Issue link: https://resources.pcb.cadence.com/i/1180264
Working with Backdrilling October 2019 5 Product Version 17.4-2019 © 1999-2020 All Rights Reserved. 1 Backdrilling Today's high-speed serial I/O technology handling 5 Gbps or higher presents new challenges for hardware engineers. Transitioning these high-frequency signals between layers greatly affects signal integrity when a portion of plated through-hole (PTH) is left unused, forming a stub. By definition, these stubs are the unused section of the barrel where the signal is not required to travel. This can be controlled in the design space by using the full length of the barrel for signal layer transitions thus keeping stubs to a minimum or with the use of buried or blind vias. If these options are not available, stubs can be removed through a board- fabrication process called backdrilling, sometimes referred to as controlled depth counter- boring. Introduction 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