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Old 6th July 2004, 02:58 AM   #11
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I can absolutely assure you that sandblasting a surface would increase its thermal resistance as I've also done this before and had to go back and finish machine on a mill that blasted(pun intended!) surface. What some may think by sandblasting the heatsinks mounting surface would be that its flatter and smoother... but not true! Blasting alumnium creates a pebbled/ textured finish that can be most uneven in itself since most blasting is done by hand, not cnc. It will definately prevent good contact between two parts because of all the tiny raised almost microscopic dings in the metal. Its great for decorative purposes though and the resulting matte finish is very nice looking after anodizing although its a champ at holding dust and lint and a pain to clean easily.

To lower the thermal resistance in the surface of an alumnium sink one should finish grind that alumnium surface on a surface grinder to a very good, almost mirror like finish. If you are the patient type that has lots of time to spare this can also be done by crefully drawfiling with a flat file across the surface. In doing this you would lower the thermal resistance quite a bit and the sil pads would be even more effevtive.

Mark
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Old 6th July 2004, 04:02 AM   #12
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I like to think of bead blasting as creating a fractal surface
which in fact has a larger surface area than described by
the square of it's boundary dimensions. Kind of like the coast
of England or the fabric of space at the sub-atomic scale.

And then I like to smear goop on it.
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Old 6th July 2004, 04:49 AM   #13
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Quote:
Is GOOP = silicon paste, white like your toothpaste? Anyone knows what's its chemical name?
Yes; Nelson was making funny by calling it "goop." It's probably usually called "heat sink compound" or "thermal grease"

Quote:
Where to buy this GOOP and sheet insulators (the one that is like paper, not the stiff mica)
Here's one page from the Digi-Key catalog:
http://dkc3.digikey.com/PDF/T042/0564.pdf
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Old 6th July 2004, 06:51 AM   #14
ARAD is offline ARAD  Germany
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In addition to a flat cooling surface, it helps to 'clean' the mounting surface of the transistors as well. Very often, the plastic around the edges is just a tiny bit higher then the metal surface. I use 600 or 800 sandpaper (wet) and polish the entire mounting surface of the transistor to the point that the copper underneath the plating shines just before mounting the devices.

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Andreas
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Old 6th July 2004, 01:08 PM   #15
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Hi !
Why nobody consider utilizing pads made of alumnium oxide ?
They are the best. Much better than mentioned one mica and silikon. I utilize them
without problems with transistors when one looses over 40W.

Regards
Jacek
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Old 6th July 2004, 01:35 PM   #16
joensd is offline joensd  Germany
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Quote:
the grease sticking out of the transistor surface
If you have the possibility after mounting the transistor to apply so much pressure that the grease thatīs too much will be squeezed away, thatīs OK.
Usually you shouldnīt use more than you need.
It obviously is only effective filling the mountain-like surface.
A whole layer of it wonīt do any good.

When I repair TVīs for example there are only cheap clamps that hold the transistor. Applying too much of that goopy stuff isnīt a good thing and I can assure you that I did it wrong in the beginning.

Jens - used to overgoop transistors and send them to semiconductor-heaven
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Old 6th July 2004, 02:11 PM   #17
Netlist is offline Netlist  Belgium
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Quote:
Originally posted by joensd

When I repair TVīs for example there are only cheap clamps that hold the transistor. Applying too much of that goopy stuff isnīt a good thing and I can assure you that I did it wrong in the beginning.
Couldn't agree more...it's artwork.

/Hugo
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Old 6th July 2004, 02:25 PM   #18
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Quote:
Originally posted by JacekPlacek
Hi !
Why nobody consider utilizing pads made of alumnium oxide ?
They are the best. Much better than mentioned one mica and silikon. I utilize them
without problems with transistors when one looses over 40W.

Regards
Jacek
The problem is their availability (and probably cost).
Do you have a reasonably-priced source?

Cheers

Andrea
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Old 6th July 2004, 08:01 PM   #19
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In addition to surface finish, the holes for TO-3 devices may
have to have the edges beveled on the mounting side to
prevent shearing of the silicon insulators. Because of that,
I've usually used mica or thin plastic insulators which have
a better shear strength. Berquist comes to mind for
insulators. I'm not fond of the mess that thermal compounds
make, so I'd like to avoid their use too.

Beryllium oxide insulators used to be popular, but there
may be restrictions on their use now; aluminum oxide is
nearly as good. You want radical? How about diamond/
silicon carbide composites? Gotta find the link for this,
but it's a real product!

Popular among CPU overclockers, Artic Silver, a non-conducting silver paste might also be helpful. Instructions caution that
it will increase capacitive coupling, if that's an issue; they
also have non-silver products and other enhancements in
the form of variable-size particles that pack more tightly to
increase thermal conductivity.

Another possibility is simply increasing the thickness of the
heatsinking anodizing and mounting the device directly, using
only conductive compound without an insulator. This is
risky but probably gives the best thermal results short of
direct metal-to-metal contact and an electrically isolated
heatsink. You'll need to consult your heatsink supplier to
see what special needs they can accomodate; it may be
easier than you think.

And, what everybody else said, too.
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Old 6th July 2004, 11:27 PM   #20
anatech is offline anatech  Canada
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Default What's cookin?

I can't help but think what happens in real life. Generally, assemblers just want to stick the transistor on the heatsink quickly. That's where sil pads come in. Grease and mica take too long, but are great in service (where many techs overtighten & cut through sil pads). Using metal oxide as your insulator turns the unit into "factory service only". Or let the destruction begin!
On a DIY level you can get esoteric if you want. In manufacturing I'd suggest keeping device dissipation within reasonable limits by speading the power around more outputs.
-Chris
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