Aging characteristics of capacitors
Time：2020.09.27
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Figure 1: Capacitor aging characteristics
A1. Aging means that the capacitance of EIA Class II capacitors will decrease over time. This is an unavoidable natural phenomenon for all ferroelectrics used as dielectric materials. The aging process is reversible, and the crystal structure of the dielectric changes with temperature and time. Aging is usually described in exponential form of capacity decay rate. The logarithm is used to express the rate of change of aging, so the decrease in capacity is very large in the first 10 hours.
Note: The dielectric loss factor (tangent) will also age, and the rate of decrease is several times the capacitance value. At the same time, EIA Class III and IV also have aging characteristics. The industry definition of aging can refer to the "official" definition of aging in EIA-521 and IEC-384-9.
Q2. What is the difference between a Class I capacitor and a Class II capacitor?
A2. The difference between capacitors for temperature compensation (EIA Type I) and high dielectric constant capacitors (EIA Type II). The temperature-compensated relative permittivity is 10-100, and the high permittivity type uses materials with a relative permittivity of 1000 to 10000, and the material systems are different.
The temperature characteristics of Class I capacitor 1 (C0G, CH) are stable. It basically does not change with temperature, voltage, and time. The temperature dependence of CH and C0G capacitors is small, and its capacitance change rate is ± 30PPM/℃. Therefore, it is suitable for circuit applications that require temperature and voltage stability.
The characteristics of Class II capacitor 2 (X5R, X7R, Y5V) are relatively large for temperature, voltage, and time changes. Compared with Class I, the temperature of the capacitor changes greatly, but it can obtain high-capacitance capacitors. It is used for rectification, side road and other purposes.
In addition, the capacitors (Class II, III, and IV) made of barium titanate as the main dielectric material are ferroelectric, so they are easy to "aging". If they are placed in an unheated or uncharged state, the capacitor's Capacity will drop. Class I capacitors are not ferroelectric, so they will not age.
Figure 2: Sequencing of electric dipoles
The temperature is below the Curie point and placed without load. Over time, the internal molecular structure will change, and this results in an orderly arrangement of electric dipoles. A structure that weakens the charge retention of molecules. The capacitance value will become smaller by 3.
Q4. What is anti-aging?
A4. Anti-aging is a heat treatment to reduce the phenomenon of aging. Simply put, it is a process of restoring re-aging, but it does not prevent the aging of components. Heating the capacitor above its Curie temperature will restore the crystal structure to its optimal disordered arrangement, thereby achieving maximum capacitance. TDK recommends heating at 150°C/1 hour for anti-aging.
Note: It is very important to record the final heating time TOLH (Time Of Last Heat) in order to measure the volume after heating.
Q5. Is "aging" just a unique phenomenon of TDK?
A5. The answer is no. Class II, III and IV ceramic capacitors produced by all manufacturers of MLCC will have "aging". Since aging depends on the structure, the rate of aging varies from manufacturer to manufacturer.
Q6. How do capacitor manufacturers compensate for aging?
A6. The test standard for capacitors has been defined in IEC-384-9. It is defined as: the capacitance value after 1000 hours of time-dependent change after the final heat treatment4. We consider the influence of aging speed in the testing process, so that the capacitance value after 1000 hours is within the allowable tolerance range for selection.
Figure 3: Example of sorting restrictions for Class II materials, K allowable deviation
Note: As time goes by, the capacitance value decreases due to aging. After heating for less than 1000 hours, it may exceed the upper limit of the capacity tolerance, and if it exceeds 1000 hours, it may be below the lower limit of the capacity tolerance.
Q7. How should capacitor users compensate for aging?
A7. Capacitor users should expect Class II, III, and IV capacitors to be within the allowable capacitance tolerance range. From the last heating, they need to be placed in TOLH for 6 weeks (1000 hours). After soldering, the adhesive is cured, and the temperature rises. Immediately after other processes, it is the process of capacitor de-aging, and it is possible that the capacitor value does not fall within the tolerance range. You may need to consider the aging factor of the capacitor and broaden the tolerance range of the capacitor in the circuit test.
