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Provided the component has been stored and used according to the manufacturers recommendations, they have an almost unlimited lifetime. The only exception is wet electrolytic capacitors which degrade slowly as their electrolyte dries out. Two years isn't a long time though, I have capacitors here which are several decades old and still in value tolerance and have low ESR.
the two years rule comes from the manufacturer who do not what to have distributor stock return with older than 2 years date code. As already said the life time is not a problem if the integrated circuit have been properly packed, stored and handled(ESD is often a problem).
Epoxy seals can be hydroscopic or absorb molecules of H2O
Each Mfg and part has a Class rating for these,
In the old days plastic parts were never rated below 0'C , hence consumer spec is to prevent ice cracking wirebonds. This problem was cured by Sumitomo, I believe.
LEDs use a different "water-clear" Epoxy which is more hydroscopic and lower glass transition temp.
The other issue is humidity absorption to prevent pop-corn effect with solder profile temps. which can shear the extremely thin gold-wire bonds, ( not explode the part) Nothing worse than a part fail for no connection in the field, en mass. A qualified process/quality engineer ought to be able to judge the corrective action.with traceability on part info.
The routine task for unsealed parts exceeding mfg shelf life for moisture is to bake the parts to remove moisture prior to soldering as per IPC std.
meaning there is no effect on the functionality of the parts even if it is stored more than two years.. what if it is not stored properly? what are the possible problem I may encounter? and how should I deal with it?
The effect on functionality depends on the kind of component we are looking at. For integrated circuits it could be bad solderability(due to oxidation of the leads), higher leakage current(due to ESD damage) or out of specification(due to threshold drifts caused by moisture). If you do not have a professional tester and test program the options to check functionality are limited. I would do first a visual inspection and then a simple supply current or leakage test(the datasheet give the test circuit and values). If the IC looks wet or rusty, or has s suspect current consumption - I would trough it away and buy new components.
visual checking with IC is enough to know that it cant be used? how about for resistor, capacitors and semiconductor? does these also apply with them? the effect in functionality? and the visual inspection?
Visual checking is the first step and then I would do measurements depend on what equipment is available. For resistors I would check the value and compare it with the code. On capacitors I would check the capacitance and apply the maximum voltage specified and measure the leakage current.
I think the bigger problem with IC's is they may look OK and even test OK in a socket but moisture tends to travel up the metal of the pins and trap in the plastic. When soldered they either fail because the wire bond detaches or fracture as the heat turns the moisture to gas and it expands. Standard procedure for ICs stored for a significant time is to bake them at well below soldering temperature (maybe 60 to 70 degrees) for as long as 24 hours before using them. It allows the moisture to leave slowly without building up enough pressure to cause damage.
For capacitors, measuring ESR is a better indication of quality than leakage. ESR creeps up as the dielectric deteriorates, value and leakage alone may not be sufficient.
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