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Mica pad query

post #1 of 30
3/17/10 at 5:25am
Thread Starter 
Quick question, is it ok to use two mica pads for extra insulation insurance? The reason I ask is because I'm still getting shorts for my intended application, probably due to a little over tightening. So would heat transfer be an issue if 2 pads were used?
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post #2 of 30
3/17/10 at 6:44am
Personally, I'd replace the damaged pad if I was sure that was why I was getting a short. I can't see any benefit to doubling the pad, so I wouldn't consider doing it.

However, if you wanted to double the pad and used a little heat transfer compound then in theory it would only have a limited effect on heat transfer, depending on how well you can get the two pads to join.

From what I know, if the join between the two pads was ideal, then they are one pad that's thicker but has the same thermal properties. Thicker material doesn't alter the fundamental thermal properties of the material - it just gives it a greater heat capacity - which just means it takes longer/more energy to heat up.

However, an ideal join is not possible so there would be some minor loss of heat conduction, depending on the efficiency of the junction. So the pad closest to the chip would be ever-so-slightly hotter than the one on the heatsink. I doubt very much if this would make a large difference to the operating temperatures because you are mating two (all-but-)identical materials with a compound designed to give maximum surface/molecular contact.

So I'd add something like 0.5 degrees per watt to be safe. Then again, I'd just replace the pad if it's the problem.

The best way to know is to try it and measure the temperature ! If you do, please let me know what the result is !
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post #3 of 30
3/17/10 at 10:25am
Check the heatsink for burrs on the holes. A slight countersink with a larger drill would do.
Also the diameter of the hole may need to be slightly larger to give more gap between the bolt/pins and the heatsink.
Also, check that when the device is screwed down, it doesn't contact both the device's metal part and the heatsink.
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post #4 of 30
3/17/10 at 12:18pm
Quote:
Originally Posted by johnwmclean View Post
Quick question, is it ok to use two mica pads for extra insulation insurance? The reason I ask is because I'm still getting shorts for my intended application, probably due to a little over tightening. So would heat transfer be an issue if 2 pads were used?
Is the problem the pad itself, or the screw you are using to secure it?

I assume you are using isolating washers..... so perhaps also consider nylon screws.
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post #5 of 30
3/17/10 at 12:31pm
I've all but stopped using the thermasil/film/mica type pads for applications that aren't on professionally machined surfaces. I've had more than one develop tiny tears due to surface imperfections and it's just not worth the risk. I use much thicker, solid isolation pads now. Three of them can be seen below.

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post #6 of 30
3/17/10 at 12:34pm
Heat transfer is still good?
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post #7 of 30
3/17/10 at 12:45pm
Quote:
Originally Posted by Beefy View Post
Heat transfer is still good?
They're good enough for Justin to use in the BHSE which seems good enough.
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post #8 of 30
3/17/10 at 12:53pm
Quote:
Originally Posted by n_maher View Post
They're good enough for Justin to use in the BHSE which seems good enough.
Yeah, I trust Justin's judgment way more than yours

But seriously, got a product link? I would be very keen to see prices. The only issue I can see is for on-board heatsink applications. The transistor legs would have to be bent rather carefully such that the legs can fit easily through the holes on the board, and the transistor can sit flush against the heatsink...... certainly doable though.
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post #9 of 30
3/17/10 at 2:48pm
Quote:
Originally Posted by Beefy View Post
But seriously, got a product link? I would be very keen to see prices. The only issue I can see is for on-board heatsink applications. The transistor legs would have to be bent rather carefully such that the legs can fit easily through the holes on the board, and the transistor can sit flush against the heatsink...... certainly doable though.
I wouldn't bother for on board sinks, those are generally machined pieces and should have a very smooth surface. I've never had an issue with pads tearing on on-board sinks like those used on the M³ and beta22. It's only when using the pads on parts that I've made (panels like the one pictured above) that I've had hole edges, even after some smoothing, have a tendency to tear the pads.

As for the actual product, here's the full range.
Standard Products - Insulators
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post #10 of 30
3/17/10 at 3:18pm
Good point. Thanks!
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post #11 of 30
3/17/10 at 5:30pm
Thread Starter 
Thanks guys, this info is really helpful. Beefy, I am using the typical recommended nylon shoulder washers with a stainless steel m3 screw, the short was from FET (several) to sink.
n_maher you're pretty much on the money, the surface is machined but not a professional grade finish, after drilling the holes I did smooth back the surface for any imperfections though, the holes are also big enough so as the mica pad did cover the complete rim with no metal viewable.
The problem came about after I re-installed new wanky hex screws, the thin mica pads cannot take the abuse of been tightened twice, I would say one installation is all they're fit for, after that if you're going to unscrew the FET replace with a new pad.
The thicker pads are a great recommendation, it's good to know there's some flexibility with different thicknesses.
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post #12 of 30
3/17/10 at 6:57pm
FWIW when I install any pad mounted device now I use loctite and only tighten things snug tight. I've monitored a couple installs like that over a few months and they stayed tight and were transferring heat just fine too.
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post #13 of 30
3/17/10 at 7:49pm
When using the ceramics do you spread a thin layer of thermal paste on each side?
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post #14 of 30
3/17/10 at 8:26pm
And if you use thermal paste, do you use
the silicone version, the non-silicone version,
the various silver,blue cpu versions?
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post #15 of 30
3/17/10 at 8:39pm
Thread Starter 
No thermal paste was used, would this help insulation as it’s non conductive? Doesn’t the pad itself deal with thermal transfer.
I’ve used paste when the pad is not needed...
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