painted heatsinks & insulating washers

[lowlevel]

I know in theory that heat sinks work better if they are black but what is the current thinking.
I note that almost all small heatsinks on the market are pianted as stock but many of the large items available as parts and almost every amp I've had the lid off in recent time the big heatsinks are plain aluminium.
Whats the current cutting edge thinking?

I have also heard many arguments in the dim past about heat transfer using diferent types of washers under you power devices.
Berilium was suposed to be real funky in the late seventies early eighties (if you didn't mind a slow lingeriing death)

I was braught up on mica washers and silicon grease.
It was considered better to omit the washer if design allowed it.
How are the grey silicon washers stacking up for transfer and long term reliability?
I have seen a few of the grey fellas blown thru in TV flyback circuits.
cheers chaps

[jaycee]

Well, it isnt paint, its anodising which is a chemical process involving nasty acids and dyes. Paint would produce a relatively thick layer that would be too poor a thermal conductor etc...

The choice du jour for mounting transistors where heat transfer is important now seems to be Kapton. The sil-pads (grey is one variety) are usually used in mass produced stuff, and there are better quality ones than the grey ones but they are normally quite expensive and it seems to work out cheaper to buy a roll of Kapton tape and cut it up into needed chunks. I haven't tried this myself yet - I use the grey sil-pads.

[kilowattski]

Actually they are not painted. They are either Anodized or Hard Coated (Hard Anodized). From what I understand Mica insulating washers and thermal transfer goo seem to work better. If you Hard Coat the heatsink, the coating is about 2 to 3 mils thick (about the thickness of the mica insulator), non conductive and has excellent thermal transfer characteristics. Simply mounting your transistor directly on a Hard Coated Heatsink with thermal transfer goo seems to be the best senario. I have done this successfully in the recent past.

[uli]

Originally posted by jaycee
Well, it isnt paint, its anodising which is a chemical process involving nasty acids and dyes. Paint would produce a relatively thick layer that would be too poor a thermal conductor etc...

The dissipation factor of painted alu is better than anodized alu.
You need to use special paint (ovens).

Uli

[Eva]

Feel free to correct me if I'm wrong...

There are two different heat transfer phenomena between heatsinks and air : Radiation and contact. Radiation is improved when the radiating surface is black, but contact transfer is reduced when using paint

Also, the thermal resistance from the transistors to the heatsink appears to be bigger if there is a layer of paint in between, so painting the entire heatsink is not a good idea

Anodizing is said to be a good compromise since it's only a fine 0,1mm layer of oxidized aluminum created by electrolytic processes, it protects aluminum from corroding and it also provides 100V or more of electrical isolation

It appears that contact heat transfer dominates in forced air cooling applications and thus anodizing or painting is not worth the extra cost. Forced cooling heatsinks are usually raw aluminium

But in natural air flow applications, the increase in Rth from anodized to raw aluminium is 15% to 20% according to some heatsink manufacturer. This makes a difference for small heatsinks, I think

[anatech]

Hi lowlevel,
Mounting transistors using just grease and relying on an anodized surface is risky. A scratch and you have a short. I still use mica and grease. It is messy and takes longer, but it works. When the sil-pads are used I still use a little grease. Just make sure the mounting surfaces are flat and smooth. Sand them with 600 grit wet / dry paper if you aren't sure. A sanding block helps (mine are titanium).
Make sure there are no burrs as well.
-Chris

[Mr Evil]

Just to clarify what has already been said: The three methods of heat transfer are conduction, convection and radiation (and evaporation is sometimes classed as the fourth, but not important here). All three are involved in a heatsink. First the heat is conducted from the device to the heatsink and through the heatsink to the fins. Then it is both radiated and convected away to the atmosphere.

Convection is the dominant transfer method. With forced air cooling radiation can be ignored. With natural convection it makes a small difference which may be worth bothering about if you want the best performance.

Two factors make radiation work better: Surface area and colour. More surface area and darker colour are better. Matt finishes have more surface area. Increased surface area also improves convection.

The surface coating, be it paint or anodizing, decreases the effectiveness of conduction through the heatsink, but as long as it is a thin layer it doesn't have to have a big effect.

The only way to be sure if a surface coating will be beneficial or not is to measure it. I just happen to have an article on thermal resistance measurement on my website.


As for washers: I prefer to go without insulation at all where possible and use just thermal paste (high quality silver-based stuff like Arctic Silver is well worth it). This may mean separate heatsinks for each transistor and each heatsink insulated from the case and each other, but it can make a significant saving in thermal resistance, allowing for smaller heatsinks overall.

For non-critical applications I like to use those sil-pad things, for simplicity and cleanliness, but their performance is relatively poor.

Hard anodized heatsinks can provide good electrical insulation without washers, but normal anodizing should not be relied on in case of scratches.

In addition to surface smoothness as anatech mentioned, mounting pressure also has a big effect on thermal resistance. You want the highest pressure possible applied as evenly as possible. Be wary of using the mounting holes on plastic devices as too much pressure there tends to lift the opposite end of the device off the heatsink.

[johnnyx]

The mica insulation on a friends power amp broke down, and was arcing between the heatsink and TO3 power transistor. The heatsink was bare aluminium. How it happened is a mystery, (there was only about 40v - unlike the high voltages in a TV circuit) but since then I have been careful to use good insulation.

[Mr Evil]

Originally posted by johnnyx
The mica insulation on a friends power amp broke down, and was arcing between the heatsink and TO3 power transistor. The heatsink was bare aluminium. How it happened is a mystery, (there was only about 40v - unlike the high voltages in a TV circuit) but since then I have been careful to use good insulation.

Could it have been dust? It can be a problem when using thermal paste that crud sticks to it. If the insulating washer doesn't extend far enough past the edge of the device, or if the thermal paste is splurged everywhere, then it is possible for enough stuff to accumulate to bypass the insulation.

[Eva]

From time to time I've seen very small solder balls and small particles of aluminum from heatsink machining trapped between the insulating silicone washers and the heatsink or the transistor. They were causing otherwise unexplainable intermittent shorts. Sometimes manufacturing quality is not as good as expected...

Once I even found a small piece of TO-220 transistor leg trapped between a TO-3 device and his insulating mica washer in a 1980s Spanish manufactured PA amplifier. That ill-fated TO-3 device was a driver instead of an output device and was not blown, but it had almost unity current gain due to years of thermal cycling to extreme temperatures. The amplifier showed hard to explain distortion and humm in one of its channels but the mica insulator resisted without causing a short