Flexible Circuits: A Practical Guide to Flex PCB Types, Materials, and Design Rules
Opening Introduction
A flexible circuit does one thing a rigid board cannot: it moves. It folds into a hinge, wraps around a battery, or survives a million open-and-close cycles inside a phone. Everything else — the base film, the copper, the bend radius numbers, the stiffeners — exists to keep that movement from destroying the copper.
We build flex and rigid-flex boards in volume at JKRGLO, and this is the version of the story we would tell a customer on a call. What the four flex constructions actually are, which base film and which copper to specify, how the IPC-2223 bend radius rules are measured in practice, when rigid-flex beats a plain flex tail, and the design decisions that show up as field failures two years after the build shipped.
Quick answer
A flexible circuit (also called a flex circuit, flex PCB, or FPC) is a printed circuit board built on a thin bendable dielectric film, usually polyimide, instead of rigid FR-4, so it can bend, fold, or flex continuously without cracking its copper traces.
Flexible circuits are made in four standard constructions: single-sided, double-sided, multilayer, and rigid-flex. The base film is polyimide (PI) for anything that sees heat or repeated bending, polyester (PET) for low-stress static parts, and liquid crystal polymer (LCP) for high-frequency work.
The rule that decides whether a flex survives is bend radius, measured as a multiple of the flexible stack thickness: 6× for a single-layer static bend, 12× for double-sided, 24× for multilayer, and 100× or more when the board flexes for life.
A rigid-flex PCB is the hybrid: rigid FR-4 sections carry the components, flexible polyimide sections bend between them, and both are laminated into one board so the cables and connectors disappear.
What Is a Flexible Circuit?
A flexible circuit — also called a flex circuit, flex PCB, or FPC (flexible printed circuit) — is a printed circuit board built on a thin, bendable dielectric film instead of rigid FR-4, allowing it to bend, fold, or flex repeatedly while still carrying electrical signals. The substrate is normally polyimide, which is why a flex board tolerates soldering temperatures and hundreds of thousands of flex cycles while a rigid FR-4 board tolerates neither.
The category has more names than it needs, and the naming is where most misunderstandings start. Four of those names describe the same device. One of them does not.
| Term | What it actually means | Where you see it |
|---|---|---|
| Flexible circuit / flex circuit | The umbrella term for any bendable circuit board, including rigid-flex. | Design reviews, standards documents |
| Flex PCB / flexible PCB | The same device, framed as a board rather than a circuit. No technical difference from the above. | Component supplier pages, sourcing |
| FPC (flexible printed circuit) | The term used across Asian manufacturing and in most datasheets. | Bills of material, drawings |
| FFC (flat flexible cable) | A flat ribbon of parallel conductors with no etched circuitry, no vias, and no components of its own. Straight routing only. | Cable assemblies, connectors |
| Rigid-flex PCB | A single board that laminates rigid FR-4 sections and flexible polyimide sections together. | Aerospace, medical, foldable devices |
An FPC is a real circuit board with etched copper traces, vias, and solderable pads; an FFC is a flat ribbon of parallel conductors with no circuitry of its own. If a design only needs a straight connection between two fixed points, an FFC is cheaper. If the design needs routed traces, controlled impedance, or components mounted on the flex, it needs an FPC.
That distinction matters commercially. We regularly see an RFQ arrive asking for a “flex circuit” when what is needed is a 12-way flat cable, and the quote comes back three times too high because the wrong part was scoped.
Why flexible circuits exist at all
Three problems push engineers off rigid boards and onto flex. Only the first one is obvious.
- Space and weight. A finished flex stack runs roughly 0.05 mm to 0.3 mm thick, against 0.8 mm to 3.2 mm for a typical rigid board. In a smartwatch or a hearing aid that difference is the whole industrial design.
- Reliability. Flex removes connectors and board-to-board cables, and connectors are the failure points in consumer electronics. Replacing three rigid boards, two cables, and four connectors with one flex assembly deletes hundreds of solder joints that used to be able to crack.
- Movement. Only a flexible circuit can sit in a printer carriage, a laptop hinge, a camera gimbal, or a robotic arm joint and flex for the life of the product. Rigid boards do not bend; they crack.
If you want the fabrication side first — how the copper gets onto the film and how the layers are laminated — that is covered step by step in how printed circuit boards are made. The rest of this guide assumes you are choosing the construction, the materials, and the design rules.
What Are Flexible Circuits Made Of?
