Back-drilling runs a slightly larger drill to a controlled depth from the opposite side of the board, removing the unused part of a plated via barrel below the layer where the signal leaves it. That stub otherwise acts as a resonant tuning stub, causing reflections and notches at multi-gigabit rates. A short residual stub, often around 0.2 mm, is left by design, and the depth is called out per net on the fab drawing.
Those 25 gig lanes exit on layer 3 of a 20-layer board, back-drilling is mandatory or the via stubs kill the eye.
Also heard as
- backdrilling
- backdrill
- back drill
Related terms
Via
Plated hole that connects copper on one layer of a board to copper on another.
Impedance control
Designing and building traces to a target impedance, such as 50 or 100 ohms, within a set tolerance.
Blind via
Via that starts on an outer layer and ends at an inner layer without passing through the whole board.