Reading a region-specific rigid-flex stackup

A rigid-flex stackup must be read by region. A layer present in a rigid island may be absent from an exposed flex span, so a single overall layer count is not enough to describe the board. Begin with the region drawing, identify its stack references, and trace each reference through the construction table.

Altium’s official substack documentation describes defining substacks for a rigid-flex design. Use this distinction when reviewing the manufacturing drawing: the global layer list and the construction assigned to one region answer different questions.

Read across regions, then down through materials

First find the identifiers for the rigid islands and flexible spans. Each should point to a specific construction. Then read from top to bottom within that construction, recording the conductive layers, dielectric layers, protective layers, and any locally applied support. Finally, compare neighboring regions to see what changes at their boundary.

For example, imagine a teaching model with six globally named copper layers, L1 through L6. Its rigid region includes all six, while its flexible span includes only L3 and L4. Calling the design “six-layer rigid-flex” leaves out the two-layer flexible construction. This example illustrates documentation logic; it is not a recommended stackup or a claim that a particular construction has been qualified.

Do not silently rename the shared copper

If L3 and L4 become “top” and “bottom” in an isolated flex view, add an explicit cross-reference to the global layer names. Otherwise, a reviewer may mistake a local view label for a different copper layer. This matters especially when a drill span, electrical net, or impedance instruction refers to a layer number.

A practical check is to choose one net crossing a rigid-flex boundary and trace its intended copper layer through each view. If the reviewer has to guess which local label corresponds to the global layer, the documentation needs clarification before output release.

Use a thickness ledger with a defined scope

A useful ledger records what each quoted thickness includes. Ask whether a number describes a purchased base material, a finished conductor, a local adhesive layer, or a finished regional thickness. Avoid adding values with different scopes and treating the result as a production tolerance.

• Record units next to each dimension, not only in an informal message.
• Identify the source and revision of every material value.
• Keep nominal thickness separate from its allowed range.
• State whether a protective or support layer is present in the region being measured.
• Resolve discrepancies between the drawing and the proposed manufacturing stack before approval.

Separate a material description from a finished dimension

A material name answers what a layer is made from, while a finished dimension answers how much space the completed construction occupies at a defined location. Keep these questions separate when reviewing a proposed stack. A purchase description may refer to an incoming film or copper-clad material. A connector drawing may instead require the thickness of the complete finished tail, including layers that are not part of that purchased material.

Create a measurement-location sketch before accepting a total thickness value. Label the exposed flexible span, the reinforced contact area, and the rigid island individually. If a protection layer has openings, explain whether the specified measurement includes a covered area or an exposed contact. The same assembly can legitimately have more than one local finished thickness, so a single value placed beside the overall outline may be ambiguous.

For each dimension, record whether it is nominal, a permitted range, or a minimum requirement. Also identify who supplies the value and which drawing or material document supports it. Do not convert a typical property into a guaranteed production limit. If the supplier’s proposed construction lists only nominal values, request the finished regional acceptance limits before using those numbers in a mechanical clearance calculation.

Work through an illustrative thickness calculation

Consider a simplified teaching ledger for a hypothetical flexible region. Assume two finished conductors, each 18 micrometers thick; a dielectric core of 25 micrometers; and two protective assemblies, each represented as a combined 40 micrometers. The nominal arithmetic is 18 + 25 + 18 + 40 + 40 = 141 micrometers, or 0.141 mm. These values are invented for demonstrating the ledger and do not define a recommended or qualified construction.

Now suppose the corresponding illustrative ranges are 16–20 micrometers for each conductor, 23–27 micrometers for the core, and 36–44 micrometers for each protective assembly. Adding the lower bounds gives 127 micrometers; adding the upper bounds gives 155 micrometers. This is a simple worst-case arithmetic interval. It assumes that all listed values describe the same finished state and that summing their bounds is an appropriate conservative model.

The exercise is useful because it exposes missing definitions. If the protective assembly number already includes an adhesive, adding a separate adhesive row would double-count it. If a conductor value describes starting foil rather than the finished conductor, the total may omit relevant processing effects. If the layers are measured under different conditions, the arithmetic may not represent a physically meaningful acceptance measurement. Resolve those issues before treating the interval as a product requirement.

Read adjacent stacks as a relationship

Do not compare neighboring columns solely by the number of rows they contain. A shared conductor can continue through two regions while outer materials change. Conversely, two similarly named rows can represent different local material arrangements. Altium’s substack documentation distinguishes common and individual material usage, which provides a useful reminder to inspect the relationships as well as the individual lists.

In the six-layer/two-layer teaching model, highlight L3 and L4 in both the rigid and flexible tables. Then annotate what happens to the neighboring dielectrics at the boundary. If the drawing leaves this relationship unresolved, add a transition section or a question for the fabricator. A clean-looking stack graphic should not be accepted as evidence that every termination, overlap, and local material arrangement has been defined.

For a branched design, repeat the exercise for each branch rather than assuming the first reviewed flex span represents all others. Two branches may share a rigid island but serve different mechanical or electrical purposes. Give each unique construction a stable identifier, then explicitly list the regions that use it. This is easier to maintain than copying several nearly identical tables that can drift apart during revisions.

Connect the stack review to electrical checks

A signal reviewer needs more than a copper count. Identify the conductor carrying the signal, the relevant reference conductor, the dielectric separation, and the local geometry assumptions. Where controlled impedance is required, label the target and the region to which it applies. The same target across rigid and flexible regions does not imply that the same trace dimensions will work in both constructions.

Keep electrical model inputs traceable to the proposed manufacturing stack. If the material designation, finished separation, or conductor geometry changes, identify the calculations that need review. Avoid silently substituting a catalog dielectric value for a frequency-dependent modeling input without checking the selected material documentation and the analysis method. Record unresolved assumptions so that a preliminary estimate cannot be mistaken for an approved production result.

Handle substitutions with an impact record

When a proposed material becomes unavailable, start with a comparison of the affected stack rows. Record the original material, proposed replacement, regional thickness implications, electrical-model implications, and mechanical questions. Ask whether the replacement changes only procurement identification or also the physical construction. The word equivalent is not a complete technical disposition unless the relevant requirements and evidence are identified.

Use a controlled revision to connect the accepted proposal to the fabrication package. Recheck dependent drawings, connector interfaces, and manufacturing notes that refer to the old stack. Preserve the earlier version for traceability, but make the current approved revision unambiguous. A useful release record states what changed, which regions are affected, who reviewed the consequences, and which output files now represent the accepted construction.

A three-person review exercise

Give the electrical reviewer the layer map, the mechanical reviewer the finished regional dimensions, and the manufacturing reviewer the proposed construction. Ask each person to identify the same R1-to-F1 boundary and explain what changes there. Record different interpretations as open issues. The exercise is successful when the documents support one consistent interpretation, rather than when all three people merely recognize the board outline.

Keep the accepted stack revision with the fabrication data. If a later material substitution changes the construction, reopen the affected electrical and mechanical checks instead of updating only a purchasing description.

Explore our stackup and materials guide and rigid-flex design guide. To review a proposed construction, send the region drawing and stackup through the inquiry form, including any unresolved layer naming or thickness questions.