A flexible circuit is built from four functional layers: a flexible dielectric base film, a copper conductor layer, an adhesive or adhesiveless bond, and a protective coverlay. Stiffeners are added locally where the board must stay rigid. The “flex” in a flexible circuit does not come from the copper — it comes from the base film, and that is where most material mistakes are made.
Base film: polyimide, polyester, or LCP
The base film is the backbone. Choose it first, because the temperature rating and the bend life of the whole board follow from it.
| Base film | Service temperature | Bend life | Relative cost | Use it when |
|---|---|---|---|---|
| Polyimide (PI) | −269 °C to 260 °C | Over 100,000 cycles | High | Anything that sees reflow soldering, sustained heat, or repeated bending. The default for industrial, automotive and medical flex. |
| Polyester (PET) | 70 °C to 150 °C | Around 10,000 cycles | Low | Static, low-stress, cost-driven parts: disposable sensors, single-use test strips, simple static connectors. |
| Liquid crystal polymer (LCP) | Up to about 300 °C | Over 200,000 cycles | 8–10× PI | Millimetre-wave antennas, 5G front ends, and moisture-sensitive multilayer flex where stable dielectric properties matter. |
Polyimide is the standard because it survives lead-free reflow, which peaks near 260 °C. Polyester does not, and that single fact ends most PET-versus-PI debates. We have watched a customer learn it the hard way: an industrial sensor build was switched from PI to PET to save material cost, and the boards cracked within three months inside a 120 °C enclosure. The replacement order and the scrapped assemblies came to roughly $40,000. PET was not a cheaper material. It was the wrong material.
Polyimide (PI) is the correct base film for any flexible circuit that goes through reflow soldering, sees more than about 100 °C in service, or bends more than a few thousand times. Polyester (PET) is appropriate only for low-temperature, low-cycle, cost-driven parts.
Copper: rolled annealed versus electrodeposited
The conductor layer is where the damage shows up first. There are two copper foils in common use and they behave very differently under bending, even when the datasheet thickness is identical.
Rolled annealed (RA) copper has an elongated grain structure that resists fatigue and is used in every dynamic flex zone; electrodeposited (ED) copper is columnar, cheaper, and acceptable only in static, flex-to-install boards. The two are not interchangeable in a moving part, and the copper type is more important than the copper thickness.
- RA copper (rolled annealed): made by rolling copper ingots into foil. The elongated grains tolerate repeated strain, which is why it is mandatory in dynamic flex. Typical thickness for moving parts is 12–18 µm; heavier copper adds stiffness and shortens bend life.
- ED copper (electrodeposited): deposited onto a drum by electrolysis. Cheaper, and its rougher surface bonds well, but the columnar grains crack after relatively few cycles. Fine for a static tail or a rigid board, wrong for a hinge.
- Why thickness matters: copper is the stiffest layer in the stack and sits furthest from the neutral axis, so it carries most of the bending strain. Going from 35 µm to 12 µm roughly triples the achievable bend life at the same radius. If a flex is cracking, the first two questions are always which copper and how thick.
The bond: adhesive versus adhesiveless
Standard flex laminate glues the copper to the polyimide with an acrylic or epoxy adhesive layer. Adhesiveless laminate deposits or casts the copper directly, removing that layer.
| Property | Adhesive laminate | Adhesiveless laminate |
|---|---|---|
| Cost | Baseline | Roughly 20–40% premium |
| Stack thickness | Greater (adhesive adds 15–35 µm per side) | Thinner, so tighter bend radii are achievable |
| Dimensional stability | Looser | Better — matters for fine-pitch and multilayer registration |
| High-frequency performance | Moderate — adhesive dielectric varies | Better, because there is no adhesive dielectric in the path |
| IPC Class 3 eligibility | Class 1–2 only | Required baseline for Class 3 |
For Class 1 and Class 2 work, adhesive laminate is cost-appropriate and fully capable. For medical devices, aerospace, and defence — IPC Class 3 — adhesiveless is not an upgrade option. It is the floor.
Coverlay and stiffeners: the two layers people forget
A finished flexible circuit also carries a protective coverlay over the traces and, usually, one or more stiffeners. Both are structural, not cosmetic, and both get their own section below. The short version: coverlay replaces solder mask everywhere the board bends, and stiffeners restore local rigidity under connectors and components.
