In many electronic devices, the capacitors are the life-limiting components. Whereas the operational life of a capacitor is dependent on both electrical factors and environmental factors, the shelf life is mostly determined by storage conditions.
The shelf life of most capacitors depends on environment factors such as humidity, temperature, and atmospheric pressure. Subjecting capacitors to harsh conditions can significantly affect their electrical properties, or even damage them completely. The effect of environmental factors on the shelf life of capacitors varies depending on the chemical composition and construction of a capacitor. Aluminum electrolytic capacitors are highly susceptible to environmental factors, especially high temperatures.
Capacitors contain chemical materials and exposing some of them to high temperatures accelerates chemical reactions. For aluminum electrolytic capacitors, it is estimated that a 10C rise in temperature can double the rate of chemical reactions. The gradual evaporation of the electrolyte when these capacitors are exposed to high temperatures causes a decrease in capacitance and an increase in tangent of loss angle (tan δ / DF). Generally, exposing capacitors to harsh environment causes degradation of their electrical characteristics.
Shelf Life for Aluminum Electrolytic Capacitors
Aluminum electrolytic capacitors are mostly used in applications that demand high capacitance values. They are commonly used for filtering applications in switching power supplies. The usable life of such devices is dependent on these capacitors.
The characteristics of aluminum electrolytic capacitors are greatly affected by harsh environmental conditions. The electrical characteristics that are affected when these capacitors are stored for a long time without charge are equivalent series resistance (ESR), leakage current, and capacitance. ESR and leakage current increase while capacitance decreases. Nevertheless, the changes are small if these capacitors are stored at room temperature. Today’s aluminum electrolytic capacitors have longer shelf life, usually around 2 years, as compared to their predecessors.
For aluminum electrolytic capacitors, the changes in ESR, capacitance, and leakage current are caused by chemical reaction between the aluminum oxide film and the electrolyte. Storing these capacitors at high temperatures degrades the sealing material. When this material is weakened, the dissipation of the electrolyte can occur, thereby affecting the electrical characteristics of the capacitor.
Changes in the characteristics of aluminum electrolytic capacitors when they are stored for a long time can also be caused by penetration of the electrolyte into the oxide film. This is the main cause of changes in leakage current. The rate of deterioration of the aluminum oxide film is a function of time and temperature.
When storing aluminum electrolytic capacitors, it is important to ensure that they are not exposed to moisture. High humidity accelerates oxidation of lead wires/terminals. This degrades solderability of a capacitor. Apart from moisture, it is also necessary to ensure that these components are not exposed to ultraviolet rays, ozone, oil, and radiations. Exposing capacitors to these conditions causes the rubber end seals to deteriorate. Weakening of the end seals lowers the overall reliability of aluminum electrolytic capacitors.
It is important to know the storage time of aluminum electrolytic capacitors before using them. Since leakage current increases with an increase in storage time, a capacitor that has been stored for a long time can have a high leakage current. The high current required to restore the aluminum oxide film of such a capacitor can damage the component. This current surge can also affect an electronic circuit.
The aluminum oxide film of a capacitor can be reconditioned by applying voltage to the component. This process of rebuilding a deteriorated oxide film of a capacitor is known as capacitor reforming. It is advisable not to use capacitors that have been in the store for an extended period of time.
Shelf Life for Tantalum Capacitors
Tantalum capacitors have a long shelf life. The electrical characteristics of these capacitors are not affected significantly when they are stored for a long period of time. Unlike aluminum electrolytic capacitors, tantalum capacitors have higher stability, and their capacitance does not decrease with time.
Many studies have revealed that tantalum capacitors can be stored for a long period of time with little or no variation in electrical characteristics. However, a small change in leakage current occurs when a tantalum capacitor is stored for an extended period of time. The storage conditions determine the changes that occur in tantalum capacitors.
Storing tantalum capacitors at high temperatures can cause a significant change in leakage current. However, the normal leakage current is regained when a voltage is applied to the device for a short time. Small or no changes in leakage current are noticed when these capacitors are stored at low temperatures. When storing tantalum capacitors, it is advisable to follow the storage instructions provided by the manufacturer.
Shelf Life for Ceramic Capacitors
The shelf life of ceramic capacitors is greatly determined by method of packaging and storage conditions. Unlike aluminum electrolytic capacitors, the dielectric material of multilayer ceramic capacitors (MLCCs) does not exhibit failures when the capacitor is stored for a short period of time. However, long-term storage of surface mount multilayer capacitors can cause aging of dielectrics (Class II dielectrics), solderability problems, and degradation of tape and reel.
When ceramic capacitors are stored for an extended period of time, a slow oxidation process can cause degradation of terminations. Storing ceramic capacitor in open air or exposing them to chlorine gas or sulfur dioxide accelerates the oxidation process. Oxidation of terminations affects solderability of multilayer ceramic capacitors.
The capacitance of ceramic capacitors constructed with Class II dielectrics such as X7R, Z5U, and Y5U reduces with time. This decay of capacitance, also known as aging, is a function of time and independent of storage conditions. To reverse aging, the dielectric is heated at a temperature above the Curie Point.
Ceramic capacitors should be stored at temperature and humidity conditions specified by the manufacturer. Before using a capacitor, you should check the recommended shelf life, date of receipt, and inspect terminations.
Conclusion
For most capacitors, the shelf life is significantly determined by storage conditions. Electrical characteristics of stored capacitors change mainly depending on storage conditions, especially temperature and humidity. For some capacitors such as aluminum electrolytic capacitors, storage temperature determines the rate of chemical reactions. Aluminum electrolytic capacitors that are stored at high temperatures lose capacitance faster than those stored at low temperatures. Exposing capacitors to moisture accelerates oxidation of lead wires/terminals. This oxidation of terminals degrades solderability. Before using a capacitor, it is important to check its receipt time. Some capacitors require reforming after they have been stored for an extended period of time without recharge. To maximize the life of capacitors, they should be stored under conditions specified by the manufacturer.
