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Best Practices: Working with Backdrilling

The procedural steps and best practices for a successful implementation of backdrill setup and processing in Allegro PCB Editor are discussed in detail in this document.

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Backdrilling is a board fabrication process that removes the unused section of plated-through
holes; typically connector pins and signal vias. Secondary, controlled depth-drilling passes
remove all electro-deposited plating material in the PTH ensuring 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 to 10 mils larger than the
original tooling. Fabricators must be careful not to drill beyond (over drill) the calculated
depths as also not to under drill, leaving unacceptable stubs. Tradeoffs between signal quality
and manufacturing costs must be considered as well as the tradeoff between signal integrity
and board testability.