A controlled-impedance requirement must connect a numerical target to a physical structure and a verification method. Specify which nets and layers it applies to, then agree the construction with the fabricator. A note saying all signals are controlled impedance leaves too many design and acceptance decisions unresolved.
Describe every required structure
For each impedance class, list single-ended or differential behavior, target, tolerance, signal layer, reference layer or layers, and representative geometry. State whether solder mask is included in the model. Distinguish ordinary microstrip, stripline and other structures rather than grouping unlike cross-sections under one width.
Approve the stack-up and trace geometry together
The dielectric spacing, material properties, copper thickness and etched shape interact. Review the proposed finished construction and any manufacturer-adjusted width or pair spacing. Confirm that the adjustment remains compatible with component escapes and the board clearance rules.
Define the coupon relationship
A coupon should represent the relevant production structure and use an agreed test method. Record what is measured, where it is measured and how acceptance is determined. A passing coupon supports process verification but does not prove that every connector, pad and via in the complete channel meets its system requirement.
Control changes on repeat builds
Store the approved stack-up with the design release and quote. Require review when the material system, copper or relevant geometry changes. A repeated part number should not hide a changed construction when the electrical design depends on that construction.
Example specification row
Use a row such as: differential interface group; target and tolerance from the interface design; signal layer L3; references L2 and L4; approved width and gap; mask condition; coupon identifier; report required. Fill each field from the reviewed design. Avoid inventing a universal target for unrelated interfaces.
Design review checklist
- Identify impedance classes and affected nets.
- Approve finished construction and permitted geometry changes.
- Define coupon, test and reporting requirements.
- Retain the manufacturing-approved stack-up revision.