteristic (in some cases) is thatthere is a slight reduction in rate of thermal transfer, but the FINAL effectiveness is about the same. IF you need a fast transfer rate, justupgrade your heatsink size / type a notch.

author: edavid
author_rank: Super Contributor
timestamp: June 27, 2013, 02:21:25 pm
content: Quote from: N TYPE on June 27, 2013, 11:51:19 amYou should never paint or powder coat a heatsink!