The official GT300/Fermi Thread

[saaya]

Source?

Unless it's based on the fact that when the chip size grows, less chips fit on the wafer, AND when the chip size grows, there is higher chance to have a defect on the chip.

So actually that makes some sense. Defect rate grows linearly, while chips per wafer decrease linearly. As these two add up, yields decrease expotentially. No?

afaik the amount of "holes" in the wafer is almost constant between a wafer with many small and few big chips... if a small chip that gets you 400 chips per wafer has a yield of 90% that means probably around 40+ defects per wafer (even if a chip gets struck it may still work with some redundant logic disabled or as a cut down version etc)

if you go for a bigger chip that only fits around 100 times on the wafer like gt300, having 40 defects per wafer means you will have a yield of 60%+ plus because the bigger the chip the higher the chance two defects happen in the same chip.

rv870 is 333mm^2 and rumored to have started with yields of ~60% it seems.
im just guessing here, but 333mm^2 should mean they can get around 175 chips per wafer, so that means 105 functional chips which means 70+ defects. gt300 should be around 550mm^2 which means around 100 chips per wafer max, and with 70+ defects, 30+ fully functional chips.

another factor is the bigger your chip, the more wafer space is wasted on the edges, but thats not a huge diference...

wafer costs are around 3000-5000 us$, so 30 chips per wafer = 100-166$ per gpu, pure die costs

for rv870 it should be around 100 chips per wafer so 30-50$ per chip cost...

these numbers are just examples, they arent accurate...
but you can see, for a rough 50% transistor increase of gt300 over rv870, the costs more or less tripple...