Define the measurement section
Draw exactly where thickness is measured and which two surfaces bound it. A section through exposed contacts can differ from one through adjacent coverlay. Mark the contact face and the support face. Use the connector-tail guide to connect this section to the selected connector’s dimensional requirement.
Reinforced-thickness budget
| Feature | Define | Review |
|---|---|---|
| Circuit section | Layers physically present at measurement | Correct local nominal total |
| Support | Material and nominal thickness | Available construction |
| Bondline | Finished thickness assumption | Process-related uncertainty |
| Acceptance | Completed dimension and method | Interface requirement and tolerance |
Keep arithmetic and tolerance separate
Sum nominal constituent values for an initial construction proposal, then assess the finished tolerance using a method agreed with the fabricator. A nominal sum is not a tolerance guarantee. Include uncertainty in the bondline and copper assumptions rather than assigning all remaining thickness to support film. The stackup worksheet can track the arithmetic by region.
Agree the acceptance method
State the condition in which the part is measured, measurement location, and any interface-specific fit check. If a connector supplier defines a finished FPC tolerance, carry that requirement into the drawing rather than only quoting its center value. Ask whether the proposed material combination can meet it and request explicit deviations before order placement. Repeat the review after a laminate, adhesive, or finish change.
Before you release the design
- Locate the exact measurement section.
- Record each constituent in one unit system.
- Separate nominal inputs from finished limits.
- Review fit after material changes.
Watch points
- The support sheet thickness is not the completed reinforced thickness.
Practical questions
Can the stiffener be selected by subtraction?
Use subtraction to propose a nominal option, but review available material combinations and the finished tolerance before treating it as a released specification.
Should every region share one thickness tolerance?
Usually the critical interfaces need their own definitions. State where each limit applies so a free span is not confused with a terminal or supported island.
Technical references
References support the material or design context. Check the exact grade, part number, document revision and project conditions before release.Updated 27 September 2026 · Technical content policy · Engineering guidance supports project review; the agreed drawing defines the build.