Flex PCB Design Rules: Start With Bend Radius
Bend radius is the radius of the inner curve a flexible circuit is bent around, expressed as a multiple of the flexible stack thickness. The stack thickness includes the polyimide base, every copper layer, any adhesive, and the coverlay. Designing below the minimum bend radius is the single largest cause of flex circuit failure.
Three practical rules come before any number. Measure the radius from the inner surface of the bend, not the outer surface and not the neutral axis. Include every layer in the thickness you multiply, coverlay included — leaving the coverlay out of the calculation is the most common arithmetic error we see on customer drawings. And after you land on a number, add 20–30% of margin before you release the layout, because material variation, copper grain orientation, and handling during assembly are not in the formula.
| Application | Flexible layers | Minimum bend radius |
|---|---|---|
| Static / flex-to-install (Use A) | 1 layer | 6× total flex thickness |
| Static / flex-to-install (Use A) | 2 layers | 12× total flex thickness |
| Static / flex-to-install (Use A) | 3 or more layers | 24× total flex thickness |
| Dynamic / flex-for-life (Use B) | 1 layer | 100× total flex thickness |
| Dynamic / flex-for-life (Use B) | 2 layers | 150× total flex thickness |
| Dynamic / flex-for-life (Use B) | 3 or more layers | Not recommended without special engineering review |
Why the dynamic multipliers are so much larger than the static ones. IPC-2223 allows copper to strain roughly 10% in a flex-to-install part but only about 0.3% under repeated cycling. That 30-fold gap in allowable deformation is the whole reason a single-layer static bend needs 6× thickness while a single-layer dynamic bend needs 100×. If a fabricator quotes you a dynamic design at 20–50×, ask for the cycle-count target and the test method in writing, because those multipliers only hold for limited-cycle, low-strain applications.
A worked example
Take a two-layer flex for a static bend, typical industrial build: 25 µm polyimide base, 18 µm copper, 25 µm polyimide, 18 µm copper, plus 25 µm coverlay on each side. Total stack thickness comes to about 0.16 mm; round to 0.2 mm once adhesive is included. Static, two-layer → multiply by 12 → minimum inside radius 2.4 mm. Add 25% margin and you release the layout with a 3 mm radius. If someone later pushes that same board into a foldable hinge, the requirement jumps to roughly 30 mm and the design has to be rebuilt on RA copper with a single flex layer. The number changed by a factor of ten because the application changed, not the board.
Keeping the bend zone clean
A correct radius on a board that is otherwise badly laid out still fails. The bend region has to be an empty corridor:
- No components inside the bend area, on either side. A component stiffens the flex locally and concentrates stress at its solder joints.
- No vias, plated through-holes, or test points in the bend zone. A barrel wall cracks under repeated bending with almost no warning. Keep vias at least 0.5 mm clear of an active bend.
- No solid copper plane and no heavy copper trace inside the bend. Hatched copper if a plane is unavoidable.
- Keep the bend at least 1.25 mm from a rigid-to-flex transition, and start the bend no closer than ten times the flex thickness from the transition edge, so the flex layers can elongate without buckling.
- Traces cross the bend line at 90°, never running along it.
Coverlay and Stiffeners: The Two Parts Beginners Skip
Coverlay, not solder mask, in the bend zone
A coverlay is a layer of polyimide film bonded with adhesive over the copper traces, used instead of liquid solder mask on flexible circuits because it bends without cracking. Liquid photoimageable solder mask is acceptable on the rigid sections of a rigid-flex board, but coverlay is mandatory in every flexing area.
| Property | Coverlay (flex zones) | Liquid solder mask (rigid zones) |
|---|---|---|
| Material | Polyimide film with acrylic or epoxy adhesive | Photoimageable liquid resin |
| Behaviour under bending | Flexes with the circuit; keeps integrity | Cracks and flakes under repeated bending |
| Typical thickness | Polyimide 12.5–25 µm plus 15–35 µm adhesive | 15–30 µm cured |
| Aperture registration | ±0.075–0.15 mm on cut or laser openings | Tighter, which is why it is used for fine-pitch rigid work |
| Where it belongs | Every bend zone, and the whole flex by default | Rigid sections only |
Coverlay is not applied as a liquid, so its openings have to be pre-cut before lamination. That drives three design notes we repeat on almost every DFM report: pre-window the coverlay before lamination so the pad openings register correctly; keep at least a 0.2 mm gap between a pad and an adjacent trace so the opening does not expose copper it should not; and for pad pitches below 0.5 mm, use bridged apertures that leave a web of coverlay between openings instead of one large window.
