This paper presents the impact of 3D assembly on a silicon ring resonator. Indeed, the flip-chip integration used to connect photonic and electronic circuits requires high temperatures processes. The cooling of such a structure induces thermo-mechanical stresses on silicon waveguides. A finite element modelling is proposed to theoretically quantify the stress distribution. The photo-elastic effect is then highlighted to explain the change in silicon refractive index. As a result, the ring resonance wavelength is shifted from approximately 100 pm. Performance of a WDM (wavelength division multiplexing) system could thus be highly degraded.