Circuit ageing degradation is becoming worse in advanced technologies, while application fields like military, medical and energy demand more reliability. Thus, reliability is one of the most important challenges of the semiconductor industry [1]. In this work, we review the physical ageing phenomena, their simulation model, and how they can be avoided. Then, we propose a synthesis methodology composed of classical circuit optimization with the reliability analysis in earlier stages. Also, the variability of the integration process technology is taken into account. We compare a classical and a reliable designed digital controlled oscillator (DCO) in order to show a reduction of 16% in the oscillation frequency ageing degradation. In this way, the reliable design makes the circuit lifetime five times longer, if we fix the maximum frequency ageing degradation at 2.0%. Finally, we present the reliability as a design criterion, advantages and disadvantages of our methodology.