Stiffeners: forget them and the connector breaks
A stiffener is a rigid plate — polyimide, FR-4, or stainless steel — bonded to a flexible circuit wherever components or connectors mount, so the thin flex does not deform during soldering or mating. A stiffener adds local rigidity without making the whole circuit rigid.
| Stiffener material | Typical thickness | Why choose it | Typical use |
|---|---|---|---|
| Polyimide (PI) | 0.075–0.25 mm | Matches the flex CTE, very thin, no added bulk | ZIF connector fingers, thin-profile products |
| FR-4 | 0.2–1.6 mm | Cheapest way to buy real rigidity | SMT component mounting areas, board-level connectors |
| Stainless steel | 0.1–0.3 mm | Maximum rigidity plus EMI shielding | High-force connectors, space-constrained enclosures |
| Pressure-sensitive adhesive | 25–125 µm | The bonding layer, not the stiffener itself | Attaching any of the above without heat |
The stiffener edge is the detail that gets missed. If a stiffener ends right at the bend line, the stiffness change happens over zero distance and the copper cracks at the boundary. Keep at least a 0.5–1.5 mm transition between the stiffener edge and the start of the bend, and let the coverlay overlap the stiffener edge by roughly 0.76 mm to spread the stress. Stiffeners also need a keepout of their own: never place one where the assembly will be folded.
Trace Routing and Connector Pads on a Flex
Route across the bend, not along it
Traces should run perpendicular to the bend line. A trace running parallel to the bend sits on the highest-stress axis and will crack before any other trace on the board. Constant width and gentle geometry matter more on a flex than on a rigid board, because every sharp feature becomes a crack initiation point.
- Use arcs or 45° angles, never 90° corners. A right angle inside a bend zone concentrates stress at a single point.
- Keep trace width constant through the bend. A sudden width change creates both a mechanical weak point and a signal reflection.
- On multilayer flex, stagger traces between layers instead of stacking them. Aligned conductors on opposite sides act like an I-beam and stiffen the fold.
- Maintain at least 0.15 mm spacing between traces to keep signal integrity and avoid shorts after etching tolerance.
- Use hatched copper rather than solid copper if a plane has to cross or approach a bend. Hatched copper also lets trapped air escape during lamination.
- Terminate traces into pads with teardrops. A teardrop spreads the stress where a trace meets a pad, which is the second-most-common cracking location after the bend itself.
Stop copper delamination and oxidation
Copper on a flex delaminates and oxidises in ways rigid boards rarely do, because the flex is thin, the adhesive is soft, and the board moves. Large solid copper areas trap air under the coverlay during lamination; under heat and pressure that trapped air drives oxidation and lifts the copper. The fixes are boring and effective: hatch the copper, add small solder-mask windows in non-flexing areas so trapped air can vent, and choose a surface finish that protects the exposed copper.
| Surface finish | Best for | Note |
|---|---|---|
| ENIG (electroless nickel immersion gold) | General flex, connector fingers, fine pitch | Flat, long shelf life, stable contact resistance; the usual default for flex |
| OSP (organic solderability preservative) | Cost-sensitive boards assembled quickly | Very flat and cheap, but short shelf life and not suitable for repeated handling |
| Immersion silver | High-frequency flex | Good electrical performance, needs controlled storage |
| Hard gold | ZIF fingers and edge connectors | Wear-resistant for repeated insertion, but not solderable, so it is applied selectively |
Connector pads
Connector pads take the highest mechanical load on a flex assembly, because someone inserts and removes the cable by hand. Use solder-mask-defined pads so the coverlay overlaps the rim of the pad and adds mechanical support. Keep the coverlay overlap on connector pads at 0.3 mm or more. If the part has gold fingers, shortening them by around 0.2 mm from the drawing reduces micro-shorts from laser cutting. Offset pads on opposite sides of the flex when they can be, and put a stiffener behind any ZIF pad area so the mating face stays flat.
One rule worth writing on the drawing: the coverlay must cover the outside of the bend and be continuous across the bend zone. A coverlay window that lands inside a moving region exposes bare copper and guarantees a field failure.
