The need for higher data capacity and throughput in applications such as wireless multimedia, IOT, big data and vehicular positioning systems are driving the use of millimeter-wave (mmW) frequencies at 28, 38, 60 and 73 GHz for the 5G mobile communication systems. Wafer-level mmW measurements of devices or circuits for 5G applications are extremely challenging due to significant system losses and calibrated state at the probe tips valid only for about 10 minutes. A new method of probe tip S-parameters calibration that incorporates power calibration will be presented, which is shown to greatly improve measurement accuracy and calibration stability up to 110 GHz. The new method allows precise and low probe tip source power biasing of -20 dBm, overcoming system losses. It also increases system calibration stability to more than 4 hours, giving microwave engineers adequate time to test their mmW devices and circuits by avoiding the need to perform re-calibration after 10 minutes of measurements.