Long-term storage of components
Sometimes electronic components have to be stored for up to 10 years. The article explains what it is and what the storage conditions should be?
Paul Austen, Senior Design Engineer at Electronic Controls Design Inc., takes the floor in the discussion of long-term storage of components: 'Most manufacturers ship their products with a moisture sensitivity level of MSL> 1 in Moisture Barrier Bag (MMB) bags, additionally containing a desiccant to keep the humidity close to zero and a Humidity Indicator Card (HIC) which is placed in the bag to confirm the humidity level. However, it must be remembered that such a sealed MMB is only valid for one year (unless the manufacturer says otherwise).
So, according to J-STD-033, the best environment for long-term storage of components is the same basic condition as in MMB, i.e. <5% RH above all. It is the best way to stop moisture absorption, contact corrosion, and prevent components from having to be annealed prior to use.
However, it is very important to prove that these conditions did last for the entire 10 years! At this point, the storage humidity and temperature monitoring system is critical to proving compliance with J-STD-033. Without data showing compliance with the required RH levels in the storage system, companies are forced to anneal the component before use, which can cause corrosion and possible damage to the components.'
What are the actual risks of long-term storage of components, reminds Fritz Byle, process engineer at Astronautics:
- Lead oxidation damages the solderability
- Likewise, the solderability is negatively affected by the growth of the intermetallic layers, which can extend down to the surface
- The elimination of moisture requires pre-heating, which entails additional risks described in points 1 and 2.
The risks described in points 1 and 3 can be achieved by excluding the presence of oxygen and moisture. The sealed MBB purged with nitrogen with the right amount of desiccant will work for about three years. 'Here it is worth noting that a previous expert points out that the pouch provided by the manufacturer only lasts one year. 'The time depends only on the quality of the bag (i.e. its permeability) and the quality of the seal. Instead of repackaging at specified intervals, storing the MBB in a nitrogen-purged cabinet will keep the MBB's dry, oxygen-free environment indefinitely.
The risk mitigation described in point 2 cannot be achieved by excluding oxygen and/or moisture. The only way to slow down the growth of intermetallic layers is to store the parts at low temperatures, but on the other hand, this is not recommended.
As already mentioned, the long-term storage of components is based on the requirements of the J-STD-033 standard. This document also specifies soak times, especially for SMT components, to be used after the storage period. Leo Lambert, vice president, and technical director, EPTAC Corporation, discusses the formal storage requirements for:
'In section 5.3 of the above-mentioned standard, the term safe storage is defined as such storage conditions that keep the floor-life clock at zero.
Section 5.3.1 refers to dry packaging, i.e. MBB, which should have a shelf life of at least 12 months from the bag closing date, indicated on the warning label or barcode. Section 5.3.2 states that if the actual shelf life is greater than 12 months but less than 2 years from the MBB pouch closure date and the HIC card indicates that annealing is not required, it is safe to reflow the components (need to be reflow). Also remember that other factors, not only moisture sensitivity, can affect the overall life of the components). And another note from Leo Lambert: The HIC, or Moisture Indicator Card that is permanently sealed in the MBB, is usually accurate for at least 2 years.
'5.3.3 gives some details on the properties of dry storage cabinets. Generally, these are storage cabinets that keep the humidity low by blowing dry air or nitrogen at 25 +/- 5 ° C. Cabinets must be able to recover to the set humidity level within one hour of routine operations such as opening/closing doors. According to point 18.104.22.168, SMD packages that are not enclosed in MBB may be placed in a cabinet with a dry atmosphere, maintained at a relative humidity of not more than 10%. These types of dry cabinets should however not be considered MBB: the storage of SMD in dry cabinets should be limited to the maximum time according to Table 7-1 given in the standard. If the time limit is exceeded, it should be annealed according to Table 4-2 to 'reset; service life (the standard can be found, for example, on the Digi-Key website).
22.214.171.124 talks about dry cabinets maintaining 5% RH. Storage in this type of dry cabinet can be considered equivalent to storage in a dry container with an unlimited shelf life. '
Leo Lambert also reminds us that moisture is one thing, but you should also remember about the solderability of component terminals, which may be affected by two other elements: the growth of the intermetallic layer as a result of solid-state diffusion and the component's exposure to contaminants deposited on its surfaces. Therefore, it is recommended to store the systems at a temperature below 30 ° C.