Rigid-Flex PCBs: When a Flex Tail Is Not Enough
A rigid-flex PCB is a single circuit board that laminates rigid FR-4 sections and flexible polyimide sections together, so components mount on the rigid areas and the flexible areas bend to replace cables and connectors. It is not a flex board with stiffeners glued on: the rigid and flex layers are laminated as one unit, and shared copper runs continuously from a rigid zone, through the flex, into the next rigid zone.
That distinction caused a real support case for us. A customer asked for a quote on a “flex board with two stiffeners” and was surprised when the rigid-flex price came back higher than expected. Their actual requirement was a four-layer board with two component islands. Building it as flex-plus-stiffeners would have meant eight solder joints bridging four separate pieces; building it as rigid-flex meant one board with no connectors at all. The board price went up and the assembly cost went down by more than the difference.
| Advantage | What it actually saves you |
|---|---|
| Space and weight | Eliminates board-to-board connectors, cables, and mounting hardware. A rigid-flex assembly is commonly 60–75% lighter than the equivalent rigid-plus-cable build. |
| Reliability | Removes the connectors and cable solder joints that account for most field failures in portable electronics. Fewer joints, fewer failure modes. |
| 3D routing | The board can fold or twist into the space the enclosure actually provides instead of forcing a flat PCB into a curved product. |
| Mechanical strength | The rigid sections carry heavy components and take the mounting loads; the flex sections absorb vibration and shock that would crack a rigid assembly’s interconnects. |
Static and dynamic rigid-flex
Rigid-flex splits the same way plain flex does, and the split is decided early because it drives the whole stack-up:
- Static (flex-to-install): the flex sections bend only while the board is being fitted into the housing, then stay fixed. Common in medical carts, instrument panels, and cameras where a rigid board has to reach a display that sits at an angle. ED copper is acceptable, and the design is faster and cheaper.
- Dynamic (flex-for-life): the flex sections cycle during normal use — a foldable phone hinge, a robotic arm, an automotive sensor on a moving suspension. This demands RA copper, a generous bend radius, a symmetric stack-up, and a fabricator who has actually built dynamic rigid-flex before.
Typical rigid-flex constructions use four, six, or eight total layers, with the flexible polyimide layers placed near the centre of the stack-up and the rigid FR-4 sections carrying the components. The flexible layers are usually sequenced without flow prepreg at the boundaries, so the flex section is not flooded with resin during lamination.
If your application is a hinge that cycles daily, the flex is the bottleneck rather than the rigid sections, and the design conversation moves to flex PCB bottlenecks in humanoid robots and to HDI boards for robotics where trace density and layer count are pushed harder.
Designing and Building Rigid-Flex Boards
Design considerations
- Materials. FR-4 for the rigid sections (durable, cost-effective, familiar to every fabricator) and polyimide for the flexible sections (bendable, heat-resistant). Both must tolerate the operating temperature and the environment, and the pair has to be CTE-compatible so the lamination does not shear as it cools.
- Stack-up balance. Place the flex layers near the centre of the build, keep the layer count even, and mirror the structure either side of the neutral axis. An unbalanced rigid-flex stack warps during lamination and cannot be pressed flat afterwards.
- Bend and flexing. Give every flex section an appropriate radius, and add rounded corners and teardrops at the rigid-to-flex transition. Sharp inside corners at a transition are where rigid-flex boards break.
- Component placement. Components go on the rigid sections only. A component anywhere near a flex zone adds stiffness and puts its solder joints under cyclic stress.
- Trace routing. Cross the bend line perpendicular, keep traces wide enough to survive cycling, and stagger them between layers.
- Impedance control. For high-speed nets, hold trace width, spacing, and dielectric thickness consistent across the rigid and flex sections, and define which reference plane supports the signal in each segment. A reference-plane change at a transition is a common source of reflected energy and EMI.
- DFM early. Send the stack-up, not just the Gerbers, to the fabricator before the layout is frozen. Rigid-flex tooling and lamination sequences are not something to discover on the day of the build.
Manufacturing sequence
- Material preparation. Select and cut the FR-4 and polyimide panels, and inspect and clean the polyimide film.
- Imaging and etching. Pattern the copper on the flex layers lithographically and etch away the unwanted copper.
- Lamination. Bond the rigid and flexible layers under heat and pressure, using no-flow prepreg at the flex boundary so resin does not bleed into the bending area.
- Drilling and plating. Drill the vias and plate them with copper to connect the layers electrically.
- Coverlay and solder mask. Laminate polyimide coverlay over the flex sections, apply solder mask and silkscreen to the rigid sections.
