Impedance Coupon
A specialized test structure is placed on the border of a fabrication panel to represent the trace geometry and dielectric configuration of the active circuit board. This time domain reflectometry coupon z measures the characteristic impedance of the conductive traces on each layer to verify that they fall within the specified design limits. The test utilizes a high speed step generator that launches an electrical pulse down the test trace, and the instrument measures the reflected signal amplitude.
This measurement allows the manufacturer to verify trace performance without probing the dense circuits of the actual board.
Wave Propagation
The electrical pulse propagates along the coupon trace and encounters variations in trace width or dielectric thickness. Reflectometry instruments track these variations in real time by graphing impedance against the physical length of the trace. This technique identifies localized defects or layer mismatches that would degrade signal integrity in high frequency applications.
Fabrication Tolerance
Discrepancies between the design impedance and the measured impedance arise from variations in the etch factor or dielectric spacing. Plating deviations of just a few micrometers will alter the trace width and cause the board to exceed the acceptable tolerance. Controlling these parameters ensures that high-speed communication lines operate without signal reflections.
