In Circuit Test Fixture Coverage and Unprobed Assembly Netlist Limits
Unprobed netlists drop structural fault coverage, requiring integrated boundary scan vectors and adjusted warranty reserves to cover escape risks.
Trace Stubs
Signal Discontinuity
High-speed circuit boards are vulnerable to a routing feature that consists of an unconnected or terminated length of conductive trace branching off from the main signal path. These routing artifacts, known as trace stubs, can act as transmission line discontinuities that degrade signal integrity by causing signal bounce along the signal line. They are often created during board redesigns, via modifications, or when test points are placed along high-frequency lines.
In multi-gigahertz digital circuits, these stubs act like antennas, generating electromagnetic interference and causing severe signal distortion.
Signal Reflection
At high frequencies, the impedance of the stub varies with frequency and can cause significant attenuation at specific resonant points. This behavior behaves as a notch filter, absorbing energy at the resonant frequency of the stub length. To avoid this, designers use simulation tools to identify trace stubs and analyze their impact on eye diagrams and jitter.
If a stub is longer than a fraction of the signal wavelength, it must be removed or shortened to prevent data errors. For example, a via stub on a thick backplane can be eliminated by backdrilling the unused portion of the via.
Remediation Technique
Shorter stubs are less damaging but still add parasitic capacitance to the trace. Design rules usually limit the maximum allowable stub length to a few mils.