AMD HD6990 Review
Under the Skin
Naturally when designing such a behemoth a lot of consideration has to be made as to the best way to ensure you can supply two GPUs with the power that they need, whilst still keeping the temperatures low enough to not make the sides of you case start liquefying.
Whilst you could have 20 GPUs on a board if yo had the time and inclination, keeping them powered would be something else altogether. As extreme an example that might be, it is an identical problem with two. If a single GPU requires 6+8 or even 8+8 PCIe power connectors, then how do you keep two rocking? With immense voltage regulators of course. Centrally mounted to ensure the smallest path possible so that neither GPU lags behind the other.
If the GTX480 taught us anything it's that huge power draw and performance is pointless without sufficient cooling.
To that end AMD have supplied the HD6990 with a centrally mounted fan blowing air across two vapour chambers, one for each GPU. The most intriguing aspect is the use of a phase change thermal compound. That sounds great and very technical, but water boiling into steam, or ice melting, are both phase changes. It doesn't mean they are desirable. Equally if all that happens when the TIM gets hot is that it drips out the card, it wont be very helpful.
Finally the nice surprise is that there are two BIOS on board the HD6990. Flicking between the two is like taking a very angry bear, then poking it with a stick. Should be a blast, and at worst allows the user a level of control over the power-draw of the HD6990.