regarding backdrilling

Backdrilling, or controlled-depth counter boring, is a process in which plating is removed from the unused portion of the via. Multilayer circuit boards are processed conventionally,with a secondary drill operation added after plating, using CNC drilling equipment with controlled depth enhancements.CNC drill files permit this process to be automated and repeatable.
One important parameter is the secondary drill diameter. This drill diameter must be greater in diameter than the primary drill, to permit removal of all the electrodeposited plated metal--typically copper with an additional surface finish. It is important to minimize this diameter in order to avoid reduction of routing channels, which otherwise could compromise hole-to-trace spacing in the pin fields.
Advantage of backdrilling is decreasing via stub length significantly reduces a particularly problematic form of signal distortion called deterministic jitter. Because Bit Error Rate (BER) is strongly dependent on deterministic jitter, any reduction in deterministic
jitter by backdrilling will significantly reduce the overall BER of the interconnect – often by many orders of magnitude. Other
key advantages to backdrilling PTH vias include: less signal attenuation due to improved impedance matching; increased
channel bandwidth;

Regards,
Satya Shankar