Microstrip Phase Velocity Fundamentals across Multi Layer Glass Stackups
Microstrip phase velocity depends on outer layer inhomogeneous dielectric fields, glass reinforcement architecture, copper roughness, and lamination compression.
TDR Phase Delay Coupon
Timing Characterization
Pulse timing structure provides a standardized test artifact for measuring the time it takes for an electrical signal to travel through a trace. A TDR phase delay coupon is located on the panel edge and mimics the routing geometry used on the actual circuit board. This structure allows technicians to verify the timing performance of the manufacturing lot without probing the functional traces.
Propagation Velocity
Reflection data from a time domain reflectometer reveals the exact moment a pulse enters and leaves the known length of the conductor. The TDR phase delay coupon helps calculate the velocity of propagation, which depends on the effective dielectric constant of the stackup. If the signal arrives later than expected, it indicates that the laminate material or the trace geometry deviates from the design intent.
This measurement is critical for high speed interfaces where the skew between clock and data lines must be minimized.
Batch Qualification
Manufacturing monitoring uses these coupons to ensure consistency across different fabrication runs. A TDR phase delay coupon is a representative sample for the entire panel, demonstrating changes in the lamination pressure or resin flow. If a coupon shows a phase shift outside the allowed window, the entire panel may be rejected to prevent timing failures in the final system.
Automated testers compare the results against a gold standard to provide a pass or fail result in seconds. Using dedicated coupons also avoids the damage to delicate pads that can occur during repeated probing of the main board.