3 ms·
This is true, re. manufacturing defects. I used to work in a DRAM fab, too, and observed similar patterns. Re. cosmic rays, it's surprising, but I think they
by josaka 11y ago
This is true, re. manufacturing defects. I used to work in a DRAM fab, too, and observed similar patterns.
Re. cosmic rays, it's surprising, but I think they can cause errors: http://stackoverflow.com/questions/2580933/cosmic-rays-what-is-the-probability-they-will-affect-a-program http://stackoverflow.com/questions/2580933/cosmic-rays-what-...
- mud_dauber 11y agoThe risk is significant: JEDEC has a standard testing methodology for them: http://www.jedec.org/sites/default/files/docs/jesd89a.pdf http://www.jedec.org/sites/default/files/docs/jesd89a.pdf
- sevensor 11y agoInteresting. Shows how my point of view colors the facts -- I never once heard one of our test engineers complain about cosmic rays, but then it was their job to blame process when defects came up. We used to try to blame dirty test heads, but that's another story. I'm sure in the real world cosmic rays actually do flip bits sometimes, but I'm equally sure that a significant proportion of the DRAM in production has latent defects. Given our observed defect rates and the limits of test coverage, it's unavoidable.
- mikeash 11y agoFor manufacturing defects, there are no doubt processes you can do to make them manifest more frequently so you can detect them. I imagine a lot of defects are temperature-dependent, so you're probably testing hot chips to force that. Errors while reading and writing can be made to happen more often by repeatedly reading and writing in a tight cycle. Errors from radio interference could be tested with artificial radio noise. The idea, I imagine, is to try to test the equivalent of X years of real-world use in only Y days of lab time. But cosmic rays are just a constant background thing. If an errant cosmic ray flips a bit, that doesn't happen any more often if you hammer on the chips or heat them up or expose them to radio waves or whatever. The only way to simulate X years of real-world use is to use them for X years, short of setting up a particle accelerator or something. (Technically, you could simulate X years of real-world cosmic ray exposure in X/N years by testing N sets of RAM in parallel, but then that requires you to have more ram under test than you give to your customers!) All the same, I'm sure more mundane explanations are behind the vast majority of errors, as you say.
- sevensor 11y agoYeah, we did hot tests as well as cold. The problem isn't whether you could detect defects given all the time and resources in the world, but whether you can maximize the probability of detection given finite resources. If you're in test, you've got to deal with an avalanche of material coming out of the fab, as well as constant pressure to get it out of the door and off the books. Upper management doesn't see you as adding value, but they'll come down on you like a ton of bricks if there's an incident and you don't catch it. It's a tough job, and I'm glad it wasn't mine.
- mud_dauber 11y agoApparently it's a sufficiently common occurence that JEDEC has a standard testing methodology for just these types of events: http://www.jedec.org/sites/default/files/docs/jesd89a.pdf http://www.jedec.org/sites/default/files/docs/jesd89a.pdf