Painted Heat Sinks

It's huge compared to the thermal conductivity of still air, so it won't change things much. The only interesting case would be with high-velocity forced air flow, where it may be worth doing the math.

Why not anodize? That's probably easier than painting.

John

Radiation effect is tiny at the temperatures seen on ordinary heat sinks. The paint's thermal resistance will likely make things worse than any increase in radiation.

John

With each change of medium is an increase in thermal resistance. CPU to thermal compound to heatsink has higher thermal resistance (conducts less heat) than CPU direct to heatsink. Thermal resistance increased due to medium changes.

Yes, you can paint the heatsink. Probably matters little the type of paint used. Medium is heatsink, through paint, then to air (3 mediums) instead of heat direct to air (2 mediums). The third medium causes thermal resistance increase. Then there is this thing called black body radiators which also would be lost in painting.

Up front number is 'degrees C per watt' which includes thermal resistance of those medium changes. That number is what heatsink manufacturers (the responsible ones) and you both worry about. Painting will only adversely increase that number.

Would not worry about pa> Have you had any experience with painted heat sinks? I have a customer

The site comes up blank in my browser, but in any case, this is where I get my dogs:

Thanks! Rich

Radiation works both ways, though, and if you've got hot stuff nearby (such as those thermionic devices or some kind of space-based thermonuclear reactor) you could get the HS 'seeing' the hot stuff rather than reflecting the IR off. So maybe they should be black on the cold side and shiny on side that sees the sun or whatever.

Best regards, Spehro Pefhany

The "color" in visible light is irrelevant, unless the HS is radiating in the visible spectrum. I suggest this is usually badness in this bidness. ;-)

Black and shiny are not colors.

Best regards, Spehro Pefhany

According to the little prog I have, a heatsink of 100 sq cm area, 50 C above ambient, emissivity = 1.0, radiates a tad under 4 watts. That's about 12.5 c/watt, and a 50C rise is pretty radical in most apps. For a 30C rise, more reasonable maybe, it's more like 2 watts.

If it's inside a non-black box, some of its radiation will be reflected back. Similarly, parallel fins mostly just radiate at each other, which can defeat most of the radiative cooling effect.

John

No, that was, "heers Phil Hobbs", as in, "Heeeere's Johnny!" ;-P

Beers! Rich

I suspect that using black paint can actually improve heat dissipation, due to increased radiation of heat as opposed to a shiny reflective surface.

Powder coat is probably thicker than standard paint, which may hinder things.

greg

Radiocity equation at the end of this paper may be insightful,

If you're using convective cooling and have cool surroundings to radiate to, glossy black oil based enamel will considerably boost the cooling over bare aluminum. Anodizing doesn't seem to do anything for the long wavelength infrared radiation needed for the lower heatsink surface temps.

On one project, I needed .35C/W cooling but the best aluminum extrusion I could buy would only give .5C/watt. By painting with black oil enamel, I got my .35C/W.

If you're using forced air on a relatively cool (less than 100C) aluminum heatsink, anodizing would be the simplest way to get color without affecting performance.

While playing with my new IR thermometer, I tried measuring the light fixture above my drafting table. The polished brass part measured well over 100C with a thermocouple but barely above room temp with the IR.

John

Ok, black or shiny in the visible range may not be so black or shiny in the far infra-red.

That may be good for the written specification, but it is not what you want for outdoor P.A. gear which may be used in strong sunshine; it will absorb more radiant energy because of it.

I'm not sure that all polished metals reflect infra-red wavelengths well, either. I have recollections of 'reflective' car tools getting very hot when left lying around in the sunshine.

Yes. And some colours that can be quite 'unexpected', can be very good IR radiators. Certain white paints, are actually good IR absorbers and radiators in the NIR, and IR sections of the spectrum, while some quite dark appearing coatings are poor (hint, normal 'anodised' aluminium is not good in this section of the spectrum...).

Best Wishes

That is why telescope domes are painted white using a paint that is dark in the thermal IR region. It mimimizes heat absorption during the day and helps the domes cool off faster at night.

Daniel Lang

Perhaps the IR thermometer's assumption of e = 0.95 was a mite off.

Best regards, Spehro Pefhany