I've read that a hard coated anodized aluminum surface results in a non-ferrous layer that can resist up to 120V. If that is true, why use grease or pads between a chip and heatsink surface?
Last edited:
The purpose of the grease is to improve thermal conductivity between the two surfaces by filling microscopic air gaps and voids. It is not for electrical isolation.
You will always need a thermal interface, even if the mating surfaces are finely machined.
Whether you can depend on a crude process like anodization for voltage isolation is another matter.
I wouldn't.
Whether you can depend on a crude process like anodization for voltage isolation is another matter.
I wouldn't.
I believe that the grease is appropriate to improve thermal conductivity between the two surfaces by filling microscopic air gaps and voids. My question was lacking specificity and should have been are pads really needed
Well, yes the anodizied layer on aluminum is an insualtor, but it is not relied upon when electrical insulation is required. It is very thin and can be damaged during assembly. Pads, along with the right mounting hardware will provide both electrical insulation and thermal conductivity improvement.
Just because I'm curious: Where do you have hard anodized radiators from?
Pretty much everything we are having to do with is anodized for decorative purposes and dyed and this is a process resulting in a rather thin oxide layer.
Hard anodizing produces a pretty thick oxide layer that is also very tough. Just like other DIYers pointed out already, I would not trust the oxide layer for insulation although it may work.
According to my experience, hard anodized surfaces come in a variety of random colors from shades of gray over gold to brown. Every batch I received had a different color. Different aluminum alloys also contribute to the color. This is why hard anodizing is not used for parts where appearance matters.
Pretty much everything we are having to do with is anodized for decorative purposes and dyed and this is a process resulting in a rather thin oxide layer.
Hard anodizing produces a pretty thick oxide layer that is also very tough. Just like other DIYers pointed out already, I would not trust the oxide layer for insulation although it may work.
According to my experience, hard anodized surfaces come in a variety of random colors from shades of gray over gold to brown. Every batch I received had a different color. Different aluminum alloys also contribute to the color. This is why hard anodizing is not used for parts where appearance matters.
Forgetting ?? that we normally use screws into drilled and tapped holes to mount the devices to a heatsink 🤣.
Shoulder washers are used to retain isolation in that case. Or clips can be used, which only contact the
isolated part of the case.
In principle, anodization is not designed to serve as an isolation in items not specifically intended for that purpose, thus you should refrain...
But, I did make a try on a particularly robust example, knowing that it might turn sour:
https://www.diyaudio.com/community/threads/chip-amp-photo-gallery.79303/post-4003957
It didn't, and this piece of gear is still working spotlessly, but your mileage may vary: I made sure that the dielectric strength of this heatsink was high enough, with a lot of safety margin.
Mechanically, the layer was (and is) extremely resistant, but I have seen other examples where it was not the case.
Anyway, if you attempt to use it as an isolator, you should not compound possible problems by applying a conductive grease
But, I did make a try on a particularly robust example, knowing that it might turn sour:
https://www.diyaudio.com/community/threads/chip-amp-photo-gallery.79303/post-4003957
It didn't, and this piece of gear is still working spotlessly, but your mileage may vary: I made sure that the dielectric strength of this heatsink was high enough, with a lot of safety margin.
Mechanically, the layer was (and is) extremely resistant, but I have seen other examples where it was not the case.
Anyway, if you attempt to use it as an isolator, you should not compound possible problems by applying a conductive grease
What conductive grease? Heatsink "grease" is not electrically conductive. It is a great electrical insulator as indicated on the attached data sheet at 5 x 10-14th ohms per cm, and a great thermal conductor at the same time.
OK, it really depends on what transistors you are using.
Some transistors have a metal backplate. The metal plate is usually the collector as well. In this case, you cannot mount the trasistor onto the heatsink without any insultation, it will cause short-circuit. You need to use insulation pads. Most insulation pads double up as thermal pads as well, so you do not need any thermal grease.
However, there are transistors which has a plastic/resin back instead of metal. They do not need insulation pads. You should apply thermal grease to improve thermal conductivity.
Btw, there are a small handful of thermal grease which are electrical conductive (most are not). They are mostly carbon/metal ones. Below is an example. I don't think it is recommended to them.
https://www.thermal-grizzly.com/produkte/25-conductonaut
An example of electrical and thermal conductive grease.
https://www.chemtronics.com/circuitworks-silver-conductive-grease
There are a small number of grease thats both electrically and thermally conductive.
https://www.chemtronics.com/circuitworks-silver-conductive-grease
Yes, this is true and I have used them before. Not for heatsinks, but to lubricate bearings on moving metal parts and simultaneously provide a path to ground for static discharge. The petroleum based grease normally used in bearings is a quite a good insulator which allowed statc to accumulate and become a problem in our device. Silver based conductive grease worked great, and although it was expensive as greases go, it was simpler and more effective than many other static control methods we tried.
This one is perfect; some are mixed with metal or carbon, and they are conductive
5 x 10 to the +14 actually, otherwise it would be more of a perfect conductor than an insulator.
Do not trust the oxide layer for electrical insulation. Always treat it as a conductive surface.
If there is issue with the insulation of the oxide, will you replace another heatsink or add an insulating washer?
You will add a insulating washer of course. So just add it in the first place and forget about the 120 volts dielectric strength.
You can avoid the washer only if the heatsink could be allowed to float. (not connected to anything electrically)
Ungrounded metal parts in a system is a cause of signal radiation and dangerous at times.
Regards.
If there is issue with the insulation of the oxide, will you replace another heatsink or add an insulating washer?
You will add a insulating washer of course. So just add it in the first place and forget about the 120 volts dielectric strength.
You can avoid the washer only if the heatsink could be allowed to float. (not connected to anything electrically)
Ungrounded metal parts in a system is a cause of signal radiation and dangerous at times.
Regards.
Some amplifier designs have all the collectors grounded - therefore no insulator and a grounded heat sink. Yet still others run the heat sink at Vcc - with a warning sticker about the exposed high voltage. Separate heat sinks for each rail of course. And some run the heat sink at speaker out (with all the collectors directly connected). Any of these avoid the need for micas/sil pads, but the most common and straightforward output stage (EF with all the transistors on one heat sink) is impossible. And none of them will assume that the anodization layer is enough. If some secondary component like the Vbe multiplier, temp sensor, or rail switch is fixed to that heat sink it has an insulator.
Type III hard anodizing can potentially be relied on as an insulator. It's much thicker than the normal stuff you find on a heatsink, comparable to the thickness of a mica insulator. Maybe ask the manufacturer what the thickness is, and if they can tell you the breakdown voltage.