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I know that heatsinks come in different fin forms and those different fin forms affect the way how they dissipate heat. However, there's something I could not get about them and it's their color.

Can someone explain how does color affect a heatsink when it comes into dissipating heat? Complex explanations are welcome because I'm really curious about this particular matter.

Jawa
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Bob Freaking
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    I think the question is where does color come from. If it is the natural material then the question is how does it conduct. If the heat-sink though is painted a color, then I would worry about the conductivity of the paint. So it does not matter, but don't spray paint yours black to look cool – Andrey Nov 08 '13 at 15:48
  • @Andrey, heatsinks are usually anodised. Same for high-quality bike frames. – Vorac Nov 13 '13 at 16:39

3 Answers3

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I suspect it's unlikely, though it depends on what you mean by colour. There are three fundamental modes of heat transfer for any material, and only one of them is directly affected by colour.

Heat is transferred from the heat source to the heat sink, and from the heat sink to the air by conduction. Most heatsinks are made of copper (heavy, and relatively expensive) or aluminium - and copper is generally left natural, and aluminum either has a natural clear covering of Al2O3, or is anodised and coloured. For conduction, material rather than colour is important.

Convection is the movement of heat through the movement of air. While passive cooling simply uses this, you can increase its efficiency by increasing surface area (which is why heatsinks are finned), or by forced convection - blowing air to carry heat away. While air is not a good conductor, convection in air is how everything from cars to heatsinks are cooled. This is very efficient, and doesn't really rely on the material properties of the heatsink, or on the colour.

Radiation is... frankly awful at transferring heat unless you're in a vacuum (It also sucks in a vacuum, but conduction and convection cannot happen). It is affected by surface colour.

Practically speaking, a heatsink being coloured is entirely for looks, and will affect cooling less than surface area, airflow, material and the contact between the heat source and heat sink.

Pang
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Journeyman Geek
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    Radiation is almost as efficient in air as it is in vacuum. IOW, it's still awful in vacuum. But convection and conduction are far worse. That's why thermos bottles use a vacuum for isolation. – MSalters Nov 08 '13 at 14:06
  • yeah, thats true. Edited my answer to reflect that. – Journeyman Geek Nov 08 '13 at 14:35
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    A more proper formulation would be "radiation is awful at transferring heat _except at very high temperatures_". The reason being, while heat conduction pretty much dissipates simply with a power proportional to the temperature difference (_P_ ∝ Δ _T_), the radiation power a blackbody emits [is proportional to _T⁴_](http://en.wikipedia.org/wiki/Stefan-Boltzmann_law)! For cooling something extremely hot, radiation is thus very effective. We usually don't call it _cooling_ in those cases, but consider the sun: an enourmous nuclear reactor that needs cooling. How, in vacuum? Radiation! – leftaroundabout Nov 08 '13 at 15:22
  • alas, its been years, and my physics is rusty. – Journeyman Geek Nov 08 '13 at 15:32
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    Not very relevant, but as a kid, I visited Edwards Air Force Base and saw an X-15. I asked the guide why it was painted black if black absorbs heat. He said yes, but it also reradiates it very well. I'm sure there are other good reasons why it may have been black, but ... – Joe Nov 14 '13 at 02:01
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This Wikipedia article has some information/discussion about that. I only repeat some links below. For the complete discussion you can look at the article.

Heat Sink Color

From http://www.radianheatsinks.com/support/faqs.html

How does the color of a heat sink impact its thermal performance?
In natural convection a black or dark colored heatsink will perform 3% to 8% better than an aluminum heatsink in its natural silverish color. This is due to the fact that dark colors radiate heat more efficiently.

In forced air applications, surface color does not increase a heat sink's performance due to the increase in convection. The color would provide cosmetic benefits only.

From http://www.globalwinusa.com/faqs/heatsink/color.html

Does heatsink color affect heat dissipation?

Black is the best thermal body in terms of being a absorber or emitter. Let's assume a vacuum situation, if the surface "A" of a black heatsink is totally covered at T1 (temperature 1) by another black body at T2 (temperature 2), the black heatsink would get the energy reflected from another black body at reflection energy Ad(T14-T24) that we call "Stefan-Boltzmann" law of thermal radiation, here refers to Stefan Boltzmann Constant, it is 5.6697 x 10-8 W/m2?K4. Therefore if based on above, T1 is the temperature obtained from the black heatsink onto CPU, T2 is the ambient temperature around CPU. So if T14-T24 is a positive value, we know black is the best heat dissipation transistor since there is no thermal source around CPU inside PC case.

From http://www.bcae1.com/heatsink.htm

Do not paint a heat sink. Most heat sinks are anodized aluminum. Painting a heat sink (especially if it's a thick coat of paint) is like putting a blanket on the amplifier. If you absolutely must paint the heat sink, use the lightest, thinnest coat of paint possible.

Rik
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  • When must you absolutely paint a heat sink? – Marcks Thomas Nov 08 '13 at 16:48
  • Yeah, i found that one funny too. Maybe if you have a (custom) glass case ;) But apparently there are system-builders wanting to do this. Reasons ranging from not spoiling "the look" like [here](http://forum.overclock3d.net/showthread.php?t=46321) to coating it with an Emissive coating like [here](http://www.candlepowerforums.com/vb/showthread.php?276982-Emissive-heatsink-coatings). – Rik Nov 08 '13 at 17:09
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In an absolute sense, yes it will affect it because a black heat sink will absorb more ambient radiant energy than a white one would. The more energy you have coming into the heat sink, the more you have to dissipate...

Now, is this measurable in the scope of CPU/GPU cooling? IDK, I suspect not.

Furthermore, in a situation such that you DO have enough radiant energy to make a noticeable difference, you probably have bigger problems than a black heat sink... such as the bonfire inside you case.

mHurley
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  • In addition, unless you have a ton of internal light sources or have a window in your casing, there will not be that much ambient radiant energy. Maybe from the GPU fan, but most ambient temperature would be the one the heat sink already just sent out, to the point where I doubt that something in or around the case sends out more ambient radiant energy than the heatsink itself. – Nzall Jan 07 '14 at 16:17
  • I know this is old, but this totally wrong. Since the goal is to remove heat from the processor, that side is going to be much hotter than the ambient environment. The black will help in the case because it radiates much more energy than silver surfaces. This is why cast iron stoves are black. Think of black as a conductor of radiant heat and silver as a resistor. If you want to keep the heat from transferring (via radiation), use silver. Example is the mylar "emergency blankets". – BytePorter Jul 09 '20 at 16:26
  • No, this is incorrect reasoning. Dark materials reflect less energy (that's what makes them dark). The potion of energy that isn't reflected is converted to heat and absorbed by the material. The reason cast iron is black is that it's covered with soot. The reason stoves are generally black is because they hide stains. Emergency blankets don't RESIST heat transfer, they REFLET heat. The difference is that RESISITORS convert the energy into heat. REFLECTORS send the energy back into the environment WITHOUT absorbing it. – mHurley Jul 09 '20 at 18:47
  • If black RADIATED energy more efficiently, it wouldn't be very useful for stealth aircraft. – mHurley Jul 09 '20 at 18:49