Heat Sink Compound on a TO220?

DreadPirate · 13th July 2008, 06:46 PM

I'm installing these output transistors to a heat sink. I've cleaned the mica insulators. Should I put HS compound (I'm using the white silicon type from RS) on BOTH sides of the insulator? The reason I'm asking is that there is clearly liberal amount on the original install between the mica and heat sink, but its not apparent that any was installed between the TO220 and the mica. Its a Marantz 2285B.

Conrad Hoffman · 13th July 2008, 07:26 PM

Always clean off every bit of old compound. It dries out and won't work well on the second installation. Apply new well mixed compound to both side of the clean mica and install.

frank1 · 18th July 2008, 01:56 PM

.... Also, it should be applied sparingly, not liberally.

Frank

AndrewT · 19th July 2008, 12:55 PM

the compound is only there to exclude the air that would be trapped in the valleys between the high spots in the mating surfaces.
The high spots should be touching each other.
If the surface finish is very rough then the quantity of compound between the non-mating surfaces will introduce significant thermal resistance.

The better the surface finish and the less compound the lower the thermal resistance of the interface.

But, you must strive to exclude ALL the air from the interface. This is far more important than any other consideration.

KCC · 31st January 2009, 06:41 AM

Hi,
Ok, since we are in TO220, I like to ask another question with regard to installation of the device. Hopefully someone can give me advise on it.
My TO220s are installed on a large heat sink.And the heat sink itself is part of chassis common ground. I see the TO220 is screwed on to the heat sink and a insulator isolated the TO220 from the heat sink. Am I correct to say then the body of the transistor should be isolated from the chassis and the mica also serve as insulator?

jitter · 31st January 2009, 08:25 AM

Yes, that's correct.

Instead of mica, other isolation pads are also in use. I like silicon best, it doesn't break.

Don't forget the isolation ring in the hole of the TO-220. Also make sure that everything is lined up nicely before fastening the TO-220 to the heatsink. Too often have I seen damaged isolation rings (and thus a short between TO-220 and heatsink) when this was not adhered to.

KCC · 31st January 2009, 02:10 PM

So I should not have read any resistance between the metal flange and the heat sink?

AndrewT · 31st January 2009, 02:43 PM

you should read near infinite resistance from transistor to heatsink.

If you read near zero resistance you have a short.

13th July 2008, 06:46 PM	#1
DreadPirate diyAudio Member Join Date: Jan 2006 Location: Northern Virginia, USA	Heat Sink Compound on a TO220? I'm installing these output transistors to a heat sink. I've cleaned the mica insulators. Should I put HS compound (I'm using the white silicon type from RS) on BOTH sides of the insulator? The reason I'm asking is that there is clearly liberal amount on the original install between the mica and heat sink, but its not apparent that any was installed between the TO220 and the mica. Its a Marantz 2285B.

13th July 2008, 07:26 PM	#2
Conrad Hoffman diyAudio Member Join Date: Mar 2007 Location: Canandaigua, NY USA	Always clean off every bit of old compound. It dries out and won't work well on the second installation. Apply new well mixed compound to both side of the clean mica and install. __________________ I used to be an audiophool like you but then I took an arrow to the knee.

19th July 2008, 12:55 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	the compound is only there to exclude the air that would be trapped in the valleys between the high spots in the mating surfaces. The high spots should be touching each other. If the surface finish is very rough then the quantity of compound between the non-mating surfaces will introduce significant thermal resistance. The better the surface finish and the less compound the lower the thermal resistance of the interface. But, you must strive to exclude ALL the air from the interface. This is far more important than any other consideration. __________________ regards Andrew T.

31st January 2009, 02:43 PM	#8
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	you should read near infinite resistance from transistor to heatsink. If you read near zero resistance you have a short. __________________ regards Andrew T.

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18th July 2008, 01:56 PM	#3
frank1 diyAudio Member Join Date: Apr 2006 Location: Devon UK	.... Also, it should be applied sparingly, not liberally. Frank

31st January 2009, 06:41 AM	#5
KCC diyAudio Member Join Date: Mar 2002 Location: Edmonton	Hi, Ok, since we are in TO220, I like to ask another question with regard to installation of the device. Hopefully someone can give me advise on it. My TO220s are installed on a large heat sink.And the heat sink itself is part of chassis common ground. I see the TO220 is screwed on to the heat sink and a insulator isolated the TO220 from the heat sink. Am I correct to say then the body of the transistor should be isolated from the chassis and the mica also serve as insulator?

31st January 2009, 08:25 AM	#6
jitter diyAudio Member Join Date: Aug 2004 Location: Eindhoven, The Netherlands	Yes, that's correct. Instead of mica, other isolation pads are also in use. I like silicon best, it doesn't break. Don't forget the isolation ring in the hole of the TO-220. Also make sure that everything is lined up nicely before fastening the TO-220 to the heatsink. Too often have I seen damaged isolation rings (and thus a short between TO-220 and heatsink) when this was not adhered to.

31st January 2009, 02:10 PM	#7
KCC diyAudio Member Join Date: Mar 2002 Location: Edmonton	So I should not have read any resistance between the metal flange and the heat sink?

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