There are numerous informative articles on the operational temperature range of electronic components and sub-assemblies, but relatively few on storage temperatures.
The datasheets of most manufacturers either show the same storage temperature range as the maximum possible operating temperatures (e.g. 0°C to +85°C) or use the “standard industrial” values of -40°C to +100°C, non-operating and without further explanation.
risks and factors
However, to correctly store electronic components or finished products, one needs to carefully understand the risks and factors that could affect the reliability of the parts before being put into use: How does the storage temperature affect the components?
What aging mechanisms come into play? Will they suffer damage if the storage temperatures exceed the datasheet values? What other environmental factors could make my parts unusable after storage?
Moisture Sensitivity Level (MSL)
Most importantly, the combination of storage temperature and humidity is a decisive factor. Even for short storage periods, a combination of high humidity and high storage temperature can lead to moisture or gas uptake inside the components.
For SMD (surface mount device) parts, the susceptibility for moisture uptake is given by the Moisture Sensitivity Level (MSL) figure as defined by the following JEDEC standard J-STD-020:
Floor Time
TOL implies the use of the “Time on Label” figure given by the manufacturer
The “Floor Time” is the time available once the sealed moisture barrier bag has been opened before the component can be put through a reflow soldering oven without the risk of “pop-corning”, where the absorbed moisture turns to steam in the hot oven and causes the component to crack, delaminate, or even explode.
If the component is left out for longer than the specified floor time, it needs to be “pre-baked,” in other words, put into a drying oven to evaporate off any absorbed moisture before being used. TOL implies the use of the “Time on Label” figure given by the manufacturer.
ambient temperature and RH
Note that the MSL levels are specified for a narrow range of environmental conditions. The ambient temperature and relative humidity (RH) values are quite high for normal indoor conditions during winter (25°C/50%), but are on the low side for some Asian or South American countries during the summer months, where the RH value rarely drops below 80% and room temperatures can exceed 35°C.
This means that the MSL floor time figure needs to be reduced or perhaps extended, depending on the geographic location of the SMD line and the season.
most common problems
Furthermore, through-hole parts can suffer from moisture uptake during long-term storage
Furthermore, through-hole parts can suffer from moisture uptake during long-term storage. Although the component itself is not subjected to such extreme temperatures during soldering, as it is shielded by PCB during the solder-wave process, thus implying a lesser risk of “pop-corning”, it can still suffer from other chemical aging processes that are accelerated by moisture and warm ambient temperatures.
The most common problems include oxidation of exposed metal parts and moisture or gas uptake, either by absorption by the encapsulation material or by capillary action at the joints.
Oxidation
Oxidation of the exposed solder pins of through-hole parts can cause “dry joints” (Figure 1), where the PCB solder does not properly wet the solder joint, as the oxidation impurities on the pin’s surface repel the liquid meniscus, and the electrical and mechanical connection cannot be guaranteed. A dry joint can be hard to spot, and worse, can cause an intermittent fault, such that the PCB assembly passes initial inspection and testing, but later fails in the field.
Certain atmospheric gases, especially sulfur, attack copper compounds, and can cause early-onset corrosion. Thus, storage facilities should be well-ventilated and far away from any drains or sewer vents (the characteristic “rotten egg” odor of sewers is caused by hydrogen sulfide gas, which is a natural product of organic decay).
moisture barrier bag
If poor solder joints are a recurring problem after component storage, the component may need to be stored in a moisture barrier bag with a desiccant gel pack or cleaned before use.
Alternatively, the pin can be “flashed” or coated with a thin layer of pure tin or gold, which are both less reactive to atmospheric moisture.
encapsulation material
Porousness of the latter materials allows moisture or atmospheric oxygen to reach internal components
The encapsulation material itself can absorb moisture or atmospheric gases, especially at high storage temperatures. Epoxy potting materials are usually very hard and highly chemically inert, but silicones or polyurethane materials tend to be softer and more porous.
Thus, the porousness of the latter materials allows moisture or atmospheric oxygen to reach internal components and cause mechanical expansion due to corrosion effects, thereby breaking the seal between the external pins or outer case and allowing further moisture ingress.
seal integrity
The seal integrity can be tested with a penetration test using a UV fluorescent dye (Figure 2). In this test, the component is immersed in a water-based dye at high pressure for a given period to see if any liquid will penetrate through any micro-cracks, pores, or bad seals.
