Salvaging Defective DRAM

An anonymous reader writes "Ever wonder what happens to DRAM that fails quality assurance testing during manufacturing? Turns out a lot of it ends up as 'downgrade' memory and ends up in OEM memory modules. Last resort: use it in an answering machine, where the sampled audio can be very tolerant of bit errors."

Comments

*Long String of Curses*

[SatanicPuppy]

You just explained a lot about my fricking answering machine! I thought that no one ever called! And now I find out it is low grade ram? My god! I may really HAVE a social life!

Did I ever wonder?

[Anonymous Coward]

Ever wonder what happens to DRAM that fails quality assurance testing during manufacturing?

No. I figured they forgot about it.

Alternatively, you could use the...

[Ari Rahikkala]

BadRAM patch.

Buying ram on the internet.....

[Ogrez]

This is the prime example of why I tell people I know not to buy ram off of the internet unless its from a major company that has good support. To many people buy 15-90 day warranty ram because its cheap, and when it fails they are upset that they have to replace it. If you pay a bit more money you get lifetime warranty ram... and why do you think they are willing to warranty it that long, because they know it works. people dont understand the testing process and think they are getting the same product buying cheap ram, as opposed to inexpensive ram...

Use memtest86

[Black Parrot]

...and read its documentation to find out how to make Linux skip any defects it finds.

recycling the chips

[v1]

I recall seeing an article awhile ago where companies were buying defective memory, and running them in these external testing units,which would identify which chip(s) on the stick were bad. I'm assuming they'd then unsolder the bad chip and recover one from another module. At that time some of those sticks had 8 chips on each side, so you could recover 15 good sticks from 16 bad ones. Considering the price of memory a few yrs ago, it was probably a worthwhile venture. Nowadays though, it's probably not worth anyone's time.

Some updates to the article...

[IvyMike]

There are some things in the article that are pretty out of date:

To reduce the test time, parallel chip testing usually is accomplished with eight to 16 chips in a row.

That's pretty low parallelism; there are memory testers out there that test over 200 devices at a time right now. And even the older, more common systems are probably testing 64 in parallel.

A special ink jet color marks the good dies.

This hasn't been true for years. Each device's pass/fail status is stored in a database, along with all other test results, and the whole process is automated enough that good die are binned out automatically. No need to physically mark the chip.

Due to the imperfection of the process, a percentage of the DRAM die contains some faulty cells.

That percentage is 100%. At modern memory sizes, you never get a perfect device without going through repair.

I have it

[acidrain69]

I've been waiting for the computer graveyard market to ramp up. Where does the rest of defective computer systems go?

It's in my closet. All of it. The whole market. I'm waiting for the entire tech market to crash, so I can flood the market.

I am seeing a lot of this

[acidrain69]

There are a lot of peeps complaining about substandard ram. If you had RTFA, you'd realize that the downgrade ram is reconfigured to skip the bad parts in the chips, so that it comes out as a normal module. Just because there is a faulty bit or 10 in a modules, doesn't mean the reast of that module is bound to fail. It could just have been an imperfection in the silicon or the circuit process.

The downgrade ram has to pass further tests to insure the detours around the bad parts worked.

Granted, I probably wouldn't use this stuff in a mission critical server, but if you are buying for a mission critical server, you should be getting ECC registered with lifetime warranties anyway. Now for a small web or file server, or even a desktop, I'd use this.

Other people have mentioned memtest86. This program is your friend. Don't even bother with BIOS POST tests of RAM, just use this every once in a while if you REALLY want to find the problems. Too bad it won't run on my alpha server :(

Re:I am seeing a lot of this

[chrysrobyn]

There are a lot of peeps complaining about substandard ram. If you had RTFA, you'd realize that the downgrade ram is reconfigured to skip the bad parts in the chips, so that it comes out as a normal module. Just because there is a faulty bit or 10 in a modules, doesn't mean the reast of that module is bound to fail. It could just have been an imperfection in the silicon or the circuit process.

You have made a statement that makes it very clear you are a very educated layman, not someone in the field. What you've said is true to the first order, but not inherantly true.

