Z Axis Permittivity Variations in Standard High Speed Multilayer Stackups
Out-of-plane permittivity in glass-reinforced multilayer stackups runs lower than datasheet figures, requiring Z-axis test methods to hold impedance.
TDR Trace Extraction
Metrological Measurement
Time-domain reflectometry calculates the localized characteristic impedance of a conductor trace by measuring the reflection of a high-speed electrical pulse. Through tdr trace extraction, engineers analyze the impedance profile of a routed trace to identify physical variations along its length. This technique allows non-destructive testing of internal trace geometry in finished, multi-layer printed circuit boards.
Reflection Analysis
A fast-rise-time step pulse is launched into the trace under test, and the instrument records the reflected voltage waveform as a function of time. Each physical change in the trace, such as a change in width, a via, or a different dielectric constant, produces a distinct reflection. By converting the travel time of the pulse to physical distance, the instrument maps the impedance of the trace from the probe point to the end of the line.
This profiling helps trace fabrication defects to specific locations on the board, such as over-etched regions or layer misalignment.
Resolution Limit
The spatial resolution of this measurement is limited by the rise time of the test pulse and the bandwidth of the test setup. Shorter rise times allow the resolution of smaller features, while longer rise times smooth out the response of localized anomalies.