Question: What is the proper hardware installation on a mounting hole when a tooth washer is used?
Answer: From Section 4.1.4 Hardware Installation –Threaded Fasteners
Both the order and orientation of mounting hardware need to be considered during assembly. Devices such as “star” or “tooth” washers may have one side with sharp edges intended to cut into the mating surface to keep the hardware from coming loose in operation. Figure 4-11 is an example of this kind of lock washer. Unless otherwise specified the sharp edges of the lock washer should be against the flat washer.
The lockwasher sharp edges cannot be on the board, they have to be on a flat washer as shown in the above photo. Therefore the sequence of the assembly on a printed circuit board is Bolt or Screw, Lockwasher, Flat Washer and then the board.
This is all defined on page 4-6 and 4-7 of IPC-A-610E.
(All images and quotes are from associated IPC Industry Standards.)