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The last design consideration that will be covered in this training module is capacitance change over time, or aging. When Class 2 MLCCs cool down below their curie point, the dielectric will spontaneously polarize, and the dipoles will align within domains and be scattered throughout the dielectric. This will cause a high permittivity and high capacitance in the MLCC. The domains within the dielectric will begin to relax and therefore will cause a slight reduction in the permittivity over time. The effect is a measurable capacitance drop around one to five percent per decade hour after the last heat above curie point. The typical process at which an MLCC sees the curie point is during the reflow or wave soldering. During this process, the MLCC will be exposed to temperatures well above the curie point. For example, if an X7R capacitor is soldered on a PCB, once the board cools down, the capacitance will start decreasing about three percent per decade hour indefinitely. This aging affect can be reversed by exposing the capacitor again above the curie point which is called deaging. An important factor to consider is that the published capacitance on the MLCC datasheet is the capacitance after a specific number of hours after the last heat above the curie point. This is called the referee time. Referee times are the number of hours after the last heat when capacitance is measured against the specification. For most X7Rs, this referee time is 1000 hours. Some higher capacitance X7Rs have a referee time of 48 hours.