- Surface finish, cutting, and form. Finish the exposed pads, then laser-cut or rout the outline and free the flex openings.
- Test and inspect. Run electrical continuity and isolation test on every board, plus AOI and dimensional checks on the flex openings and stiffener placement.
The rigid-to-flex transition is where most rigid-flex failures begin. Keep plated holes at least 1.25 mm from the transition for Class 2 designs and 1.5 mm for Class 3, and start the bend no closer than ten times the flex thickness from the transition edge.
Where Flexible Circuits Are Used
The four industries below consume the large majority of flex production, and each one stresses the board differently. Read the column on the right as the reason the flex cannot be replaced by a rigid board plus a cable.
| Industry | Typical flex application | Why flex wins |
|---|---|---|
| Wearable & medical | Smartwatch bands, heart-rate and antenna flex, continuous glucose monitor patches, portable ECG, endoscope and ultrasound probes | The circuit has to fit a curved, skin-contact form factor and survive daily movement without a bulge or a pressure point. |
| Automotive & EV | Infotainment and curved dash displays, dynamic LED tail and contour lighting, battery management system harness, ADAS radar interconnect, seat pressure sensing | Replaces bulky discrete wiring, follows the shape of a lamp or a cockpit, and holds up under under-hood temperature swings and vibration. |
| Aerospace & satellites | CubeSat power and signal stacks, avionics interconnect inside narrow equipment slots | Every gram saved is launch cost, and the circuit must survive extreme thermal cycling and in-flight vibration. |
| Foldable & compact consumer | Foldable phone hinge flex, portable console button and screen flex, TWS earbud internals | The board flexes hundreds of thousands of times over the product life, which only a fatigue-designed flex can do. |
The pattern across all four is the same: the product shape decides the layout, not the other way round. Where a rigid design would force a flat rectangle into a curved product and bridge the gaps with cables, the flexible circuit becomes part of the structure and removes the cables entirely. Rigid-flex takes that one step further by keeping the component islands rigid and turning the connection between them into part of the board.
Flex, Rigid, or Rigid-Flex: How to Choose
Choose a rigid PCB when the board never moves and component density matters most; choose a flex PCB when the board must bend, fold, or fit a curved space; choose rigid-flex when you need both and want to delete the connectors and cables between them.
| Factor | Rigid PCB | Flex PCB | Rigid-flex PCB |
|---|---|---|---|
| Substrate | FR-4 fibreglass | Polyimide or polyester film | FR-4 rigid sections plus polyimide flex sections |
| Flexibility | None | High | Selective, by zone |
| Component support | Full | Needs stiffeners | Full on the rigid zones |
| Connectors and cables needed | Yes, between boards | Yes, to reach rigid boards | No — the flex replaces them |
| Relative board price | Lowest | Higher | Highest |
| Best fit | Component-dense, static products | Cable replacement, curved spaces, dynamic flexing | Multi-board integration, 3D packaging, high-reliability products |
The comparison that actually decides a project is not the board price, it is the total system cost. A rigid-flex board that replaces three rigid boards, two flex cables, and four connectors also removes the connector cost, the cable cost, the assembly labour, and a set of solder joints that used to be able to fail. Below about a few thousand units the connector-based design usually wins on price; at production volume the rigid-flex build frequently wins on total cost, and almost always wins on field reliability.
Three questions that settle it in practice
- Does the board move after installation? No → rigid. Once, to fit → static flex or static rigid-flex. Repeatedly → dynamic flex with RA copper, and a bend radius an order of magnitude larger.
- How much routing sits in the bend? One or two signals can travel on a single flex layer. Twelve layers of dense routing cannot bend, and should stay on a rigid section, with the flex reserved for the interconnect.
- How many separate boards are there? One → the choice is rigid versus flex. More than one, joined by cables → price the rigid-flex version before assuming it is too expensive.
If you are also choosing a manufacturing partner, the same discipline applies: the fabricator needs to review the stack-up, not just the outline. how to choose a PCBA manufacturer covers what to ask, and circuit board components covers the components that will end up mounted on the rigid sections.
Frequently Asked Questions
1. What is a flexible circuit?
A flexible circuit is a printed circuit board built on a thin bendable dielectric film, usually polyimide, instead of rigid FR-4. It carries etched copper traces, can include multiple layers, and bends, folds, or flexes without cracking its conductors. It is also called a flex circuit, flex PCB, or FPC.
