> 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.

The 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.

The 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.

That 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.

Thermal wear? I thought it was mostly based on write cycles. How can that be 'thermal'?

The write cycles are realised by moving electrons through a material. This creates heat, which leads to gradual degradation of the material.

Does 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?

I 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-bake...

On 1): It's because each cell is essentially a consumable with a limited number of state transfers. It's similar in that way to how a solid state accellerometer still has a moving part inside it that can break or wear out over time.

EDIT: More reasons and methods for failure of "solid state" circuits and systems: https://news.ycombinator.com/item?id=14765868

I understand that there is something that makes the cells stop working over time. blattimwind's reply is the first actual explanation I've ever seen of what that something might be.

On the subject of capacitor death. Is there a robotic device that can take in a circuit board and swap out all the [electrolytic] caps?

i've never seen or heard of an automated board rework system. thinking about the steps and things i've had to do to manually rework boards, your machine would need not only be able to apply force to pull parts off of boards without damaging the board, but also be prepared to restore pads / through holes to usable states after desoldering parts, before new ones could go back in.

there's a reason companies don't repair circuit boards in consumer electronics.

I've done it myself, save restoring pads -- to me it's just the sort of fiddly, finnickety thing that it seems like robots sold be good at. And, they're not going to burn their fingers or run out of hands to hold things!

Robots are good at repetitive tasks and economy of scale. Doing a tricky action for ten thousand boards is something that robots are good at; doing a different tricky action on each of a hundred boards is not.

It doesn't seem plausible to have a business case where you'd get be able to get a large quantity of identical boards (that are otherwise good!), replace caps on them, and be able to sell them for much more than you got them - i.e. that the boards haven't become obsolete in that time. If there's no mass production, there's not much use for automation.

Yes, and it mostly speaks Ukrainian, or Chinese.