SUMMARY: AMD’s design specs for heatsinks reveal one incredible challenge for heatsink manufacturers and even more of a challenge to overclockers.
Those of us that have been overclocking Durons and T-Birds who come out of the Intel world are somewhat taken aback at the temperatures we see. Totally spoiled by CPU temps ranging from below ambient to maybe 5C above (water cooling), seeing CPU temps in the high 30s to low 40s is unsettling.
More unsettling to me has been the initially inadequate response by heatsink manufacturers to cooling these hot-blooded beasties. We saw a lot of recycled Intel spec’d heatsinks purporting to be AMD cooling solutions. Initially, it looked like Alphas were the only heatsinks up to the challenge, and that because they have more “overhead” than any other heatsink around.
Andy Lemont is a heatsink designer who really knows his stuff – you’ll hear more about him as time goes on. He sent me an email with the graph shown below:
Now let’s get into it a bit. Heatsinks are measured by C/W; I did an article maybe a hundred years ago on using Intel’s thermal diode temps to measure heatsink efficiency (HERE). Generalizing the formula to determine heatsink C/W yields:
Heat Sink Efficiency (C/W) = (CPU Die Temp – Fan Intake Temp) / CPU watts
It’s really pretty simple – measure CPU temp (in Centigrade), subtract out ambient temp and divide by the CPU’s power output: C/W.
Once you know a heatsink’s C/W, you can then estimate the CPU’s temp rise under various loads; if you know watts and ambient temp (here temp at the fan intake), you plug them into the formula and it tells you how well this particular heatsink should cool the CPU. Simple.
Now if you look at the graph, hypothetically let’s assume the Alpha PAL 6035 is represented by the middle dark blue line. Let’s say you have a Duron 700 at 1.6 volts – about 29 watts. Reading off the graph, if your ambient temp is 25 C, the Alpha should cool the CPU about 35C – 10 C over ambient. Not terrible.
I doubt few of us are content to run the Duron at spec, so lets push in to 1 GHz at 1.85 volts – now it’s putting out about 50 watts. The Alpha now should cool the CPU to about 43C, an 18C increase over ambient. That’s the Alpha story. A less capable heatsink (top line on the graph) will result on about a 28C rise over ambient. How many of you were seeing temps in the mid to high 50Cs?
So DUH! You’re telling me lousy heatsink won’t work well?
Not exactly. What I’m telling you is that if we look at what’s coming from AMD, the cooling challenges we face are considerable. Look again at the graph and read out to 70 watts. An Alpha class heatsink will yield a temp rise over ambient of 25C, putting the CPU into the 50C range.
Now if we look into AMD’s Thermal, Mechanical and Chassis Cooling Design Guide, we see some very interesting numbers:
First off, if you’re wondering how hot is too hot, AMD’s specs state that Maximum Die Temperature for CPUs is 90C/194F under 1100 MHz and 95C/203F over 1100 MHz. In comparison, Intel’s max out at about 65C/149F. Basically you can just about boil water on an AMD CPU.
No way we run at anything close to this. No way you are going to get any significant overclocking at these temps. But now let’s look at what AMD is planning and what they expect for heatsink performance:
Sink-to-ambient thermal resistance:
For 55W processor at 90ºC: 0.44º C/W For under 1100 MHz
For 55W processor at 95ºC: 0.48º C/W For 1100 MHz and over
For 76W processor at 90ºC: 0.32º C/W For under 1100 MHz
For 76W processor at 95ºC: 0.35º C/W For 1100 MHz and over
Cooling an overclocked 76 watt CPU is going to require some heavy duty heatsinks, and I doubt we have seen yet what these are. Take a close look at what AMD’s spec implies:
At a minimum, 76 watt processors REQUIRE something like an Alpha PAL6035 to cool them.
Now think about overclocking a 76 watt CPU: You can easily see that if you push it, you’re looking at cooling something like 125 watts. I don’t know how air-cooling is going to cope with this load. The graph shows straight lines – this is valid up to a point. At some point, heatsinks hit a wall and the lines go ballistic – straight up. Heatsinks are designed over a certain range; once you go outside the range (as we overclockers do), the heatsink will not deliver.
All this suggests that the cooling challenge we face is considerable; Socket A air-cooled heatsinks have to do a lot of cooling in a very small footprint. Overclocking Socket A CPUs places an additional cooling burden that, I think, will require more innovation than brute force cooling solutions like Delta fans.
Water cooling is clearly a better solution but more difficult and expensive to implement. Peltiers are even more difficult to implement, and air-cooling a total burden of something approaching 200 watts is a daunting challenge.
We are either going to be running CPUs a lot hotter than we are used to or will wind up paying a lot more for air cooling solutions. If overclocking a 76 watt CPU is feasible, then expect it’s going to be more expensive than what you’re paying now and water cooling may not look as “extreme” as it is today.
As always, drop me a line with other views – If I’m off base here, tell me.