Just searched for a datasheet on white LEDs. Found this, it ios for the most common 5mm 20mA white LED

And i found something weird in it :

(2) Storage
a. The LEDs should be stored at stored at 30 C or less and 70%RH or less after being shipped and the storage life limits are 3 months.
b. If the LEDs are stored more then 3 months, they can be stored for a year in a sealed container with a nitrogen atmosphere and
moisture absorbent material.
c. Please avoid rapid transitions in ambient temperature, especially, in high humidity environments where condensation can occur.

http://www.surplustronics.co.nz/library/Mega-White-LED.pdf



So they say that the storage life for LEDs is 3 months - i guess it has something to do with the epoxy package not being sealed and moisture going into the LEDs on the interface between the epoxy and lead wires

But : The LEDs are supposed to last many years when they are in use, and they are not potted in anything when installed in a device... How does the effect damage LEDs in storage, but not damage LEDs which are installed and in use (whether powered on or off for most of the time) ?

thats to keep them dry for hot air or vapor soldering.
if moisture gets in they may explode when heated.many parts have this issue.
i use old stock led's all the time but solder with a plain old weller station.never a problem.
but with hot air/vapoer process the board and all parts get to solder melt temp.trapped moisture=BANG!

It is not only about LED, but about all components to survive the (mainly reflow, but any type) soldering.
There are two principal problems:
- Corrosion damaging the soldering surface, so they do not form a reliable joints anymore. If this happen, the components are wasted.
- Package materials soak the humidity and when the parts become heated during the soldering, the humidity evaporate and the vapor pressure damage the components (their mechanical integrity - usually mentioned as the "popcorn effect"...). The humidity seeps in usually around metal/plastic borders. When this happen, the parts have to be dried by slowly heating up in an oven for a few hours. This process is called "baking".


The rules usually are:
- virtually Infinite storage time of unopened original sealed packages (well, replace the "infinite" by the lifetime of the packages). These packages keep the inner atmosphere very dry (<10%RH)
- When opened, store for maximum few months in temp 20..40degC and RH<70% (exact numbers could vary a bit)
- When the humidity indicator inside the just opened bag exceed the 20% limit, or temperature or humidity during opened bag storage was outside the above limits, or the storage time of opened packages was longer than above, but still shorter than about a year (solderability limit), baking is necessary before mounting (the "baking" is an exact procedure aimed to remove all humidity)
- When the solderability limit is exceeded, reliable soldering is not guaranteed, so such components are in fact unusable