Power planes are capacitive at low frequencies and develop resonances according to their cavity dimensions. By reducing the plane thickness, the downward impedance slope is pushed to lower values, while the modal resonances get gradually suppressed. The Q of these resonances is related to the effective loss of the composite material of dielectric and metalization. In this paper, we show how the total area and aspect ratio, together with laminate thickness, impact the high-frequency impedance profile. Simulated results will be correlated to test structure measurements.