The reliability and charging/discharging dynamics of wideband (1.5-14GHz) phase shifters made of MEMS capacitive switches using Aluminum Nitride (AlN) as dielectric are originally reported. Phase shifter lifetimes exceeding 109 cycles are achieved in hot-cycling (+5dBm RF power). Dynamic tests were done for the first time under ambient conditions (5000 humidity) over 2x109 cycles with no major degradation of individual switches performances. It is demonstrated that the phase shifter is very robust (no permanent failure or stiction) and can withstand environmental effects as well as high temperature variations, without the need of expensive hermetical packaging. The excellent reliability is attributed to the slow dielectric charging (a square-root time law) and fast discharging mechanism of AlN (an exponential time law proposed and validated in our work). We extend the validity of charging and discharging models from single device to arrays of parallel MEMS capacitors. ©2009 IEEE.