Material Removal
Removal of unwanted copper from a plated through-hole in a printed circuit board is a mechanical process used to eliminate unused portions of vias. This process of backdrilling employs a drill bit slightly larger in diameter than the original hole to bore out the copper cylinder to a precise depth. The action stops short of the active signal layer to leave the functional connection intact.
Board Fabrication
Depth-controlled drilling demands high-precision equipment to avoid penetrating the trace layer that carries the high-speed signal. In the production line, the board is aligned using optical targets, and the drill depth is set using electronic touch-sensing or mechanical Z-axis control. A tolerance of plus or minus one hundred micrometres is common in this step to ensure the remaining copper does not bridge or disconnect.
Penetration beyond the target trace layer will sever the transmission line, causing a scrapped board. Runouts that are too shallow leave behind a stub that degrades high-frequency signal performance.
Signal Quality
Minimising signal reflection is the primary objective when designing with high-speed lines that operate above five gigabits per second. Unused via stubs behave as transmission line extensions that reflect energy back to the source, causing signal attenuation. With backdrilling, these stubs are reduced to less than two hundred and fifty micrometres in length.
This alteration improves the impedance profile of the channel and prevents destructive interference in the receiver.
