As the rumour of the 5GHz AMD FX part gets traction on the enthusiast forums, well-known AMD power user The Stilt from Finland replied in a very clear fashion on the news. He calls it a poorly constructed hoax. The Stilt, known for his very deep insights in overclocking AMD processors - he developed the TCI K² OC software - stated it's physically not possible for AMD to launch this kind of overclocking chip in retail package.
Issue #1: What cooler to use?
A 4CU/8C OR-C0 operating at 5000MHz would have TDP & ETP of 180 - 200W+. The maximum temperature for a FX (in high frequency & power condition) is around 65 Celsius, higher than that will retard the clockability.
Let say the ambient temperature of the case is at constant 40 Celsius.
- TCPUMax (65) - TAmbient (40) = 25 degrees Celsius.
- TCPUMax-TAmbient-Delta (25) / TDP (180) = 0.1388 C/W RTH (the required thermal resistance of the heatsink).
In case you use a relatively high performance TIM, such as Shin-Etsu X23-7783D (which has thermal conductivity of 6.0 W/m K):
- AM3+ heatspreader area = 0.0016 square meters.
- TIM layer thickness: 0.000025 meters (0.025mm)
0.000025 / (6.0 * 0.0016) = 0.002604166 K/W or C/W (thermal resistance of the TIM layer).
In total you would need a cooler with minimum thermal resistance of 0.136195834 C/W. Any suggestions?
Issue #2: What mainboard VRM to use?
At 5000MHz the required Core VDD would be around 1.475V - 1.500V. At the given TDP & ETP range it would mean 120 - 136A of current draw. The "margin" (IDDSpike) is around 20-25% on AM3+ designs, so the motherboard should be able to deliver 144 - 170A for short periods of time. The power requirements alone leave the selection of motherboards quite narrow.
Since the VRM efficiency on a Crosshair V Formula-Z in these conditions is 77-80% (at most) it means the VRM will dissipate 36 - 46W of heat. That is a massive amount of heat for a relatively small, passively cooled aluminium heatsink to dissipate.
- TVRMMax (135) - TAmbient (40) = 95 Celsius.
- TVRMMax-TAmbient-Delta (95) / VRMETP (36) = 2.63888 C/W (2.06521 C/W at 46W VRM ETP).
Any passive heatsinks you can fit on VRM with such of thermal resistance? No.
Backed up with inside knowledge on the silicon production quality and processes, he is confident that AMD cannot release a part that is guaranteed to run stably at a rated frequency of 5000MHz with conventional cooling methods. Conventional means air or normal water cooling solutions. The required improvement in terms of power consumption of the Vishera die would be so enormous it is very unlikely that AMD could pull it off. After all, if they would have silicon that needs so much less power, it would already be on the shelves.
I have to admit that the news of a 5GHz retail SKU might have made me a bit over-enthusiastic - I am a sucker for high frequencies - but the information as provided by The Stilt seems fairly solid. I do wonder what or who that "good authority" Hexus.net claims to have provided the information really is. Trustworthy? You decide.