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Old 15th February 2007, 07:14 PM   #1
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Default Painting Heatsinks..I Know..Bad Idea

Ok, I have a big ol' pile of heatsinks from an industrial motor controller that are painted an ugly grey. I realize that the best thing to do, from a performance point of view, would be to strip 'em and leave them "nude", but I have WAF to deal with.

My questions are:

1) Is there a paint designed for thermal transfer?

2)By what factor should I de-rate the sinks painted?

3)Do you think any less of me for considering such a noob blasphemy?

Let the flaming begin

-Casey
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Old 15th February 2007, 07:44 PM   #2
wboyd is offline wboyd  United States
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My question to you would be...What are you planning on using them for?

Also, have you considered having them powdercoated?

If the heat is not "extreme" and you feel like it will not cause that paint to loosen and fall off then you should be fine. I have a couple of chip amps that have painted heatsinks, and they are fine. I did however remove any paint around the area where I mounted the chips.

There is also heat resistant paint (mainly available for grills, etc) that can be found at your local Home Depot, hardware store, etc.

Just my .02 there will be other theories...some will be more precise...

Wayne
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Old 15th February 2007, 07:47 PM   #3
cpemma is offline cpemma  United Kingdom
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The commercial black ones are generally paint (rather than anodised & dyed) so you're in good company.

Strip the old paint off first and use matt black spraycan from a car place (a car in the sun can get too hot to touch). The bare aluminium may need etching or priming to improve paint adhesion and knock-resistance.

If you check IR's AN-1057 there's not much wrong with paint for a passive-cooled sink.

Yes, I wouldn't paint the semiconductor mating surface.
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Old 15th February 2007, 07:51 PM   #4
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Heatsinks are usually anodised, you better check if it really is paint as anodising is not easy to strip off.

If it is paint and you can strip it off, you will first need to prime the alloy with etching primer. Normal primer will simply fall off after a while. Then on top you use whatever paint you like. Normal car paint should be fine as the heatsinks should never be hotter than the hand can stand.

By painting I think the thermal efficiency would be reduced by a similar amount to that when a mica washer is used.

Oh yes, nude heatsinks are not the best thermally speaking, matt black is the most efficient colour to get rid of heat.
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Old 15th February 2007, 08:25 PM   #5
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Heat escapes from a heatsink via photons and they don't much care about the color of the heatsink. To absorb heat use flat black color but to get rid of heat plain old aluminum is fine.

I found some auto bumper/trim flat black spray paint that is self etching and works great on bare aluminum. I can't tall any difference in the efficiency of a sink that has a thin coat of spray paint over one that is bare.
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Old 15th February 2007, 09:05 PM   #6
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Well slap my a** and call me Nancy

I read the application note by IR that cpemma linked to (table attached), and according to them, paint is more effective than anodizing

Seems this old fart can still learn something

The "bumper paint" seems to be the ticket, though I'll do a mild acid (vinegar) wash first.

Thanx guy's.

-Casey
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Old 16th February 2007, 04:48 PM   #7
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Strip the paint and have them anodized.

Use the anodizing as your insulator, and you can use non-insulated components, lowering your overall thermal resistance, improving cooling.

I think that will get you better results than picking the perfect paint.
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Old 16th February 2007, 05:02 PM   #8
hermanv is offline hermanv  United States
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Quote:
Originally posted by Narcisse91
Strip the paint and have them anodized.

Use the anodizing as your insulator, and you can use non-insulated components, lowering your overall thermal resistance, improving cooling.

I think that will get you better results than picking the perfect paint.
I don't think standard commercial anodizing is thick enough to prevent shorts caused by tiny bumps or burrs. Those aluminum TO-3 insulators you see are hard anodized with thicker than normal anodizing.

Even if you measure carefully for shorts there is little guarantee that shorts won't develop later as the parts grow and shrink with heat cycles.

It's just my opinion, but I'd be very carefull with this idea.
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Old 16th February 2007, 05:13 PM   #9
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I'm not sure of different levels of anodization and where the each one reaches it's limits - definitely something to consider - but a shop that does the anodization should know this. I know some will withstand a couple thousand volts before they break down, higher than most FETs will be operating.

Using anodization as an insulation suffices in commercial, industrial and even military applications (the most particular when it comes to long term durability), so for a home project, I would think it would be fine.

When you anodize, you actually embed the material down into the aluminum. So any shorts that would develop would have to go through the thickness of the anodized material.

One of my next amps is going to be built in a chassis that was designed to hold FETs this way, using just metal clips to hold fets against an anodized surface.


But hey, it's at your own risk, so if you're not comfortable with the idea, don't do it.
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Old 16th February 2007, 08:46 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
that table in post8 is worth reading.

Heat gets out of the sink in two ways:
radiation and conduction.

Conduction to the laminar layer of air in direct contact with the surface accounts for the majority of the dissipation capacity of a heatsink.
The faster the air travels past the sink the thinner the laminar layer. A passive cooler has slow air flowing past the surfaces (that's why it's important to have the major dimension of the sink fins in a vertical direction) resulting in a thick layer and we all know that air is a poor conductor of heat. Blow the sink and dissipation improves markedly, even a slow blower or a strong draught makes a significant difference.

Radiation accounts for less than 10% of the dissipation capacity of a passive heatsink that has been designed to expose a large surface area to the cooling air. A fan blown sink relies on only a tiny proportion of it's dissipation capacity from radiation (probably less than 2% or 3%).

Radiation can be improved by choosing a high emissivity surface.
The gains in emissivity from 0.7 to 0.94 are significant PROVIDED that the conduction is not compromised. Adding a thick layer of insulating coating will reduce conduction and since this should account for at least 10 times the radiation then take care with any added insulation.
A gain of 2.7% in radiation [(0.94/0.7-1)*8%] and risking the loss of 10s of percent in conduction should be approached cautiously. Plastic coatings (dipped or electrostatic) can be very thick, undercoat adds to thickness, brushed on coatings are usually thicker, etc.

Why are the fanned CPU coolers almost always plain aluminium? Because the sink relies on about 99% of it's capacity from conduction through the very thin boundary layer and the fast moving airflow that carries away the heat (turbulence is the computer chips friend, that's also why they are so noisy).
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