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Mica pad query - Page 2

post #16 of 30
3/17/10 at 9:44pm
I use paste, always, and just the stuff Radio Shack used to carry which I think is silicone based.
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post #17 of 30
3/17/10 at 9:45pm
Kevin's referring to the ceramic pads that n_maher said he's used.

Edit: Excellent thanks! I have a crap load of arctic ceramic paste here that I can use =)
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post #18 of 30
3/17/10 at 10:44pm
Thread Starter 
Quote:
Originally Posted by n_maher View Post
I use paste, always, and just the stuff Radio Shack used to carry which I think is silicone based.

Why? If the mica pad performs heat transfer and insulation duties at the same time, I don’t see a reason.

EDIT, sorry crossed wires, n_maher were you referring to ceramics pads only?
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post #19 of 30
3/17/10 at 11:07pm
From memory, mica is a poor heat conductor. It needs the paste.
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post #20 of 30
3/17/10 at 11:58pm
Thread Starter 
Quote:
Originally Posted by Beefy View Post
From memory, mica is a poor heat conductor. It needs the paste.
I will add the paste, a job worth doing is worth doing right!
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post #21 of 30
3/18/10 at 12:03am
The thermal compound is there to fill in all the microscopic air pockets between the heatsink or device and the pad (air is a VERY poor conductor of heat). The silpads are a special phase change substance which becomes more like a liquid when hot to fill in the same gaps, so doesn't require thermal compound.
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post #22 of 30
3/18/10 at 12:11am
Quote:
Originally Posted by srserl View Post
The thermal compound is there to fill in all the microscopic air pockets between the heatsink or device and the pad (air is a VERY poor conductor of heat).
This, and it seems to me to be the better safe than sorry approach and I find it actually helps hold things in place during assembly which is a bonus.
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post #23 of 30
3/18/10 at 1:47am
when using mica sheets, don't forget that you need goo on both sides, like srserl alluded to.
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post #24 of 30
3/18/10 at 4:18am
Thread Starter 
Thanks everyone again for all your help and suggestions, this is really good knowledge, I'm glad I posted the query, my sinking skills will be on overload this coming weekend.
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post #25 of 30
3/18/10 at 10:14am
You could try something from Chomerics.

-> http://www.chomerics.com/products/therm_chotherm.htm
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post #26 of 30
3/24/10 at 8:34am
Thread Starter 

*Update*

Unfortunately I was not able to use the ceramic insulators n_maher recommended, they were a little too thick and would have left the FET legs too short to be solderable to the underside of the boards it’s a very tight fit, but definitely a consideration for my next project. I found these instead:

BERGQUIST|2015-54|THERMAL PAD, TO-220 | Farnell Australia

The material is boron nitride loaded silicon elastomer, and they are at least twice as thick as the standard pads at 0.38mm. I felt a lot more comfortable with these pads, although one did tear quite easily from over tightening, as n_maher stated a snug fit is all that’s needed.

The old FET’s were removed, although I hadn’t blown any, I felt I had traumatized the legs several times and wanted to wipe the slate clean for a totally new installation. I applied a liberal dose of thermal paste to the underside of each FET and pad with a clean makeup brush for an even layer.







The two boards now measure no shorts and power up just fine, thanks again for your help!

EDIT: The underside of the board has a nylon washer and M3 nut followed by a standoff, I’ll use some loctite on the standoff’s to limit any chance of assembly becoming loose.
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post #27 of 30
3/24/10 at 9:57am


Is it me or does it look like you are not getting good contact between the FET and the angle?
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post #28 of 30
3/24/10 at 12:15pm
He is using some rubber type pads.
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post #29 of 30
3/24/10 at 12:18pm
Quote:
Originally Posted by nikongod View Post
He is using some rubber type pads.
Right, it just seems like an awfully short distance for the edge to be flaring up that much.
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post #30 of 30
3/24/10 at 3:56pm
Thread Starter 
It’s a combination of the pad bending and distorting from tightening as they are quite squishy, and the angle of that particular photo, the FETs are in good contact with the sink. BTW they were all tightened first, before soldering
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