CRJ-700 Lead Technician reported that the incorrect length fasteners were used during the replacement of a malfunctioning AOA sensor; resulting in an air leak in an RVSM critical area.

Narrative

[The incident] occurred during [a] regular overnight maintenance shift. Action was the replacement of a left angle of attack sensor. During replacement of a panel associated with this sensor in the pressurized part of the aircraft; improper fasteners with an excessive grip length were used by someone under my supervision; which I failed to notice due to performing other maintenance tasks in the hangar. The excessive grip length caused the screws to bottom out against their nut plates and record sufficient torque for installation before the panel was completely in place. My post-installation inspection of the work did not catch the improper installation; which was not visible once the panel was installed and sealed. This problem was discovered when mechanics pressurized the aircraft for other maintenance and felt a leak from the AOA panel as they passed by. They installed a new AOA sensor and noticed that the screws had an incorrect grip length; so they replaced the attaching hardware to correct the problem.The causes of the event include: a lack of knowledge about fasteners by an employee with about 7 months of experience; my failure to train this employee on types of fasteners; lack of good standard procedures for verifying the correct fastener.This incident will result in everyone with limited experience getting a lecture and demonstration of different fasteners used on our aircraft. Also; procedures will be updated to require everyone to compare old fasteners with new ones for each installation. In the event of any doubt about whether the correct fasteners are available; technicians are to consult a Crew Lead.