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Registered Member #1526
Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216
See, that was my initial reaction too, it`s just that when I re-focussed I considered again what we know: that a heatpipe of size definitely has much higher thermal conductivity than a piece of coppper of size, despite double the number of material interfaces => there is room to manoeuvre.
From looking again at the XtremeSystems thread I see that someone mentioned `vapour chamber` blocks on some GPUs. I think now that this is what I`m arriving at maybe...?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
That's true, but the point is that a long, thin copper rod has pretty lousy thermal conductivity, so it's not hard to beat.
A heatpipe works by vapour phase cooling: the alcohol boils at the hot end and condenses at the cold end, where a wick carries it back to the hot end.
Because the heat is carried by moving alcohol vapour, the delta-T and hence the effective thermal resistance hardly depends on the length and shape of the pipe. The heat just has to get from the pipe wall into the liquid at one end, and then from the vapour back into the pipe wall at the other end. In between, there is essentially no temperature difference except what's required to drive convection.
To sum up: The "Heatpipe beats solid copper" assertion really only holds for long, thin, heatpipe-shaped things that wouldn't be any good if they were made of solid copper. For a large enough cross-sectional area and short enough distance of heat travel, a solid copper slab will beat a hollow copper box full of alcohol vapour, and a slab with fins will beat a separate heatsink connected by a heatpipe. The main use of heatpipes is to cater for odd-shaped scenarios like inside laptops, where the heatsink can't be bolted right to the top of the CPU.
Registered Member #1526
Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216
I think that`s where my thoughts are - in that nether region where we define what `heatpipe-shaped` means. I guess the proper empirical research in this area has been done already though. I`m happy right now, no more niggling :o)
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Water directly on the CPU is probably not the best idea, the waterproofing of the die interconnects is never 100% so if you are using say one of the newer hybrid chips with bare die it could have reliability problems long term.
I had this happen once, had a dead laptop which did the 3BOD ie CPU/RAM fail. As no CPU was available I oven cooked the CPU for an hour at 125 Celsius as this procedure is recommended for SMDs prior to rework. I'll be damned, it worked!
Had previously reseated CPU, RAM and even did a low temp reflow of the GPU, no workie.
EDIT: Another approach might be to vacuum evaporate copper directly onto the core as serpentine patterns.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Threadsurrection!
wrote ... but what I`m thinking is that it`s the interface to the water that`s the problem, that`s why the direct to die doesn`t work because below a certain limit of area, the water just won`t take up the heat fast enough. So then you increase the area as discussed until you reach the sweet spot where the water can pick up the energy fast enough, in return for moving the water further from the die. Once the energy is in the water it`s problem solved.
It's not just an interface problem, it's also a flow-rate problem. The temperature rise in the water is the heat power / (volumetric specific heat * flow rate). If you bring the water closer to the die you're pumping it through a smaller cross-section. To keep the same temp in the water you need the same flow rate which requires higher pressure due to the smaller pipe area. Bringing the water closer to the die means you can reduce the flow rate a bit since you don't have the deltaT over the heat spreader, but I'm guessing the require pressure will still be very high.
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CPU waterblock idea...
For a large enough cross-sectional area and short enough distance of heat travel, a solid copper slab will beat a hollow copper box full of alcohol vapour, and a slab with fins will beat a separate heatsink connected by a heatpipe. The main use of heatpipes is to cater for odd-shaped scenarios like inside laptops, where the heatsink can't be bolted right to the top of the CPU.
