Everything stated above makes sense to me, but there are also additional factors like solder, oxidation etc.
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Naturally an assembly of components, like pcb, cannot have longer lifespan or shelf life than the individual components used on it; however it would seem like the assembly should have shorter lifespan than shortest lifespan of individual components, since assembly is an item of higher complexity and may be more prone to faults or additive behavior of faults.
That's why i was curious if anyone did any research into lifespan of assemblies.
Your concern seems to be that the ingredients used to
assemble a circuit board (PCB substrate, traces, solder) are shorter-lived than the actual components? I don't think that is an issue in practice, and certainly not if the complete device is stored under conditions which are favorable for precision opto-mechanical assemblies: Moderate temperatures, limited humidity, no permanent vibrations.
A lot of the feedback you received above is based on actual experience -- people do use and repair old computers, test and measurement equipment, audio electronics etc. Some components are more prone to failure than others, as discussed above; but I don't recall ever seeing, or reading about, PCBs disintegrating or traces just corroding away on their own. (Unless a battery or electrolytic capacitor has spilled their corrosive guts...)
Solder joints may deteriorate into "cold joints" or develop cracks if parts run very hot, expand and shrink a lot under temperature cycles, or are subjected to mechanical vibrations without being properly supported. Again, none of these should occur in a precision (and low-power) device like a photography camera.
Cameras for very low-light use, e.g. in astronomy or high-end microscopy, use actively deep-cooled image sensors, and are somewhat likely to suffer from problems in that area sooner or later. The high temperature differences between operation and storage will cause mechanical stress on solder joints, and also on the seal of the (often evacuated) chamber which encloses the sensor.
Edit: Oh, and contact surfaces which are exposed to the environment can be a weak spot of course: IC sockets, connectors to internal or external wiring, contacts inside switches. That's where oxidation
can bite... .