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Heatsink - Plain old copper VS Silver plated copper.

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dmg

Thu Sept 16 2010, 03:30AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

I have a few slabs of copper laying around, which where originally meant to be used for some high powered motor drives, but I think of using some of them as heatsinks,

After searching around on the net, I couldnt find much results that give me an idea how much dissapation these plates have, which brings me to the next question,

Which will have more heat dissapation, the exposed copper, or the oxygen free silver plated copper?
Has anyone tried this before? or does anyone know of a source or literture or charts on how these two can be compared to eachother?
or is this just a pipe dream, and it really wont affect dissapation of heat to a great extent?

Thank you

Dave Marshall

Thu Sept 16 2010, 03:50AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

First off, 'Oxygen Free' anything is pretty much just BS marketing. If there were appreciable amounts of oxygen in the material, it would be copper oxide, or silver oxide. I've heard claims that the manufacturing process is different, whatever. For the purposes of the vast majority of users, there is no discernible difference. If you start building your own semiconductors or satellites, then maybe it would be something you'd investigate.

All that being said, silver has greater thermal conductivity than copper. The silver plate will conduct the heat more efficiently. Estimating dissipation for a specific slab of material might be fairly straight forward if there are no fins or the like. I imagine it gets pretty convoluted pretty fast when you start increasing surface area beyond a simple cube. If some careful googling doesn't turn up an equation, I suspect one of these guys in the Power Electronics industry might be able to shed a little light on that.

-Dave

dmg

Thu Sept 16 2010, 03:56AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Okay, thank you for clearing that up a bit dave,

fin wise, I wanted to mount these plates on bigger, finned AL heatsinks, these are 1/4" thick plates, so I had hopes that they will aid in cooling power devices. (I planned on just clamping them tightly with bolts, and using thermal compound in between the CU and AL)
as youve said, the oxygen free part may be questionable, I didnt know. (got these plates for free)

so far my largest plate of silver plated copper is 1/4" thick, messuring 5" X 7.5"
Ill try to ask a few companies and people if they have any information on formulas.
Do you personally think its even worth the effort then?

...

Thu Sept 16 2010, 04:17AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Adding a copper heat spreader on an aluminum heatsink is a bad idea from a corrosion/thermal expansion standpoint, but it does greatly improve the thermal properties when you are using physically small (ie, to-246, isotop) devices that dissipate a lot of power. Aluminum really isn't a great conductor, you can have tens of degrees in temperature drop across a 1/4" thick piece of aluminum sheet.

The layer of silver won't appreciably change the thermal properties of the heat sink, I am curious as to why they would have sliver plated it in the first place--tin or gold is much more popular due to the better corrosion resistance. Maybe for a high frequency application of some sort where a thin layer of silver would appreciable change the resistance due to the skin effect?

There are papers out there on heat sink design, aavid has some good resources, this paper goes into the details if you aren't afraid of math

dmg

Thu Sept 16 2010, 04:26AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Why yes, as a grade 11 student, I am terrified of math!

But Ill venture into this territory anyway...

Thank you "..." this arcticle should be an interesting read.

klugesmith

Thu Sept 16 2010, 04:31AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Aluminum really isn't a great conductor, you can have tens of degrees in temperature drop across a 1/4" thick piece of aluminum sheet.

Aluminum conducts 5/8 as much heat as copper of the same dimensions,
and almost twice as much as copper of the same weight and length.

Bjørn

Thu Sept 16 2010, 04:34AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

A thin layer of anything will not improve your heatsink when it comes to conducting heat through the heatsink. If it made a big difference you could just as well use the thin layer alone and save some money. The contact between the heatsink and the device is critical, concentrate on that.

A heatsink with as shiny surface will be worse than a black one. I have a table somewhere that shows up to 30% higher efficiency on the black ones. Just as a mirror will not absorb radiation a mirror finish heatsink will tend not to radiate it.

2Spoons

Fri Sept 17 2010, 02:21AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Bare copper will be superior to silver plated, due to the vastly different emissivities. Convection heat transfer will be the same, but the radiated heat will be lower for silver (e of 0.02) vs copper (0.05) or better still oxidised copper (e of 0.7)

Adam Munich

Fri Sept 17 2010, 02:29AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
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What you want is anodized aluminum. Ever wonder why high quality Al heat sinks are anodized black? The radiant capacity is much higher than an unanodized sink. Anodization is pretty easy to pull off at home too.

Here's a paper on the subject. Anodization makes a whole world of difference.

IntraWinding

Fri Sept 17 2010, 02:44AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

2Spoons wrote ...

Bare copper will be superior to silver plated, due to the vastly different emissivities. Convection heat transfer will be the same, but the radiated heat will be lower for silver (e of 0.02) vs copper (0.05) or better still oxidised copper (e of 0.7)

I agree. And in fact I think the silver plating will actually act as a slightly thermally insulating layer over the heatsink surface since it isn't a perfect heat conductor.

There's also a similar problem with coating a heatsink in an emissive (black) material. You need to be sure the improvement in radiation outweighs the insulating effect of the added layer. On that basis I figure black paint would be bad, but a very thin chemical blackening of the metal would be good.

In practice the only black heatsinks I've seen are either painted or black anodized. I suspect the effect of the finish is rarely tested and more an aesthetic choice.

Also bear in mind that the surface needs to be emissive at the dominant wavelengths for black body radiation at temperatures only a little over ambient, so well into the infrared. Something that looks black to the eye may not be 'black' where it counts. I don't suppose the dyes used in anodizing are optimised for anything other than appearance.

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