Is there any evidence that chips degrade in such a way? I've got servers that are 8 years old that ran full bore and they still work fine?
As far as I can see, the answer is yes[1]:
The relationship between integrated circuit failure rates and time and temperature is a well established fact. The occurrence of these failures is a function which can be represented by the Arrhenius Model. Well validated and predominantly used for accelerated life testing of integrated circuits, the Arrhenius Model assumes the degradation of a performance parameter is linear with time and that MTBF is a function of temperature stress.
However, the dramatic acceleration effect of junction temperature (chip temperature) on failure rate is illustrated in a plot of the above equation for three different activation energies in Figure 2. This graph clearly demonstrates the importance of the relationship of junction temperature to device failure rate. For example, using the 0.99 ev line, a 30° rise in junction temperature, say from 130°C to 160°C, results in a 10 to 1 increase in failure rate.
Wikipedia has an overview of the the Arrhenius equation[2].
Now, as you probably know most complex chips like GPUs and CPUs have built-in thermal management which prevents the junction temperature to rise above some limit, which one would assume is set at a point which reasonably guarantees a decent lifetime.
However, according to this, chips that have experienced less heat should, on average, live longer.
* Dried up electrolytic capacitors
* Bad solder joints exacerbated by lead free solder.
* Failed high current transistors (typically regulation mosfets or triacs in appliances)
some gpus are rather hard to get a replacement fan for because it's not like they use an off the shelf modular 60 or 80mm 12vdc fan.
The higher end card companies in some cases will just ship you a replacement fan if your email to them is pleasant. Or sell it to you for a modest price.
I just looked and I can get replacement fans for my specific GPU for less than $20 including shipping.
The bigger problem is VRM wear, you never know when it will fail. Said TIM drying out affects the VRM components the most. One failed capacitor or resistor and the whole GPU can get fried (seen it happen many times).
Is there a good way to spot this sort of thing from a distance when evaluating stuff off eg eBay?
SMD elements can fail either open or closed/shorted, the latter is more likely to cause a worse failure but it's not exclusive. Due to protection circuits elsewhere they can fail without visible marks so it's hard to diagnose.
Thermal pads dry out because of heat first, their quality second, with load coming in last imo. So the fact that it's been used for mining alone doesn't mean much.
The conditions under which a card was used and the time matter more - if only sellers told you the average temperature, case type, ambient temperatures, whether the cards were undervolted/overclocked and how long they actually ran...