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Working with Backdrilling in Allegro PCB Designer: Best Practices
Overview
Backdrilling is a board fabrication process that removes the unused section of plated
through-holes, typically, connector pins and signal vias. A secondary controlled depth
drilling pass(s) removes all the electro-deposited plating material in the PTH ensuring
that the signal stubs are minimized. Stubs are the source of impedance discontinuities
and signal reflections, which become more critical as data rates increase. Backdrilling
can be performed from either side of the PCB and to multiple depths. Drill sizes used for
backdrilling are typically 6-10 mils larger than the original tooling. Fabricators must be
careful not to over drill beyond the calculated depths or to under drill (leaving
unacceptable stubs). Tradeoffs between the signal quality and manufacturing costs as
well as between the signal integrity and board testability must be considered.
