Cell Processor Yields
The Inquirer referred to an interview with Tom Reeves (VP of semiconductor services and technology at IBM) over at Electronic News, and it's an interesting read, particuarly in reference to the PS3. Here's an excerpt:Electronic News: Let’s look at design for manufacturability from a different standpoint. IBM has said it needs seven of the eight cores on the Cell processor to work for Sony’s Playstation. Will there be an aftermarket for chips with fewer operational cores?
Reeves: There are a lot of chips with six cores operational, and we’ve been thinking about whether we should really throw all of those away. We also have a separate part number for chips with all eight cores good. The stuff that’s going to be for medical imaging, aerospace and defense and data uses eight cores.
Electronic News: But might it be the less-expensive version of Playstation 3?
Reeves: It could, but I don’t think Sony has thought about offering that. That doesn’t mean there aren’t good uses for a chip with four SPEs [synergistic processing elements].
Electronic News: What’s the defining factor that makes some chips better than others?
Reeves: Defects. It becomes a bigger problem the bigger the chip is. With chips that are one-by-one and silicon germanium, we can get yields of 95 percent. With a chip like the Cell processor, you’re lucky to get 10 or 20 percent. If you put logic redundancy on it, you can double that.
Okay, maybe I'm misunderstanding something here, but isn't a yield of 10-20% somewhat worse than nightmarish? They're using logic redundancy, so take that up to 20-40%, but isn't that still absolutely horrible?
Plus what about this: There are a lot of chips with six cores operational, and we’ve been thinking about whether we should really throw all of those away. I think you can infer some interesting possibilities from that comment. If the yields really are that horrible, could Sony announce that the PS3 is only going to use six cores instead of seven? They've already reduced the number once, from eight to seven.
That may not be possible, and I know nothing about how the Cell is programmed, so it's possible that reducing the available cores might cause massive reworking of games already in development. I don't know. But I don't see people lining up to use the Cell in other applications, and that's a huge loss to take if 60-80% of your manufacturing output can't be used in your primary product.
The full interview is here.
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