Back Drilling Rigid PCB

A rigid PCB with back-drilled vias to remove the unused via stub left after a signal exits an inner layer. Stubs act as quarter-wave resonators that drop signal energy in narrow bands, and they are one of the dominant via-related loss mechanisms above 10Gbps. Back drilling removes the stub by counter-boring the via from the back side to a controlled depth, leaving only the active portion of the barrel. Stub depth is held to tight tolerance with depth-controlled drilling, and remaining copper barrels are inspected by cross-section to confirm clearance and breakout. Typical uses are AI-server compute boards, switch and router line cards, 25/56/112G SerDes backplanes, optical-module evaluation boards, and 5G base-station backhaul where channel insertion loss and resonance have to be tightly managed.