The present disclosure relates to a debonding apparatus. In some embodiments, the debonding apparatus comprises a wafer chuck configured to hold a pair of bonded substrates on a chuck top surface. The debonding apparatus further comprises a pair of separating blades including a first separating blade and a second separating blade placed at edges of the pair of bonded substrates. The first separating blade has a first thickness that is smaller than a second thickness of the second separating blade. The debonding apparatus further comprises a flex wafer assembly configured to pull the pair of bonded substrates upwardly to separate a second substrate from a first substrate of the pair of bonded substrate. By providing unbalanced initial torques on opposite sides of the bonded substrate pair, edge defects and wafer breakage are reduced.