|PowerColor AX6990 4GBD5-M4D Video Card|
|Reviews - Featured Reviews: Video Cards|
|Written by Bruce Normann|
|Sunday, 20 March 2011|
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Closer Look: PowerColor Radeon HD 6990
The PowerColor AX6950 4GBD5-M4D is based 100% on the AMD reference card, which is a completely new design. As of the moment there are no AIB partners that are game enough to design and produce their own version of this card. At best, I expect one or two may offer a different cooling solution, but the board design is probably going to remain the same for the foreseeable future. Let's start at the top.
The first thing you notice when you pick up this video card is that it's very long, and relatively thin, or at least not thicker than any other reference card. The second thing you notice is the weight and the fact that's it's both better balanced and distributed along its length. None of this should be a surprise if you think about it for a second, but no one stops to think when they're unboxing the most powerful graphics card in the world. Oops, did I let the cat out of the bag? Too late, I think AMD got there first.
The fan shroud is a completely sealed-off design; there are no extra relief vents anywhere to be seen. Both ends are relatively open and the airflow path is as uncluttered as they could make it. The straight through design of the fin assemblies and the lack of restrictions at both ends help out on the noise performance as well. NVIDIA did the same thing with their GTX 580, with the same general effect.
The single 80mm radial fan is placed right at the center of the card and pushes air out in two directions, along the length of the shroud. The two GPUs are spaced out a fair distance from the center and each has its own dedicated fin assembly sitting atop it. The component level on the PC board is cooled by full size cast aluminum heat spreaders on both the front of the card and the back. The one on the front catches the heat from all the Voltage Regulator Modules, as well as half the installed GDDR5 DRAM. The other half of the 4GB total is mounted on the back surface of the board and they are pretty much the only major heat producers located there.
The cooling fin assemblies do away with heatpipes completely, and use full-sized vapor chambers instead. They operate on the same principle as heat pipes: using specially selected refrigerant fluids that change phase at the operating temperature involved, and naturally occurring convective currents. As the gas rises, it is cooled by the aluminum fins and returns to its liquid state. This type of evaporative cooling is one of the most efficient heat transfer methods and is used in lots of high temperature applications such as nuclear power plants and inside the exhaust valves of high performance combustion engines. In that case, sodium is used as a very high temperature coolant inside the valve stem, transferring heat away from the valve face which is getting pummeled by a couple thousand explosions every minute the engine is running at normal highway speeds. The liquid inside these vapor chambers is more like the FreonTM that's inside your typical refrigerator or air conditioning system.
The back of the card is almost completely covered with a black anodized heat spreader, with cutouts for each GPU, and the tensioned skeleton backplate that holds the mounting screws for the heatsink/fin assembly. It's too bad no one has figured out a good way of removing heat from the back side of the board, right underneath the GPU. This section gets plenty hot, but there are over a hundred small capacitors and resistors in this area that you don't want to short out. Thermal tape to the rescue?
The thermal interface material (TIM) used by AMD on this card is a new "Phase Change" type of material that few people have had hands-on experience with. AMD warned all the media contacts that the performance of the card would be permanently degraded if it was disassembled, due to the unique properties of this TIM. Many thanks to W1zzard over at techPowerUp, who showed no fear and disassembled his card to report on this mystery TIM. Info from the US Patent Office (#6,620,515) calls this class of materials: "A phase change composition comprises: a matrix comprising an organofunctional silicone wax, and a thermally conductive filler. The composition can be used as a thermal interface material in electronic devices. The composition is formulated to have any desired phase change temperature." That's a pretty broad description, but the bottom line is that the usual grease-based carrier material has been replaced with a wax-like substance that thins out better at typical operating temperatures. This minimizes the impact of the carrier material and allows the filler material, which is many times better at conducting heat, to make better contact with the surfaces of both the heat source and sink.
I have never had the thermal performance of a video card degrade after I've taken it apart and reassembled it with a smaller amount of high quality TIM paste, but this time I think I've met my match. The thermal design of this card is so close to the edge of meltdown that I don't want to mess with it. Show me a full coverage waterblock that I can bolt on, and then I'll take it down to its bare essentials. For those braver than I, who end up succumbing to their curiosity and DIY spirit, please read the thorough discussion of best practices for applying TIM available here.
The layout on the front and back of the printed circuit board is very unusual, just as you would suspect, given the design brief. The current paths for power are made as short as possible by grouping the main voltage regulator module sections between the two GPUs, and by locating the highest current ones feeding the two GPUs along the top edge of the board, where they are closer to the power input connectors. As the highest of high-end graphics cards, the PowerColor Radeon HD 6990 is a necessarily complex and tightly constrained product. The designers at AMD had very little design flexibility when faced with the overwhelming amounts of power and heat that are involved here.
One final thing we need to look at is the installation of this oversized card in a modern gaming case. Owners of the previous model HD 5970 are well aware that the length of the card could be a problem when choosing a PC chassis. I measured 305mm from front to rear on my sample. Thankfully, this number doesn't need to be fudged with some imprecise allowance for PCI-E power connectors, as the two 8-pin connectors are located along the top edge. In my test bed PC case, which is a CM Storm Sniper, I had 35mm of extra room left over between the far end of the card and the drive cage framework. I had no problems getting the card in or out with this much clearance, especially since I didn't have to worry about accessing any power connectors on the end of the card. And yes, I know my cable management is a mess, and yes that is painter's tape on my CPU cooler. You wouldn't believe the amount of air that comes barging out from the gap between the fan housing and the fin assembly without it.
In the next section, let's take a more detailed look at some of the new components on this non-reference board. I didn't do a full tear-down, but AMD graciously provided some detailed images that will allow us to see everything of importance.