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Attaching Aluminum bar to heatsink - thermal paste or pads?
Attaching Aluminum bar to heatsink - thermal paste or pads?
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Old 11th November 2014, 11:49 PM   #1
john65b is offline john65b  United States
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Default Attaching Aluminum bar to heatsink - thermal paste or pads?

Hey all, trying to attach an Aluminum Bar (18" long x 3.5" wide x 1/2" thick) to a set of three heatsinks in a Krell KSA-300 Chassis. Surfaces are nice and flat on both the aluminum flat bar and heatsink face. Originally, Krell had some goofy flexible metallic pad or something there, but was removed after I sandblasted the heatsink face. This interface does not need to be electrically isolated, like the connection of output device to Aluminum bar - I plan on using a the Thermon Pad to connect the output devices to the Aluminum bar. Surface of heatsink is 4" x 2.5" wide....

Question is what to use? Thermal paste, Thermon Pad, or nothing?

The thermon pads I have are around 2W/mK conductivity (I think)....I bought a bunch of these yellow pads 2.5" x 2.5" x 1/16" thick....use them where I need the electrical isolation...but don't think I should use it in this particular application...
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Old 12th November 2014, 12:04 AM   #2
CBS240 is offline CBS240  United States
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Just paste it. Works for me and it is cheap, easy. After you secure the screws or whatever fasteners you are using, take it apart to see how flat the surfaces really are. If the paste pools up in spots, you can sand it down so as to make better flat metal contact.
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Old 12th November 2014, 12:23 AM   #3
rayma is offline rayma  United States
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Just paste it. Works for me and it is cheap, easy. After you secure the screws or whatever fasteners you are using, take it apart to see how flat the surfaces really are. If the paste pools up in spots, you can sand it down so as to make better flat metal contact.
If the surfaces aren't machined, you may be better off using the thermal pads with paste.
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Old 12th November 2014, 12:25 AM   #4
john65b is offline john65b  United States
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I believe that the voids filled with thermal grease or pad is a LOT LESS conductive than the metal to metal. If I use paste, I will have entire face area conducting through the paste, limiting heat transfer, while using metal to metal (no thermal grease) will have tiny voids, but a majority of the face is in direct contact....yielding the best heat transfer...

Surfaces appear machined...

Am I wrong here??
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Old 12th November 2014, 12:28 AM   #5
jkuetemann is offline jkuetemann  Canada
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Graphite interface material is similar to a silpad but more efficient for places where insulation isn't needed. Cleaner than paste too.
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Old 12th November 2014, 12:36 AM   #6
rayma is offline rayma  United States
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I believe that the voids filled with thermal grease or pad is a LOT LESS conductive than the metal to metal.
If you think that both mating surfaces are machined and clean, try placing them together and sliding them around.
You'll be able to tell. If you fasten them together bare, use reasonable torque on the fasteners to avoid warping.
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Old 12th November 2014, 12:40 AM   #7
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Does the surface reflect light, like a mirror if looked at down the end? Shows flatness to some degree...

What holds the two pieces together?

I'd put the grease in there, if you squish two pieces together that are flat enough, there will be no grease remaining between the two pieces anyhow.

got some pix?

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Old 12th November 2014, 12:46 AM   #8
john65b is offline john65b  United States
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The heatsink and bar pieces are fastened together by a bolt in middle. I have fastened the pieces tight and tried to slip a piece of paper between - all the way around - nice and tight...Since aluminum is so malleable, I am thinking a single kitchen aluminum foil rectangle sandwiched between the two faces...maybe two layers of foil...
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Old 12th November 2014, 01:03 AM   #9
rayma is offline rayma  United States
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Originally Posted by john65b View Post
The heatsink and bar pieces are fastened together by a bolt in middle.
It would be better to use two or three fasteners on each piece.
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Old 12th November 2014, 02:01 AM   #10
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by john65b View Post
I believe that the voids filled with thermal grease or pad is a LOT LESS conductive than the metal to metal.
So what?
"Voids" by definition means "NOT metal to metal contact".

Quote:
If I use paste, I will have entire face area conducting through the paste, limiting heat transfer,
No, once you apply pressure paste flows away, only stays where there's voids.

Quote:
while using metal to metal (no thermal grease) will have tiny voids, but a majority of the face is in direct contact
Think again.

Only way to approach full contact is to apply progressively finer abrasive paste between both surfaces and move them around, applying pressure, until you grind away differences.

You should use rouge polishing in the last step, and even so .....

Or if you think to "crush your soft aluminum foil" to make it flow and fill the gaps, remember you'll need some 40 Tons per square inch.

Compared to alternatives, thermal grease looks attractive.

I wonder what have they been using for the last 60 years.

Unless you do so, forget it.


....yielding the best heat transfer...

Surfaces appear machined...

Am I wrong here??
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