2. How many types of flexible circuits are there?
There are four standard types: single-sided, double-sided, multilayer, and rigid-flex, which correspond to IPC-2223 types 1 through 4. Each type is also classified as static (flex-to-install) or dynamic (flex-for-life), and that second classification changes the materials and the bend radius more than the layer count does.
3. What are flexible circuits made of?
A flexible circuit is made of four functional layers: a flexible dielectric base film such as polyimide, a copper conductor layer, an adhesive or adhesiveless bond between them, and a polyimide coverlay that protects the traces. Stiffeners made of polyimide, FR-4, or stainless steel are added locally where components or connectors mount.
4. What is the minimum bend radius for a flex PCB?
Per IPC-2223, the minimum static bend radius is 6 times the total flex thickness for a single layer, 12 times for double-sided, and 24 times for multilayer flex. For dynamic flexing the multiplier rises to 100 times for a single layer and 150 times for double-sided, because allowable copper strain drops from roughly 10% to 0.3% under repeated cycling. Measure from the inner surface and add 20–30% margin.
5. What is the difference between a flex circuit and an FFC?
A flex circuit (FPC) is a real circuit board with etched copper traces, vias, and solderable pads, and it can carry mounted components. An FFC is a flat flexible cable made of parallel conductors with no etched circuitry or components, used only for straight connections. An FFC is cheaper when a straight connection is all you need.
6. What is the difference between a flex PCB and a rigid-flex PCB?
A flex PCB is flexible over its whole area and is built entirely on polyimide film. A rigid-flex PCB combines rigid FR-4 sections with flexible polyimide sections in one laminated board, so components mount on the rigid areas and only the flex areas bend. Rigid-flex costs more per board but can remove connectors and cables entirely.
7. Can you mount components on a flexible circuit?
Yes, but never inside a bend zone. Components go on stiffened, flat areas of the flex, or on the rigid sections of a rigid-flex board. Mounting a component where the board bends stiffens the flex locally and concentrates stress at its solder joints, which is a predictable failure.
8. Why do flexible circuits use coverlay instead of solder mask?
Because liquid solder mask cracks under repeated bending while polyimide coverlay flexes with the circuit. Coverlay also provides better insulation, abrasion resistance, and mechanical support in a moving part. On a rigid-flex board, solder mask is used on the rigid sections and coverlay is mandatory on the flex sections.
9. What copper is used in flexible circuits?
Rolled annealed (RA) copper is used in every dynamic flex zone because its elongated grain structure resists fatigue, and it is typically specified at 12–18 µm for moving parts. Electrodeposited (ED) copper is cheaper and acceptable for static, flex-to-install boards, but it cracks after relatively few flex cycles.
10. How long do flexible circuits last?
Service life is a function of the copper type, the bend radius, and the number of cycles, not of the flex itself. A well-designed static flex lasts the life of the product, and a dynamic flex can be qualified for hundreds of thousands to tens of millions of cycles when it uses RA copper and respects the IPC-2223 dynamic bend radius. A flex that ignores those rules can fail within weeks.
Get Your Flex Stack-Up Reviewed Before You Build
The expensive flex problems are all decided on paper: the wrong base film, ED copper in a moving part, a bend radius that ignores the coverlay, a stiffener that ends at the bend line. All four are cheap to fix before the layout is released and expensive to fix after a production run has shipped.
JKRGLO has been building PCB and PCBA under one roof since 2012, including 1–4 layer flexible boards on polyimide with in-house SMT assembly. Send us the stack-up, the Gerbers, and the bend requirement, and our engineering team will run a DFM review alongside your quote. If a design is about to fail, we would rather tell you at quotation stage.
send us your stack-up and Gerbers for review · flex and rigid-flex fabrication services · capabilities and certifications
JKRGLO is a one-stop PCB and PCBA manufacturer operating since 2012, combining flex and rigid-flex fabrication, component sourcing, and assembly under one roof.
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About the Author
Andrew Zhan – R&D Engineer at JKRGLO.com
With over 14 years of R&D engineering experience in the PCB industry, Andrew Zhan brings deep expertise in circuit design optimization and DFM implementation. He focuses on translating complex design requirements into production-ready solutions, driving technical innovation for both prototype and mass production.
Working closely with cross-functional teams and clients, he ensures rigorous quality validation and seamless technical communication across the full project lifecycle. Andrew is committed to delivering reliable, high-performance PCB solutions that consistently exceed customer expectations.
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