Stub Geometry
Unused vertical copper conductive channel segments extending beyond signal layer transition points form unintended open-ended transmission line branches. Fabricators and signal integrity engineers evaluate micro-via stubs during high frequency board design and signal testing. This physical defect excludes functional microvias that terminate cleanly on targeted signal or reference layers.
Resonant Effect
Multilayer board routing often transfers high speed signals between non-adjacent layers using laser-drilled microvias. When blind microvias or stacked via structures extend past the destination signal layer, micro-via stubs act as quarter-wavelength resonant stubs at specific frequencies. High frequency signal energy traveling down the trace enters the stub segment, reflects off the open end and re-enters the primary signal path with a phase delay.
This phase delay creates deep notch filtering resonances in the channel frequency response, severely attenuating specific signal frequencies. Controlled back-drilling or proper blind microvia depth planning eliminates unnecessary copper via tails before final layer lamination. Signal degradation increases with stub length, making short microvia stubs critical for multi-gigabit data transmission.
Layer Discontinuity
Quarter-wavelength notch filtering effects severely degrade high speed serial eye openings. Eliminating unused via length restores channel bandwidth and minimizes signal reflections. Precision laser drilling prevents stub creation in high density interconnect designs.
