Well...
Let's make a list of things that absolutely cannot possibly be used:
Energy Storage, Power Generation
- Battery: All types
(Arguably, a Zamboni pile not only
could last extremely long, but may even be
the only proven multi-century technology! Even so, note the power density is extraordinarily minuscule.)
- Supercapacitors, electrolytics, polymers (all age limited, susceptible to dry-out or moisture ingress).
- Solar cells (PV) -- dubious; good enough for aerospace, but most are known to degrade over time; also, dust accumulation in dry climates.
-
RTGs -- that is, if made with conventional fuels and materials. With suitable choices, they could last very long indeed, however, and the added weight and space of lower density fuel, extra shielding and poor efficiency converters would be tolerable in a terrestrial application (like the millennium clock thing).
Active Components
- Flash memory, EEPROM (and the vast array of programmable devices which integrate these): charge decays over time, and programming causes incremental damage.
- Fine pitch, low voltage ICs. Besides being more sensitive to ESD / overvoltage, they're more sensitive, thermodynamically speaking. (One of the reasons a PC CPU shouldn't go over say 80C, or a GPU over 100C, versus a mil spec transistor over 200C.) I would be fine with, say, CD4000 series logic at 5V (well within ratings), despite the ESD sensitivity. 74/LS logic should be okay too, but will take a whole lot more power. Of course, you can't go wrong with discrete transistors.
- ESD, EMC, life in general: everything must be several times below ratings, all circuits well shielded, filtered and protected, with current limiting (of the "big dumb resistor" kind) and multi-level transient protection everywhere. Most protection devices (MOVs, GDTs, etc.) are wear components, but with current limiting, sufficiently large TVSs should be okay I think?
- What else?
Things that are okay include a surprising variety of mechanical things, but they all have to be magnificently engineered with top-notch materials. Oxidation, moisture, and lubrication (migration / degradation) are the greatest challenges here. Examples of energy storage / power generation might include springs (well below ratings -- nearly infinite fatigue limit), weights (gravity isn't going away any time soon, but mind if it moves), and motors (induction motors are essentially indefinite; permanent magnet motors may be susceptible to demagnetization either over time or due to very strong ambient fields). Electrical energy reserves would be best implemented as massive stacks of film or ceramic capacitors -- very bulky, but won't dry out.
Tim