How Do You Design a Rigid-Flex PCB in Altium Designer?

Detailed Explanation

Rigid-flex PCBs combine rigid, component-mounting sections with flexible interconnect sections in a single continuous board — replacing what would otherwise be separate rigid boards joined by connectors and cable. Altium Designer supports this as one PCB document with a variable layer stackup, rather than requiring separate board files stitched together afterward, using the Layer Stack Region tool as the central mechanism. Everything else in the workflow — schematic entry, library components, the Update PCB Document/ECO process — is the same as any standard Altium board; the work specific to rigid-flex is almost entirely in the PCB document's mechanical definition. See the Altium topic for the full set of Altium workflow guides.

Layer Stack Regions

Altium's core rigid-flex mechanism is the Layer Stack Region: a defined area of the board where a different subset of the overall layer stack is physically present. A typical two-rigid-section, one-flex-section design has three regions — two "rigid" regions carrying the full layer count (signal layers, ground/power planes, solder mask, silkscreen) and one "flex" region where only the flexible dielectric and its copper layer(s) continue through, with the rigid-only layers (typically FR4 core, solder mask over the flex area, and any planes not needed on the flex) dropped out. The main Layer Stack Manager defines the overall stack across every possible layer; each Layer Stack Region then specifies which of those layers are actually present within its boundary, and Altium's 3D preview renders the resulting variable-thickness board so the mechanical transition between regions can be checked visually before fabrication.

Bend Lines and Bend Areas

Two related but distinct mechanical objects define where the board actually flexes:

• Bend line — a reference line marking the theoretical fold axis, used primarily for clearance checking and documentation call-outs on the fabrication drawing.
• Bend area — the real zone of the flex region that will flex or fold, sized to reflect the actual minimum bend radius the flex material and its copper layers can tolerate without cracking, not an idealised zero-width line. Trace routing rules — such as routing traces perpendicular to the bend axis rather than parallel to it, which distributes the flex stress across many traces instead of concentrating it along a single trace's length — are applied within this area specifically.

Both are defined against the board outline and layer stack regions, and both need to be finalised before routing begins in the flex zone, since retrofitting bend geometry after traces are already routed usually means re-routing whatever crosses the bend area.

Component and Via Placement Restrictions

The flexible substrate cannot reliably support component placement and reflow soldering without added local stiffening, so components are placed on rigid regions almost without exception — see the FAQ above for the local-stiffener exception. Vias inside a bend area are a similarly common restriction: a plated through-hole via sitting inside a repeatedly-flexed zone is a stress concentration point and a common site of copper fatigue cracking over the product's flex-cycle lifetime, so most rigid-flex design rule sets prohibit or heavily restrict vias within the bend area, keeping any layer transitions on the rigid sections either side of the flex zone instead.

Coverlay and Stiffeners

Where a rigid PCB uses solder mask over exposed copper, a flexible section uses coverlay — a flexible polyimide film laminated over the flex circuit's copper in place of conventional solder mask, since rigid solder mask would crack under repeated flexing. Coverlay openings are defined for exposed pads exactly as solder mask openings are for a rigid board, but as a distinct layer in the stackup. Stiffeners — small localised additions of FR4, polyimide, or stainless steel bonded to specific areas of the flex section — reinforce connector-mounting zones or any component location, and edge-launch areas that need enough rigidity for insertion or handling without extending a full rigid region over that zone. Both are defined per-region within Altium's layer stack management, alongside the rigid/flex Layer Stack Regions themselves.

Fabrication Output for Rigid-Flex

Rigid-flex fabrication data needs the same base deliverables as any board — Gerber/ODB++/IPC-2581, drill files, and Draftsman fabrication and assembly drawings — plus the layer-stack-region and bend-line geometry made explicit for the fabricator, since a rigid-flex fabricator needs to know exactly which layers are present in which zone and where the material transitions occur. This additional documentation is one of the reasons rigid-flex fabrication drawings are typically more involved than an equivalent rigid board's, and why working closely with the target fabricator's own rigid-flex design guidelines early — rather than finalising the stackup independently and hoping it matches their process capability — avoids costly re-spins.

Practical Examples

A wearable health-monitor product uses a two-rigid-section, one-flex-section design: the main MCU and radio on one rigid section, a battery and charging circuit on the other, connected by a flex region routed around the product's curved enclosure — eliminating the board-to-board connector and ribbon cable that a two-separate-boards approach would otherwise require.

A folding or hinged industrial device routes a rigid-flex interconnect directly through the mechanical hinge itself, with the bend area's trace routing oriented perpendicular to the fold axis and no vias placed anywhere within the flexing zone, so the interconnect survives the product's full rated number of open/close cycles without a conductor fatigue failure.

Design Considerations

• Finalise bend lines and layer stack regions before detailed routing begins. Routing the flex zone against a bend geometry that later changes typically means re-routing everything that crosses the affected area — treat the mechanical definition as a precondition for flex-zone routing, not something to adjust afterward.
• Route flex-zone traces perpendicular to the bend axis wherever the design allows it. This distributes flex-induced mechanical stress across many short trace segments rather than along the length of a trace running parallel to the fold, meaningfully improving flex-cycle survival.
• Avoid vias inside the bend area entirely. Route any needed layer transitions onto the adjacent rigid sections instead — a via inside a repeatedly-flexed zone is a well-known site of copper fatigue cracking over the product's service life.
• Add local stiffeners under connectors and any component that must sit near a flex boundary, rather than assuming the coverlay alone provides adequate mechanical support during connector mating cycles or handling.
• Engage the fabricator's rigid-flex design guidelines early, since minimum bend radius, stack-up material choices, and coverlay registration tolerances vary meaningfully between fabricators and are considerably harder to renegotiate after the stackup is finalised than before. Zeus Design designs rigid-flex boards in Altium Designer for space- and weight-constrained products, from stackup definition through to fabrication liaison.

Common Mistakes

• Defining the bend area as a zero-width line rather than a properly sized zone. This produces a design that looks correct on screen but doesn't reflect the actual minimum bend radius the flex material can tolerate, risking a fabricated board that cracks at first flex.
• Placing a via or component footprint that clips into the bend area because the Layer Stack Region boundary wasn't checked against final component and routing placement — a straightforward DRC-catchable mistake if the rigid-flex-specific rules are actually enabled and scoped to the bend area, not just the layer stack region boundary.
• Treating coverlay openings like an afterthought copy of the solder mask layer rather than defining them explicitly for the flex stackup, producing exposed-pad mismatches that only surface at assembly.
• Finalising the stackup without checking it against the target fabricator's rigid-flex capability, discovering only at quote or DFM review stage that the specified bend radius, material stack, or coverlay registration tolerance isn't something that fabricator can actually build.
• Retrofitting bend geometry after routing is largely complete, rather than defining Layer Stack Regions and bend areas before detailed flex-zone routing begins, which usually forces a costly re-route of everything crossing the affected area.