Mica pad query
Mar 18, 2010 at 12:45 AM Post #17 of 31

nattonrice

500+ Head-Fier
Joined
Apr 10, 2008
Posts
979
Likes
30
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 =)
 
Mar 18, 2010 at 1:44 AM Post #18 of 31

johnwmclean

Aka: capone, bignurse.
Joined
Apr 28, 2008
Posts
2,909
Likes
52
Quote:

Originally Posted by n_maher /img/forum/go_quote.gif
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?
 
Mar 18, 2010 at 2:58 AM Post #20 of 31

johnwmclean

Aka: capone, bignurse.
Joined
Apr 28, 2008
Posts
2,909
Likes
52
Quote:

Originally Posted by Beefy /img/forum/go_quote.gif
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!
 
Mar 18, 2010 at 3:03 AM Post #21 of 31

srserl

100+ Head-Fier
Joined
Mar 5, 2008
Posts
248
Likes
105
Location
NorCal
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.
 
Mar 18, 2010 at 3:11 AM Post #22 of 31

n_maher

Resistorous Conflagorous
Joined
Nov 20, 2004
Posts
8,405
Likes
31
Quote:

Originally Posted by srserl /img/forum/go_quote.gif
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.
 
Mar 18, 2010 at 7:18 AM Post #24 of 31

johnwmclean

Aka: capone, bignurse.
Joined
Apr 28, 2008
Posts
2,909
Likes
52
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.
 
Mar 24, 2010 at 11:34 AM Post #26 of 31

johnwmclean

Aka: capone, bignurse.
Joined
Apr 28, 2008
Posts
2,909
Likes
52
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.
 
Mar 24, 2010 at 12:57 PM Post #27 of 31

n_maher

Resistorous Conflagorous
Joined
Nov 20, 2004
Posts
8,405
Likes
31
dbc433e0d3.jpg


Is it me or does it look like you are not getting good contact between the FET and the angle?
 
Mar 24, 2010 at 3:18 PM Post #29 of 31

n_maher

Resistorous Conflagorous
Joined
Nov 20, 2004
Posts
8,405
Likes
31
Quote:

Originally Posted by nikongod /img/forum/go_quote.gif
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.
 
Mar 24, 2010 at 6:56 PM Post #30 of 31

johnwmclean

Aka: capone, bignurse.
Joined
Apr 28, 2008
Posts
2,909
Likes
52
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
smile_phones.gif
 

Users who are viewing this thread

Top