Figure E-1: Backdrilling Process; Figure E-2: Backdrilled Vs Non-Backdrilled Channel Characteristics - Xilinx RocketIO X User Manual

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Appendix E: Serial Backplane System Design
One solution is backdrilling or counter-boring. Counter-boring is accomplished by drilling
out any unused portion of the PTH via from the underside of the backplane. Figure E-1
shows an illustration of how backdrilling is achieved.

Figure E-1: Backdrilling Process

Removing the unused part of the via can lead to vast improvements in the quality of the
data being transmitted and received. The plot shown in Figure E-2 illustrates the
improvement that can be achieved by carefully removing most of the via stub. Because
backdrilling is not an exact science, reliable design dictates that there always remain a
small amount of unused via that cannot be backdrilled. The smaller the via stub that
remains, the farther the resonance frequency of the via can be pushed out.

Figure E-2: Backdrilled vs Non-Backdrilled Channel Characteristics

A second way to reduce via effects is to use a blind via or a buried via. Building a
backplane with blind and/or buried vias requires the PCB manufacturer to build and drill
the backplane in multiple stages. First, a standard PTH via is used to connect the signal
from the top layer of a board to the bottom of a board. A second PCB is then attached to the
first to complete the backplane, effectively moving the signal layer from the bottom layer
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