Heatsink mounting woes

[Aeternam]

I have a pretty big heatsink that I ripped out of a PC which I'd like to use it to cool a MOSFET.

Now as you can see the transistor doesn't fit out of the box, I'll need to fixate it somehow on the body of the sink. But I don't know how. Drilling is not an option since the thermal transfer pipes are going right through the main block (or are they?). The wife offered her glue gun, but I'm afraid that that'll get pretty messy once this thing starts to heat up. Smelly, too

I was thinking a metal plate, cut to size, to squeeze the transistor in place?

What do you guys think? Any ideas?

[rrinker]

 Someone has to ask - why does that little mosfet need such a giant cooling device?  Seems a proper type for the package would do a better job with less bulk. I suppose you could always bock up the mosfet and then screw down that PC cooler and use the springs to maintain tension between the contact surface of the cooler and the mosfet, like it does with the actual CPU, but I don't think the efficiency would be there since you'd be going to the body of the mosfet and not a heat spreader like sits on top of a CPU die.

[Aeternam]

Someone has to ask - why does that little mosfet need such a giant cooling device?

Because it looks bad ass !!

I suppose you could always bock up the mosfet and then screw down that PC cooler and use the springs to maintain tension between the contact surface of the cooler and the mosfet, like it does with the actual CPU, but I don't think the efficiency would be there since you'd be going to the body of the mosfet and not a heat spreader like sits on top of a CPU die.

Hm yes that's what I was thinking. Hadn't thought about the efficiency though but I guess you're making sense...

[David Hess]

I suppose you could always bock up the mosfet and then screw down that PC cooler and use the springs to maintain tension between the contact surface of the cooler and the mosfet, like it does with the actual CPU, but I don't think the efficiency would be there since you'd be going to the body of the mosfet and not a heat spreader like sits on top of a CPU die.

Hm yes that's what I was thinking. Hadn't thought about the efficiency though but I guess you're making sense...

The top of the CPU package operates as a heat spreader but you will only need to include that if the power is really high.  Heat pipes have a maximum power density before film boiling prevents proper operation.

Overmold packages are designed to also be "clipped" into place so use a block on the top of the MOSFET to stiffly push it against the bottom of the heat pipe essentially making a giant clip that the existing heat pipe mounting screws can use.  That is pretty much how the CPU cooler is designed to work anyway.

I have occasionally seen this kind of thing done with a cutout inside a printed circuit board allowing the transistor to contact a massive heat sink or heat pipe *under* the printed circuit board.

[Mark]

Drill + tap a hole in the metal X shaped base and screw in a short bolt to act as a peg to hold the mosfet in place. 
Apply a small amount of good quality heatsink paste to the FET. 
Remove (or shorten) the standoffs so that the FET can be held between the heatsink and the base. 

 

[Whales]

Wow you have a lot of heatpipes in a small area.  Yes they generally extend all the way through the heat block you attach the hot thing to.  Some CPU coolers machine this back and partially into the heatpipes, so that you mount the heatpipes directly to your CPU.

If you can't find another solution: consider drilling it out.  If you aim well then you'll only kill one of the heat pipes 

[bktemp]

Using the existing mounting hole isn't the best way of mounting power transistors. For best heat transfer, apply the pressure directly at the area where the die is mounted to get the lowest thermal resistance at that area.
Using the existing pressure plate is probably the easiest solution, because it requires only to shorten the standoffs.

[Jeroen3]

You get the PCB shape of the socket. And get an aluminum oxide isolator, a thick one, and you can press-fit the transistor against the heatspreader using PCB material. I've done this for a socket 775 server cooler. Solid mount, pcb is only very slightly curved.

The other method involves milling copper slab, which is expensive.

[Ampera]

If you don't care about getting the MOSFET apart from the heatsink, use a thermal glue. It works like regular glue, but it's also a thermal paste.

You can also do something fancy, but that's what I would do personally.

I would also never do something this crazy, but I'm still young and my brain hasn't rotted enough.


Getting there...... 

[edpalmer42]

How much power will the Mosfet be dissipating?

With this type of heatsink, the only heat transfer is through the heatpipes.  This requires a certain amount of heat to vaporize the liquid inside the tubes.  If you don't have enough heat, the liquid doesn't vaporize and almost no heat will be transferred.  You could have the very odd situation of an overheated transistor with a huge heatsink!

Ed

[Nozzer]

I've never seen such a strange (and beautiful looking) heatsink. It reminds me of a set in The Alien. I don't know how big it is, but that would make a potentially attractive stand, with a few modifications, for a high end, hifi top loading CD player!

[rrinker]

 Quite common actually, for those that want something better than the supplied stock cooler on their CPU but don;t need the ultimate in watercooling. I have a simialr but not as tall one in my one machine. I don't overclock at all any more so it was mostly because it has a larger but slower turning fan than the factory cooler and thus is quieter. That it's a few degrees cooler really doesn't matter to me. My newest machine just used the factory cooler, a monster like that won't fit in the mini-ITX case I built it in.

[rdl]

I found a heat sink similar to that sitting next to the dumpster a couple of years ago. It looked never used, was still in the box and had all the mounting parts and stuff. I grabbed it and sold it on ebay a few weeks later for $20 profit. CoolerMaster brand I think, been awhile so not sure.

[Whales]

How much power will the Mosfet be dissipating?

With this type of heatsink, the only heat transfer is through the heatpipes.  This requires a certain amount of heat to vaporize the liquid inside the tubes.  If you don't have enough heat, the liquid doesn't vaporize and almost no heat will be transferred.  You could have the very odd situation of an overheated transistor with a huge heatsink!

Ed

I'm not sure if this is how things work.

Regardless of the amount of heat being generated, won't the liquid will start to absorb energy and change phase at about the same temperature threshold?

[David Hess]

The practical lower limit for a water heat pipe is about 20C so it will work fine.  Power devices commonly operate with a case temperature of about 75C and processors with a case temperature below 50C.

[grifftech]

shove it between 2 fins