How is Backdrilling done in Layout

[lax_ct]

In my design i am using press fit connectors for which i need to be backdrill the pins.The layer count in my design is 18 Layers.Do i need to backdrill all the layers?As of my knowledge backdrilling is done from bottom for this scenario but will it be done on the other side as well?How do they perform the backdrilling in the manufacturing plant

 

[marce]

Backdrilling is used to reduce the length of vias, so the board is drilled from the rear with a bit larger than the via plating, to a pre-determined depth, just below the last layer a signal trace exited from the via. If you backdrilled all mlayers you wouldn't have any connectivity.