Say which board structure the coupon represents
List the signal layer, reference layers, trace width, spacing where relevant and material construction for each required coupon family. Identify single ended and differential structures separately. A generic request for an impedance coupon leaves too much room for a convenient structure that does not address your critical routes.
Discuss whether neckdowns, launches or other discontinuities require separate evaluation. A uniform coupon and a complete application channel answer different questions, so do not combine their acceptance criteria without defining the measurement boundary.
Agree on the measurement setup
Tektronix's TDR guidance explains that fixtures, probes and measurement interpretation affect impedance results. Request the conditions needed to interpret the report, such as the test structure identification, setup and evaluation region, using a procedure agreed with the supplier.
Ask how launch effects and unusable portions of the waveform are handled. The goal is a reproducible comparison with the stated acceptance requirement, not a single unqualified number that different operators could obtain from different regions.
Connect the report to the delivered hardware
Specify how coupon identity maps to the fabrication lot or panel and how results will be retained. Agree on sampling and disposition of failures before production. These are procurement decisions that must be explicit; they should not be inferred from the presence of a coupon drawing.
- Identify each required layer and line geometry.
- Record targets, tolerances and applicable measurement procedure.
- Define report content, traceability and sampling agreement.
- Preserve the approved artwork and stackup with the results.
Interpret passing results within their limits
A passing coupon supports the measured construction under the agreed method. It does not independently verify every route, connector, via transition or complete channel on the board. Use it with the project's design analysis and other necessary electrical measurements.
If results are unexpected, compare the approved geometry, actual construction and measurement setup before changing the design. A disciplined investigation can distinguish a model mismatch from an export error, fabrication variation or inconsistent measurement boundary.
Adjacent-layer access, one connection at a time
Dielectric · geometry + material
Core or build-up dielectric
Illustrative only. Copper thickness, dielectric construction and via spans require a reviewed stackup.
Common questions
Does one coupon cover every impedance on the board?
Only if the agreed plan justifies that representation. Different layers, references or geometries can require distinct structures and acceptance records.
Can a coupon passing impedance prove low insertion loss?
No. Impedance and loss are different characteristics. Define the additional measurements and channel requirements needed for the application.
References & further reading
Method & assumptions · General design guidance. Validate the proposed construction and acceptance criteria for your project.