This paper reports degradation characteristics of low-k dielectric (FOX) in multi-level metal structures (comb-type capacitors) submitted to moisture stress. A large increase of leakage current (>105) and capacitance (up to ×3) is observed after moisture stress when only FOX is used as lateral dielectric, while moderate degradation takes place when an oxide liner is placed between FOX and metal lines. Enhanced moisture induced degradation is found on previously probed dices with respect to virgin devices. Systematic electrical measurements, combined with SEM analysis, are performed to find out the moisture diffusion path. When contact pads are damaged by previous probing (owing to the mechanical weakness of FOX in the pad stack), they provide a direct entry path for enhanced humidity intake. Humidity is also shown to enter through wafer border. Using a SiO2 liner combined with FOX improves considerably the resistance to moisture degradation.