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A back-drilled via is a normal through-hole via in a PCB that connects two signal layers, but the dangling stub hanging down from the last signal layer to the bottom of the board has literally been drilled out. A drill bit with a diameter a few mils larger than the drilled and plated hole drills a "blind" via to within a few mils of the lowest signal layer. This removes the copper stub and prevents a potentially disastrous resonance in the 5 to 10 GHz range. All high-performance backplanes use back-drilled vias.