4 ms·
> I've seen lots of writeups of how wear-leveling works, etc., but never a good physical explanation of what is actually wearing out over time. SSDs are flash
by blattimwind 8y ago
> I've seen lots of writeups of how wear-leveling works, etc., but never a good physical explanation of what is actually wearing out over time.
SSDs are flash are basically EEPROMs. In an EEPROM one bit is stored in a dual-gate MOSFET. One Gate is a normal gate, the other is floating, i.e. it just is a small conductive island. The information is stored by (quite literally) shooting electrons through the insulation of the floating gate into it. They're then trapped on the gate; if you turn the second gate on, the transistor conducts iff the floating gate is also turned on. This shooting action happens to damage the insulation, which at some point is degraded enough that it can't keep the electrons trapped on the floating gate. Hence the electrons leave, together with your information.
> For devices manufactured after about 2000 there's tin whiskers,[3] which began to be a problem because of RoHS requirements.
The solder joints themselves usually don't form whiskers, which mostly grow on pure tin-plated surfaces, e.g. the pins of components (which previously used lead). Using lead-free solder is mostly co-incident with whisker risk, not the cause of the majority of problems.
- 0db532a0 8y agoThe electrons aren't shot through the insulation. They end up in the floating gate by quantum tunneling. The insulation by its name is non-conductive, and must be so that the electrons stay put. The degradation happens through general thermal wear.
- userbinator 8y agoThe exact wear mechanism doesn't really matter (you must be referring to "hot carrier injection") --- the point is that to record data, electrons are being forced through a material which gradually wears it out.
- 0db532a0 8y agoThat was also my point. I was just pointing out that the idea that electrons shoot through the insulation to end up in the floating gate was not true.
- Dylan16807 8y agoThermal wear? I thought it was mostly based on write cycles. How can that be 'thermal'?
- 0db532a0 8y agoThe write cycles are realised by moving electrons through a material. This creates heat, which leads to gradual degradation of the material.
- Dylan16807 8y agoDoes the individual gate get that much hotter than the rest of the device? If I keep my SSD 40 degrees cooler with a freezer, will it last an indefinite number of write cycles?
- 0db532a0 8y agoI just looked this up, and it turns out I was completely wrong about how the wear is caused. Apparently it’s due to a build-up of electrons remaining in the floating gate over time, and not general thermal wear. It turns out that heating can even reverse this wear. Explained in this link: https://arstechnica.com/science/2012/11/nand-flash-gets-baked-lives-longer/ https://arstechnica.com/science/2012/11/nand-flash-gets-bake...