The component is then dismantled and placed under a UV lamp. Any liquid penetration will show up as the dye fluoresces.
Micro-cracks and seal failures
A less drastic but more long-term thermal cycle stress can occur if the part is then stored in an unheated warehouse
Micro-cracks and seal failures can also occur due to environmental temperature cycling. If a component is manufactured in a warm, humid climate and then transported in the hold of an airplane at -40°C, any internal moisture can shock-freeze and fracture the hermetic seal as the ice expands.
The part can go through several thaw/freeze cycles as it is transported through different flight or road traffic distribution centers before arriving at its final destination, thus causing the defect to propagate. A less drastic but more long-term thermal cycle stress can occur if the part is then stored in an unheated warehouse over several summer/winter seasons.
several hot/cold cycles
Thus, for instance, if a manufacturer states that their part has a storage temperature and humidity range of -40°C to +85°C @ 50%RH, it does NOT mean that the part can be safely cycled between these storage temperature limits.
If a part is stored at a low temperature to reduce ionic or atomic aging processes (Figure 3), then it must be warmed up very slowly and brought to room temperature before being used. Sustained hot or cold storage temperatures are preferred over several hot/cold cycles.
SMD components
What happens if the storage temperatures are exceeded? SMD components mounted on an internal PCB will have a different rate of thermal expansion or contraction than the substrate itself, so at extreme temperatures, the mechanical stress can cause the solder to break or the component to crack.
Encapsulated components (diodes, transistors, etc.) can usually withstand lower temperatures, as the case gives mechanical support to the pins, but they may still fail at temperatures below -40°C, as they often contain metallic lead frames, and copper has a high coefficient of thermal contraction.
Electrolytic capacitors
Electrolytic capacitors rapidly lose their capacitance during cooling, and at -40°C
At very low temperatures, the most difficulty arises with components that rely on the movement of ions or liquid chemical processes. These include electrolytic and some types of ceramic capacitors. At low temperatures, such activity “freezes out”.
Electrolytic capacitors rapidly lose their capacitance during cooling, and at -40°C, they may have only 10% capacitance compared to their room-temperature value.
cryogenic temperatures
At cryogenic temperatures (i.e., below about −65°C), the electrolyte will freeze, thus causing permanent physical damage. Moreover, the capacitor equivalent series resistance, ESR, and dissipation factor, Tanδ, deteriorate rapidly with temperature, with the ESR and Tanδ values increasing rapidly below freezing point and rising to x100 or x10, respectively, by the time -55°C is reached.
These effects imply that the capacitor has a completely different performance characteristic at lower temperatures than at room temperature, so if a product containing these parts is moved from a cold storage space into a warm room and then power is abruptly applied, there is a high chance of switch-on failure.
copper windings
The copper windings in these components will contract at low temperatures
Other components that are sensitive to very cold temperatures are wire-wound inductors and transformers. The copper windings in these components will contract at low temperatures and place a mechanical strain on the ferrite core, which will become more fragile at lower temperatures as the binder material between the ferrite grains becomes less flexible.
If a very cold product containing a ferrite part is accidentally dropped, the mechanical shock could very easily crack it, even if it is “safely” encased inside a potting encapsulation (Figure 4). Therefore, products and sub-assemblies taken from cold storage should be handled with extreme care.
conclusion
The shelf-life of a RECOM encapsulated DC/DC or AC/DC power supply module is ten years
In conclusion, if a component or a sub-assembly is stored in a benign environment where the humidity and temperature remain relatively stable or change only slowly, then the shelf-life can indeed be very long. Under ideal conditions, the shelf-life of a RECOM encapsulated DC/DC or AC/DC power supply module is ten years.
However, if any such product is put into use after such a long storage time, then it should be gradually acclimatized to room temperature and visually inspected to ensure that there is no corrosion on the pins and connectors.
current global supply chain problems
If any such product contains electrolytic capacitors, it should be powered up gradually with a limited current supply (a process called “reforming”), to allow the aluminum oxide dielectric insulation to recover before being stressed with the full input voltage.
The current global supply chain problems are causing numerous distributors and suppliers to dig back through their inventory of old stock to meet immediate customer demand. As long as the components have been correctly stored and gradually brought up to room temperature before applying power, the parts may still be perfectly usable.