Copper/Aluminum heatsink - yay or nay?

[thylantyr]

I can't remember where I seen this concept, was it home computer CPU heatsinks or was it Rockford Fosgate, whatever


Traditional transistor on heatsink

------------------------
aluminum fins
------------------------
aluminum plate 1/4" thick
------------------------
transistor
------------------------


Copper method
------------------------
aluminum fins
------------------------
aluminum plate 1/4" thick
------------------------
copper plate 1/8" thick
------------------------
transistor
------------------------


What is the pro's/con's of using copper sandwich?

[Nisbeth]

I haven´t got much theoretical knowledge to back this up but I´ll give it a shot anyway:

Copper is better at transporting the heat away from the silicon chip, whereas aluminium is better at radiating it to the surrounding air (besides being cheaper). This means that using a copper plate lowers the thermal resistance between the transistor case and the actual heatsink (the fins) and also ensures that the heat sink will be used as effectively as possible because the copper is better at transporting the heat to the "far corners" of the aluminium heatsink.

My 0,02 etc.

/U.

[thylantyr]

does any exotic home amplifier companies use this
method? or is it just extreme overkill - heh

would you do it if money wasn't an issue?

[Lisandro_P]

Ok here's the deal. You have two principal characteristics to look for when you're choosing heatsink material; radiation (given by the colour of the sink's surface) and thermal conductivity (given by the metal).
The thing with cooper is that's an EXCELLENT electrical & heat conductor, which means that heat is quickly removed from the power devices, but it's red/orange which means it radiates poorly. Most sinks are built in Al, which, while it's not such a good heat conductor as Cu (yet still good enough), it's cheaper and can be readly anodized to any colour you like.

Now, i'm building my sinks using Al extrusions for the surface area and 2mm thick Cu sheet to bolt them to the Al, hoping to get the best of both. Hopefully it'll work!

[roddyama]

Lisandro,

Now, i'm building my sinks using Al extrusions for the surface area and 2mm thick Cu sheet to bolt them to the Al, hoping to get the best of both.

That sounds like a great idea. The copper will rapidly pull the heat away from the devices and than transfer it to the aluminium heat sink over a much larger area. This will make the whole heat sink more efficient.
There must be a downside or manufacturer would have used this method to save cost with smaller heat sinks (less weight). In any case it should make for cooler running FET's or BJT's with a given heat sink.

It would be interesting to hear from anyone who tried this before.
Rodd Yamas***a

[tiroth]

Originally posted by Lisandro_P
Ok here's the deal. You have two principal characteristics to look for when you're choosing heatsink material; radiation (given by the colour of the sink's surface) and thermal conductivity (given by the metal).
The thing with cooper is that's an EXCELLENT electrical & heat conductor, which means that heat is quickly removed from the power devices, but it's red/orange which means it radiates poorly. Most sinks are built in Al, which, while it's not such a good heat conductor as Cu (yet still good enough), it's cheaper and can be readly anodized to any colour you like.

Er...kind of. From what I understand, Cu conducts heat better than Al but transmits it less well (air/metal interface transmission). Emissivity (=radiation) also enters into things but I believe this is a small contributer when compared to convection.

When you go to a dual Cu/Al design you have to balance the interface losses. This really is rocket science!

[Lisandro_P]

Originally posted by roddyama
There must be a downside or manufacturer would have used this method to save cost with smaller heat sinks (less weight). In any case it should make for cooler running FET's or BJT's with a given heat sink.

Well, for starters you're introducing an interfase between the Al & Cu, which must make excellent thermal conection; otherwise it's a no-go situation. So everything must be well soldered (could it be done?) or bolted & greased to get it to work. In any case, it wouldn't be cheap anymore... But again, you see the weirdest things, so i dunno... i once ran across *SILVER* heat sinks for PC hardware (video cards). Silver is even better than Cu at conducting heat, but i'm not willing to pay it!
On the other hand, it's no unusal to find large sinks using a Cu sheet to help distribute heat quicker, specially in very big ones.

My goal was to get good heatsinking at a moderate price. I already bought and cutted the Al extrusions, and will get the Cu sheet this week. It's a lot of metalwork (a LOT), but i'm not spending over US$50.

[Lisandro_P]

Originally posted by tiroth
Er...kind of. From what I understand, Cu conducts heat better than Al but transmits it less well (air/metal interface transmission). Emissivity (=radiation) also enters into things but I believe this is a small contributer when compared to convection.
When you go to a dual Cu/Al design you have to balance the interface losses. This really is rocket science!

I was convinced that the heat transmision in contact with air was given solely by surface area and not sink material... or am i mistaken? Radiation transmision can build up to 15-20% of the total i readed once, and it does make a notable difference.

And about the rocket science thing... yeah, you're right I once tried to get into the physics of this thing and ended up settling with ESP's sink calculator. Didn't have the courage

[Circlotron]

I don't think you can extrude copper the way you can aluminium, at least not at reasonable pressures anyway, so aluminium wins on cheapness there. Don't stop at a 2mm sheet of copper as a thermal "spreader", 5mm would be a lot better. For all the devices that have their collectors / drains electrically connected, you could mount them *directly* to the copper and get much improved thermal contact, and put the insulating layer (that now has a much greater cross section and therefore a correspondingly lower thermal resistance) between the copper and the aluminium. This is what I have done with my CDA amplifier, see the pic at thread -> http://diyaudio.com/forums/showthrea...5&pagenumber=2

GP.

[Lisandro_P]

I'm using 2mm because its the only thing avaliable here at a reasonable price. If i could get thicker sheets i'd use them, but even with 2mm it should have enough thermal inertia.

I like the idea of insulating the copper directly a lot, through i have to see if this is feasible with my h.s. arrangement. Where did you get the insulator sheet?