Controlled-Impedance PCB: What to Put in the Request

An impedance request needs more than “50 ohms” in the order notes. The fabricator must know which structures are controlled and how they relate to the proposed stackup. A clear table reduces the chance of a correct number being applied to the wrong layer or trace type.

Identify each controlled structure

List the target impedance, allowed tolerance, single-ended or differential type, signal layer and reference layer or layers. Add the relevant nets or a clearly marked region. Keep trace width and pair spacing separate: a differential pair specification cannot be described by width alone.

Use the layer names in the actual fabrication package. Labels such as “inner signal” become ambiguous when the stack changes. For example, a request may identify a specific pair group on L3 with the intended adjacent reference layers and point to the corresponding drawing.

Share the calculation basis

Provide the proposed dielectric thicknesses, material assumptions and copper information used by the designer. Include the calculation or field-solver inputs where available. Rogers' design tools illustrate the relationship between material properties, geometry and impedance; a result calculated for one stack should not be transferred to another without review.

The overall board thickness does not uniquely define the dielectric spacing around a signal layer. Ask for the proposed production stackup early enough that geometry changes can still be evaluated.

Agree who may change the geometry

State whether the fabricator may propose width adjustments and who approves them. Review any proposed change against routing space, pad transitions and circuit requirements. Avoid giving blanket approval to modify all traces when only selected nets are controlled.

• Name the controlled nets or geometry classes.
• Record targets and tolerances without mixing differential and single-ended values.
• Attach the proposed layer sequence and material details.
• Request review of any stackup or geometry difference.
• Agree whether coupons, measurements or reports form part of the order.

Close the approval loop

Keep the accepted stackup and impedance table with the released files. If changes require a new layout export, replace the archive as a complete revision rather than leaving two possible versions attached to the order. Ask for clarification when the quote does not say what verification is included. A design target and an agreed production measurement are separate parts of the handoff.

Technical reference: Rogers: impedance design tools. Supplier-specific process limits in the reference are not DIYIPCB manufacturing commitments.