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Working with Backdrilling in Allegro PCB Designer: Best Practices
Purpose
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 can greatly affect 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 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. The procedural steps and
best practices for a successful implementation of backdrill setup and processing in the
Allegro PCB Designer are discussed in this document in detail.
Terms
Backdrill Secondary drilling process that removes the unused
section of Plated Through-Holes
Counter-bore
Alternative term for backdrilling
Gbps
Giga Bits per Second
PTH
Plated Through-Hole
Press Fit
Connector whose pins are forced (spring loaded) in
PCB, not soldered
RMB
Right Mouse Button
Stub
Unused section of a plated through-hole resulting in
signal quality issues (reflections, impedance
discontinuity)
