Let us consider a 8-layer PCB routing under progress. Let us assume we are routing routing a high speed signal from first layer to third layer in a PCB. In this scenario, there shall be a via from top to bottom layer and via is breakout to get routed in third layer. The remaining section of the via from third layer until bottom layer is not needed and actually acts a stub. As this is a high speed signal, any stub acts as impedance continuity causing signal integrity issues. The signal integrity issues could lead to EMI finally. So, the additional stub is removed in such a way it doesn't affect the signal and eliminates stub. The back drilling shall be done such that the diameter of the drill is slightly more than the diameter of the via. Even the back drilling shall be done within a mils of the routing layer.
The major advantage of back drilling is that the need for blind and buried via can be eliminated. Blind and buried via are nightmares for the test engineers as well as to companies as they cost more.
For low frequency boards, back drilling is not required.
Back drilling is also called CCD which stands for Controlled depth drilling.
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