An accurate material characterization method for microwave printed-circuit-board substrates is proposed by combining the microwave ring resonator method with iterative full-wave simulations. A detailed error analysis is performed quantitatively to compensate the discrepancy between the estimated value and the actual permittivity. The improved estimation accuracy is a necessity for some highly-precise microwave devices. In addition, the permittivity of FR4 substrate at 109 degrees C operating temperature as a function of time is investigated to validate its long-term stability.