Rookie question about thermal pastes

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SpeakerBob

SpeakerBob

Member
Joined 2002
#1
Rookie question about thermal pastes / Rectifier Q

Hi all,

I've had to unbolt this heatsink to move my PCB so I can replace the speaker relay on the board. About to put it back together and thought maybe this paste is insulating the rectifier from the chassis ?

I've looked up the datasheet and it states 'Case to Terminal Isolation Voltage 1500V' which I think means it doesn't need insulating. However I'd rather double check with you guys before blowing my amp up 😀

Thanks,
Rob.
 

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#2
You are right. The datasheet says the case is insulated to 1500V, so no need to insulate it from the heatsink bracket.

But the paste is not there to insulate it anyway, it is there for good thermal contact between the bridge and the metal. Neither surface is infinitely flat, so the paste is there to fill any voids and help the heat transfer from the diodes to the metal.

Jan
 
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#10
By the way, the trick is to use enough thermal grease, but no more than that. It is just meant to fill microscopic voids, not to be a thick layer between the transistor and the heatsink. That's because its thermal conductivity is not very large, even though it's much greater than that of still air. Maybe Grarea's CPU compound is less critical than ordinary thermal grease in that sense.
 
#11
My :2c: regarding the grease type - In the recent past, the highest rated thermal conductance types for use with CPUs, contained enough silver powder to also be highly electrically conductive and a trap for inexperienced DIYs. Even when the external excess is carefully wiped away, it could still result in electrical shorts or failures from excess around the pins of TO3 power transistors. As a rough indicator of the grease type, usually, the darker the paste colour, the more silver content and conductance but there is no substitute for always checking your work with a multimeter.

Ideally, if the insulator pads (mica or Kapton etc.) are clear of excess paste and smears there should not be any shorts but this is by no means reliable. I still see botched repairs, often by very capable but unwise folk. CPU upgrade accessories may also be easier to buy than quality, alumina based thermal grease but unless there is an explicit claim that the type you choose is electrically non-conductive, don't risk it.
 
#12
Thanks.
I went to look at thermal grease.
lol, where on earth does one start?
I went to Farnell's, typed in thermal grease and there was a hundred options (literally, bang on a hundred)

Is there a 'go to'?
I would prefer a slightly better one as I won't be using loads (he says, with a long list of 'to do' and longer 'want' list)
I am a noob, so I need all the help I can get 🙂


Thanks.
 
#14
Ian Finch said:
My :2c: regarding the grease type - In the recent past, the highest rated thermal conductance types for use with CPUs, contained enough silver powder to also be highly electrically conductive and a trap for inexperienced DIYs. Even when the external excess is carefully wiped away, it could still result in electrical shorts or failures from excess around the pins of TO3 power transistors. As a rough indicator of the grease type, usually, the darker the paste colour, the more silver content and conductance but there is no substitute for always checking your work with a multimeter.

Ideally, if the insulator pads (mica or Kapton etc.) are clear of excess paste and smears there should not be any shorts but this is by no means reliable. I still see botched repairs, often by very capable but unwise folk. CPU upgrade accessories may also be easier to buy than quality, alumina based thermal grease but unless there is an explicit claim that the type you choose is electrically non-conductive, don't risk it.
Click to expand...

Ian, in this case there was no need for electrical insulation. The case of the bridge is itself insulated to 1500V (data sheet). So the need is only for thermal contact between the case and the heatsink. No insulation washer needed.

Marcel's point is well taken: use only the minimum necessary as a thick layer of paste worsens the thermal impedance.

Jan
 
#18
You would need to thoroughly "dry" such a concoction before assembly because any IPA solvent remaining in the compound would vaporise and build up pressure as the case heated up above 80°C. The resulting vapour pressure could also displace the film of compound from where it is most needed, below the die location.

It's certainly innovative thinking but along with the problems of different oil media used to make the various heatsink compounds and even getting dust and small fibre particles in the mix, it would be a risky practice for DIYs and also professional techs who may not be aware of those issues or how to avoid them.
 
#19
A thin film contains a little alcohol, by the time I get to fix the part it has dried out.
Clarification: I recommended alcohol to be used as a solvent for cases where a thick layer of thermal paste has to be removed, or is particularly sticky.
A drop of alcohol, applied with a syringe is direct from the bottle, helps to smoothen out patchy paste when you have to attach a replacement part to the heat sink..
I use my finger to spread it out...it dries out well before I fix the heat sink back.
 
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