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I want to use a 330nF X7R ceramic capacitor for an analog circuit. Obviously the capacitance value is very important for the correct working of the circuit.
I haven't found on the datasheet if these kind of capacitors has an important capacitance decay during time. For example, does the capacitance decrease after 1000 hours of usage?
Class II dielectrics experience a phenomenon called ageing, and it is simply a decrease in capacitance over time due to crystalline changes that occur in all Class II dielectrics (X7R, X5R and Y5V). This is caused by the relaxation or realignment of the electrical dipoles within the capacitor.
This phenomenon is affected by time, temperature, voltage (voltage has a minute effect).
Class I dielectrics (NP0 – COG) do not exhibit this phenomenon as they are stable over Time, Temperature and voltage. The drawback for these dielectrics is that the εr is relatively low as compared to the Class II dielectrics.
Exposure to 150c (above Curie point) for 1.5 hours is sufficient to return the capacitor to its original value. The soldering process is not necessarily an effective De-Aging process but the capacitance value will be raised.
Capacitance measurements may be erratic in the initial 10 hours after testing. This is due to the initial capacitance value, dielectric type and the time between reflow and the capacitance measurement.
It's not at all obvious that the capacitance value is "very important".
If you need high stability with time and temperature use an NP0 type of capacitor rather than X7R. Eg. Kemet C2220C334J5GACTU. It will be much bigger and much more expensive.
Murata specifies variation of less than +/- 12.5% after 1000 hrs operation under "durability":
To find this spec you need to download the "Detailed Specifications Sheet" rather than the "Characteristic Data".
This example was for a 330 nF 0805 X7R part.
X7Rs do age. The rate is linear with the log of the hours, fast at first, then slower and slower. One chart I remember showed about 1% per decade. So, 1% at 10 hours, 3% at 1000 hours, 5% at 100,000 hours. I would expect this to vary brand to brand and for different values.
C0Gs have very low aging, similar to film caps.
