Electronics > Projects, Designs, and Technical Stuff

DC load using a CPU cooler

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metacollin:
I came to this thread kind of late, but I've been reusing CPU heatpipe/sink assemblies with and without fans for years with FETs and transistors with all sorts of packages.  Doesn't matter if they're surface mount or have a screw hole, I use both.  Use whatever package has the lowest junction to case thermal resistance. 

This is what I've learned:

1. Do not use thermal interface compound (TEC).  It's terrible and using it often ends in ritual suicide. TECs do little beyond fill in imperfections between two flat surfaces with something that isn't air.  But otherwise, it's basically about as bad at conducting heat as a solid, non-air filled material/over priced goop can be. Toothpaste performs nearly as well as thermal grease while smelling significantly better (spearmint is my favorite).  There is less than a 10% performance difference between quality thermal paste and shitty toothpaste.  And TEC dries out or otherwise deteriorates, though much more slowly than toothpaste (years vs. hours).

2. Do not drill holes in the heat spreader, it's a huge dickchafe and you'll probably breach a heat pipe and effectively ruin the entire assembly's ability to shed those phat watts. 

3. Brackets? Clamps? Maybe even bears, oh my? Don't bother, and it will be a huge pain in the dick for inferior thermal performance. 

4. Hot air heats heatsinks as effectively as cool air cools them.  Heatguns aren't just for heatshrink and bacon.     

5. The important bit - Just solder the damn FETs directly to the copper heat spreader.  They're *made* to be used that way.  No holes, no brackets, no bullshit, just bitchin' awesome thermal performance and a bond as strong as the heatspreader is bonded to the rest of the assembly and heatpipes.  And you won't have to commit Seppuku to restore honor to your family. (The last one was a joke, of course.  Probably.)

Use low temperature Bismuth-Tin solder.  Sometimes you can find it in paste form in small quantities, but its quicker and easier to just make your own.  Find that old bag of bismuth ingots and that bar of grade A tin you have stashed somewhere in your garage, melt them in an aluminum container on the stove, mixed 42% Tin, 58% Bismuth, Sn42/Bi58.  This is the same thing bonding the heatpipes to the cooling fins and heatspreader.  It melts around 140°C, give or take.  You can tweak the temperature by moving the ratio tinward. Remember, heatpipes work in reverse, just blow a heatgun through the part a fan would blow air through and the heat pipes will eagerly shit all of those watts into the copper heat spreader where you want to solder your FET, and you'll have things up to temp and wetting nicely in tens of seconds.  Just, uh, make sure the whole thing doesn't fall apart due to gravity or something.  Also don't attempt this with normal solder, you'll vent one of the heatpipes and it WILL shoot a jet of methanol directly at your genitals.

Those wattage and SOA curves in the datasheets are all done with those mofos soldered DIRECTLY to some big ass hunk of copper.  If you solder a FET to copper, it adds negligible thermal resistance and, depending on the total thermal resistance of the junction to heatsink/heatpipe assembly to ambient, you will actually achieve the specs in the datasheet.  Now, often the wattage is assuming you cast level 99 Heatsink of the Infinite and have a heatsink with 0 thermal resistance cooled by unicorns breath, but if you run the numbers for your non-magic non-unicorn breath cooled heatsink, you'll get exactly what all the thermals predict.  The heatsink is the limiting factor if its for a CPU, it probably won't handle much more than 100W shed to ambient with a fan, but a TO-247 will dump 100W into those sweet sweet heat pipes without even breaking a sweat, no problem.  Single package.  With a bigger heat pipe assembly, 200W out of a single package is totally doable, I've done 240W out of a TO-264 soldered to a big ass aluminum heatsink (solder wets aluminum wonderfully, just sand it in an aquarium or similar and fill it with CO2 using dry ice or whatever is handy, this will remove the surface layer of aluminum oxide and prevent a new one from forming instantly.  The aluminum oxide surface layer is what makes aluminum seemingly unsolderable, but actual unoxidized aluminum wets nicely).  Not even copper or heatpipes, just shitty old aluminum. 

