#TIL When a Class 2 ceramic capacitor is heated above its Curie temperature (130 °C), the crystal structure changes. Soldering the cap causes a sudden increase of capacitance beyond spec (so don't attempt to hand-matching ceramic caps!), followed by a logarithmic decline (aging). This is reversible, resoldering the cap or baking the board can "reset" an aged cap back to the initial state.
"Referee time" is the time for a cap's capacitance to decline back to its standard spec value (X7R cap ages slowly, referee time is 1,000 h., X5R is only 48 h.). A DC bias not just reduces the effective capacitance, but further accelerates aging in a nonlinear way.
As usual, Class 1 NP0/C0G capacitors are not affected by these effects. #electronics