Understanding Glass Weave Effects in High Speed Differential PCB Stackups
Glass weave skew causes intra-pair phase delay in high-speed differential pairs, requiring spread glass, dual-ply prepreg, or off-axis panel rotation.
TDR Skew Testing
Measurement Verification
Time-domain reflectometry evaluates the timing alignment between conductors in a differential pair by measuring the arrival of reflected pulses. Accurate tdr skew testing identifies physical length mismatches or dielectric variations that cause signal delays. This process involves sending a fast-rising edge down the traces and observing the reflected waveforms.
Any difference in the time of flight between the two conductors indicates a skew.
Quality Control
Fabricators use this diagnostic method to ensure that high-speed boards meet the specified timing tolerances. During tdr skew testing, the probe interface must be carefully managed to avoid introducing parasitic capacitance. The results allow for the verification of serpentine length matching and substrate uniformity.
Compliance with these standards is necessary for the successful operation of multi-gigabit interfaces.
Equipment Setup
High-resolution oscilloscopes with specialized modules are required to resolve small timing differences in the picosecond range. The calibration of tdr skew testing equipment ensures that the measurements are accurate and repeatable across different production lots. Software tools automate the calculation of skew and impedance from the reflected signal data.
This data provides a detailed record of the electrical characteristics of every tested board. Automated testing stations integrate these measurements into the final inspection workflow for mass-produced electronics.