Microcontroller shelf life
Actually, I found an interesting document from TI: Component Reliability After Long Term Storage).

I will just quote some interesting parts here:


  
  Each year the semiconductor industry routes a significant volume of devices to recycling sites for no reliability or quality rationale beyond the fact that those devices were stored on a warehouse shelf for two years. 
  Results show that current packaging material (mold compound and leadframe) is sufficiently robust to protect the active integrated circuits for many decades and permit standard reflow solder assembly beyond 15 years. 
  Standard packing materials (bags, desiccant, and humidity cards) are robust for a 32
  month storage period that can be extended by repacking with fresh materials. 
  Packing materials
  designed for long term storage are effective for more than five years.
  


Background:


  
  The origins of date code age restrictions are not well documented, but it is probable that limitations of the packing materials for moisture sensitive components and post storage solderability of SnPb or Sn finishes contributed to the
  concerns of customers that led to the shelf life restrictions
  


Device risks:


  
  Device functionality and parametric performance after extended periods of shelf
  storage
  
  
  No failure mechanisms have been identified that would compromise the electrical performance or circuit reliability of LTS devices.
  
  Exposure to the ambient atmosphere for extended periods of time may oxidize the lead
  surface impacting solder wetting during assembly.
  
  
  Aging studies have shown that NiPdAu lead finish devices pass solderability requirements beyond 8 years. Actual testing of LTS devices indicates that storage for 15
  years does not compromise solderability.
  
  Moisture absorbed in the epoxy matrix of devices categorized as MSL 2 to 6 could
  vaporize during reflow solder assembly and crack the package.
  
  
  Devices targeted for LTS are packed in special metallized bags that are sealed with desiccant and a HIC.
  
  Devices stored for extended periods may exhibit corrosion of bond pads or interconnect
  metallization
  Devices that incorporate non-volatile memory may suffer data retention issues after long storage intervals.
  


Packing material risks:


  
  The static dissipative properties of tubes or tape and reel may degrade over time
  resulting in potential ESD damage to LTS devices
  Storage bags may leak, allow moisture to enter, and cause problems for MSL
  Label adhesives may fail or the ink marking could fade making lot history or device identification difficul
  


Conclusion:


  The shelf life of LTS devices as determined by solderability, SEM visual, SEM spectral analysis, optical microscopy, MSL performance, solderability, and decapsulation/visual is >15 years.
  IC packing material shelf life is limited by moisture diffusion through the MBB. A standard MBB maintains satisfactory moisture levels for 32 months. LTS bags control moisture levels
  beyond 5 years.


Terms:


Humidity Indicator Card: (HIC) – A card printed with a moisture sensitive chemical (cobalt chloride) that changes from blue to pink in the presence of water vapor.
Long Term Storage: (LTS) – Storage of devices in an uncontrolled indoor environment for
more than two years.
Moisture Barrier Bag: (MBB) – Storage bag manufactured with a flexible laminated vapor
barrier film that restricts transmission of water vapor.