Wafers have what can be measured as "defect density", and observe a phenomena called "defect clustering". Defects are not always hit or miss, open or short, some of them are latent or resistive. As the part ages (diffuses), electromigrates or observes hot electron effects, all parts will decrease in quality. Downgrade RAM, so to speak, would be most likely to have additional cells fail due to the above effects -- because it had failures that made it marginal in the first place. Testing methodologies at higher quality manufacturers build in guardbands to make sure that nobody ever experiences the defects when used in-spec. (This is why many overclockers lose their chips after only a year or two, they cause latent defects to surface and suddenly the chip won't even operate at nominal frequencies; the guardband effect also explains to a great degree why many chips can be overclocked in the first place.)

I'm not dis'n you, just trying to fill in a few more holes.

How to identify DIMMs using bad RAMs

[udif]

It's quite simple. Really.

DRAM chips are usually have either 4, 8 or 16 bits per word. In order to construct a DIMM, 64 bits are needed. This means that with 4 bit DRAMs, you need 16 chips, with 8 bit DRAMs you need 8 vhips, and with 16 bit DRAMs you need 4 chips. usually you will see only the 4 or 8 bit DRAMs, because these occupy less board area for the same capacity. 16 bit DRAMs are only used for low capacity DIMMs.

When your DIMM supports ECC, it's 72 bits wide, which makes it more complicated. Usually its made of 18, 4-bit chips, or 9 8-bit chips.

(back in the 30 and 72 pin SIMM days, when memories were 8 or 32 bit wide, you could see ECC SIMMs that use 3 chip for 2x4+1=9 bits, or 2x16+4=36 bits).

If you see DIMMs with 12 chips, This is usually a cheap OEM SIMM using partially good DRAMs.

The Best way to identify such a DIMM, is to write down the marking on ALL the chips on it, and look them up in the internet. You then sum up all the DRAM bit widths, and see what you come up with:

If its 64 bits, its a normal DRAM.

If its 72 bits, its probably an ECC DIMM.

If its more, it's probably a DRAM using partially good DRAMs.

I just figured it was at Fry's

[Artifex]

Seriously, I've had some of their OEM memory as part of a package deal, and it was very nasty stuff.

What's worse, before they would take it back, they wanted to "test" it, testing being limited to a couple runs of PC-Doctor, which is totally lightweight.

To make a long story short, they refused to take it back the first time, later it blew up my motherboard. They replaced the motherboard (it was part of the package) and sent me home, where I discovered my Athlon XP was also damaged. I took it up there, and they wanted to run PC-Doctor on it, but the "technician" (hah!) cracked the CPU while putting it in a "test board," so "oops, I guess we're replacing that."

P.S. One of the guys at the return desk who I got to know quite well told me, when I asked him why the "test boards" they were using always changed, that he thought they were boards that belonged to customers. Whether that meant boards in for repairs, or returned boards, I don't know or care - either is bad news.

P.P.S. This was at the Fry's in Wilsonville, Oregon. There is also an idiotic troll in the service department there who, after ignoring me waiting at an empty counter for 10 minutes while he chatted on the phone, wanted to charge me for a "missing" monitor stand on a monitor I was returning, refusing for 15 minutes to look in the bottom of the box under the styrofoam because monitor stands always come attached to the monitors, didn't you know? He finally looked when I demanded to talk to the manager, and of course it was there. I had a long discussion with the manager anyway over his, and their, incompetence (I reminded him of the memory fiasco) but the troll was still lurking there the last time I dropped by for consumables, which is all I will ever buy from Fry's, now. You can't miss him - he looks like he'd feel more at home in a raincoat, instead of his cheesy lab coat, roaming a playground on a sunny day.

Re:well

[wik]

Not to mention, give you hell at the airport. The security guys in Pittsburgh told me to put my keys in the little bucket, then when they looked closer, told me to put them through the X-ray machine.

They were looking at the old 256k SIMM PCB (all chips removed) and asking "is that a computer chip"? Funny how they pointed at that and missed my Intel keyring fob with a real processor die on it.