Dielectric thickness impact on Time Dependent Dielectric Breakdown (TDDB) of Ultra-Low-κ (ULK) (κ=2.7) and porous ULK SiCOH (κ=2.55) was systematically investigated using Constant Current Stress (CCS) method on our 64nm pitch double patterned metal layers. Invariance of J²t_BD suggests that the NBlock-IMD interfacial Cu diffusion is the dominant failure mechanism at stress conditions. Applying a power law dependence of J²t_63.2 on the physical spacing convincingly demonstrates that the critical Cu density required for breakdown depends on the dielectric thickness. By normalizing the Time to BreakDown (t_BD) according to each individual device's characteristic spacing, the β value obtained is much closer to expected intrinsic value. Results and analysis thus show that great care is needed when taking into account the impact of dielectric thickness scaling on calculating the total fail rate and on extrapolating current TDDB data to future technology generations, where much tighter BEOL pitch is likely needed.