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.

The concern is solderability.

If not stored correctly oxidation on the pins can cause problems in the solder process.

Try ordering a few to begin with and see how it goes.

Your EMS might have some tricks to deal with components that have been on the shelf too long.

I would not worry too much about the chips functioning.

What I would be relatively concerned about is solderability- that’s the main reason behind the relatively short “shelf life” numbers. A more aggressive flux may help- there may be some recommended procedures used by low volume high-rel folks who may have little choice about using NOS (New Old Stock) parts. At the other extreme, I’m told one car maker has a 6-month limit on parts - reflecting the perceived risk of unreliable PCBs vs. their ability to dominate suppliers.

Also, if you are not 100% sure they were stored in a low humidity cabinet, a specified bake procedure is called for to drive moisture out of the plastic packages and prevent damage in the reflow process- it’s unnecessary for hand soldering.