Some quick examples attached.  In order: 1. A big ass fet soldered directly to a big ass cooler assembly salvaged from an MSI graphics card. 2. One of those shitty chinese 100W LEDs, soldered directly to the spreader.  3.  For a modest reduction in wattage, I quite like laptop coolers.  This one cost me $2.50 USD on ebay.  4.  My ghetto home made Solder Slab™ of low temp Sn42Bi58 solder, with little nibbles from my iron evident at the edge. 5. Finally, a CPU heatsink actually being used as such, AMD processor soldered directly to it.  Yes, it still works.  It's running FreeBSD as I type this.

Spikee:
I tried the soldering method on my arctic freezer 7 pro rev 2 but it was a no go.
I have a top of the line soldering iron with around 240W of capacity (JBC) but the heatpipes of the cpu cooler are to good. After a fuckload of time the temperature only achieves around 60-80 deg C.

I have tested the Arctic silver thermal adhesive up to 600W dummy load using 4 mosfets and it works fine. The only hassle scraping it off when you don't need it anymore. Just normal thermal compound and a press fit heatsink is the best solution.

Jeroen3:

--- Quote from: Spikee on September 25, 2014, 10:36:53 am ---I have a top of the line soldering iron with around 240W of capacity (JBC) but the heatpipes of the cpu cooler are to good. After a fuckload of time the temperature only achieves around 60-80 deg C.

--- End quote ---
As described above, you must use the heatpipes in reverse by heating up the fins using hot air. All energy will channel and concentrate back up to the heatspreader plate. Making it incredibly hot very fast.

It's quite interesting to test if soldering is a lot better. Intel recently changed from solder to glue between the heatspreader and the chip die. This seems to have decreased the performance.

microbug:
@Jeroen3 Yes, I heard the same story about solder being better.

I can see that solder would be better once it's there. How would the thermal connection between the cpu cooler and the heat spreader be when soldered together? Also, would this be doable with the Atten 858D hot air gun and Hakko FX-888D soldering station (my soldering equipment)?

@Joenuh, multiple gains on the op-amp is one way of doing it. It would make sense as you are unlikely to need high resolution at high power. The reason, however, that I am not using 1 ohm shunts - apart from the heat, which has to go somewhere - is that in constant power mode with a current source of 20A (the max current) connected to the input, they would limit the minimum power to 200W (I2R = 400*0.5 = 200). On a 300W load, that's almost useless.

EDIT: No reply yet from IXYS...

metacollin:
Thermal paste is often still adequate, but will up your over all thermal resistance and you'll have less headroom.  It just seems like once you've gone through all the trouble of mounting something with sufficient force, drilling a hole, cleaning the surfaces, blah blah, you could have had a much easier time just using the soldering method.  I am really not sure why its not used more (in prototyping/personal project contexts, I am clueless about how well it would scale to some sort of volume production setting, probably not well though).  I started doing it simply because it was *much* easier and quicker. 

Yes, it's totally impossible to get it to temp using an iron.  I use a $20 1500W heatgun from amazon.  Hot Air is terrific for heating any kind of surface-area heavy thing like a heat sink.  The radiator fins suck up the heat if you blow hot air between them like crazy.  All I'm saying is anyone interested should give it a try, and especially if you get ahold of or make some low temp solder, and you'll never go back.  I used to drill and tap stuff and mount it the classic way, but haven't done that in ages, simply because I'm, ahem, a convenience enthusiast and the soldering method got the most done for the least amount of effort. 

But seriously, I've yet to encounter a heat sink large enough that a cheapie heat gun couldn't bring up to temp with relative ease.  Also, it's a lot easier to hit 140° than 180° or higher for lead-free SAC alloys. 

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