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.