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Old 25th August 2010, 04:05 AM   #1
GregH2 is offline GregH2  Australia
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Default Quick heatsink question

Hi All,

I have some large heatsinks from modushop for my aleph project however they are anodised on all surfaces and have no machined surface for mounting of components. Is it ok to mount the fets to an anodised surface or do I need to sand a bare patch?


Your advice is appreciated!
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Old 25th August 2010, 06:40 AM   #2
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you need to use paste or thermal pads either way. Sanding the anodization flat would get you maybe 10% more heat throughput but it's not the highest thermal resistance in the circuit, if I remember rightly. The die to package resistance is the limit in a well designed heatsink system.
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Old 26th August 2010, 12:54 AM   #3
Russ H is offline Russ H  United Kingdom
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Hi Swordfishy,

Do not worry about the anodising, it will have negligable effect on thermal performance (infact, depending on the colour and size it can offer an improvement.....but again, its very small)

DO NOT start to sand the anodising away, you will introduce a rougher surface finish, requiring a thicker thermal pad or more thermal paste that before......and thermal pads / pastes are a thermal resistance to the link between case and sink. You will probably find the heatsink (im assuming an extrusion here) will have a surface finish in the region of RA 0.3 - 0.8 if its from a good company, such as Bluecore Heatsinks
This will allow you to use as thin a thermal pad as possible (or less grease)
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Old 26th August 2010, 03:02 AM   #4
GregH2 is offline GregH2  Australia
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Great, thanks guys.

Always like to hear that I don't need to do more work
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Old 26th August 2010, 03:16 AM   #5
Eric is offline Eric  United States
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Quote:
Originally Posted by Russ H View Post
Hi Swordfishy,

DO NOT start to sand the anodising away, you will introduce a rougher surface finish, requiring a thicker thermal pad or more thermal paste that before......and thermal pads / pastes are a thermal resistance to the link between case and sink. You will probably find the heatsink (im assuming an extrusion here) will have a surface finish in the region of RA 0.3 - 0.8 if its from a good company, such as Bluecore Heatsinks
This will allow you to use as thin a thermal pad as possible (or less grease)
Hmmmm... I don't think this advice can be universally followed. On the heat sinks I purchased, the anodize left rather noticeable ridges on the otherwise flat surface of the heat sink. Sanding a smooth spot with 400 grit paper and some WD-40 worked very well. I then used a sil-pad between the transistor and the sanded spot on the back of the sink.

The difference between sanding and not sanding was very clear by scratching the surface with your finger nail.

Eric
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Old 26th August 2010, 03:38 AM   #6
GregH2 is offline GregH2  Australia
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Quote:
Originally Posted by Eric View Post
....left rather noticeable ridges on the otherwise flat surface of the heat sink.

Eric
Yeah, this is what I am dealing with. However the ridges seem to be the result of a poor extrusion not the anodising which is a very thin (almost transparent) layer.
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Old 26th August 2010, 08:41 AM   #7
Russ H is offline Russ H  United Kingdom
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Hi Guys,

The anodising will be microns in thickness, so I very much doubt it is leaving ridges.
The ridges are more than likely coming from very cheap extrusion meathods. I am the thermal engineer at a heatsink company, and standard extrusion practises should achive RA of around 0.3 - 0.8 for a high quality extrusion, just over 1 for a standard extrusion. Both will carry a max flatness tolerance of 0.4. Our standards are used acoss mass consumer electorincs.

I have attached a image of one of our heatsinks with black adnodise for your reference

Click the image to open in full size.


Click the image to open in full size.
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Old 26th August 2010, 10:29 AM   #8
Russ H is offline Russ H  United Kingdom
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Obviously if your heatsink is badly extruded and you have excessive ridging then Erics recommendation of sanding with WD40 would help a great deal, but a heatsink should not be supplier with sure poor surface finish, especially on the thermal coupling areas.
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Old 26th August 2010, 02:51 PM   #9
Eric is offline Eric  United States
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Perhaps I mis-spoke. I agree that the ridges are most likely a result of the extrusion process, not the anodize process. Another set of higher quality sinks that I have are very, very smooth. Either way, the back of my sinks have ridges that are larger than they should be for good thermal contact. Sanding for just a few minutes works wonders in this situation.
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Old 26th August 2010, 06:25 PM   #10
Itsmee is offline Itsmee  England
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You could use a thick alloy plate (heat spreader) to mount the output devices on, then bolt the plate (with a little thermal paste) to the heatsink, alternatively, have the heatsink skimmed at a machine shop
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