What is a Rigid Flex PCB?
Rigid flex PCBs represent an advanced evolution in printed circuit board technology, combining the structural strength of rigid PCBs with the adaptability of flexible circuits (FPCs). By integrating rigid and flexible substrates into a unified structure, engineers can achieve both mechanical durability and controlled flexibility within a single interconnection system. Among all PCB configurations, rigid flex boards demonstrate exceptional resistance to harsh environmental conditions, making them particularly suitable for demanding applications in industrial control systems, medical devices, consumer electronics, and military equipment.
At QFPCB, we continue to expand our rigid flex PCB manufacturing capacity, with rigid-flex products now accounting for more than 23% of total production volume. This strategic growth reflects increasing market demand for high-reliability interconnect solutions. The core advantage of rigid-flex PCBs lies in their ability to combine the high strength, multilayer routing capability, and structural stability of rigid boards with the bendability and space-saving characteristics of flexible circuits. Designers can eliminate traditional wire harnesses and connectors, thereby reducing interconnection complexity and improving system integration.
Compared with conventional wiring harness assemblies, rigid-flex PCBs offer significantly longer service life, improved mechanical reliability, and enhanced environmental resistance. The integrated structure minimizes risks associated with wire breakage, oxidation, and delamination, leading to greater long term stability. As a result, rigid-flex technology plays a critical role in enhancing overall product performance, reliability, and compact system design.
Key Considerations for Rigid-Flex PCB Design
Designing a reliable and high performance rigid-flex PCB (R-FPCB) requires engineers to evaluate electrical, mechanical, and manufacturing factors simultaneously. The following considerations play a critical role in ensuring optimal performance, durability, and manufacturability.
a. Bend Radius
Engineers must carefully define the bend radius of the flexible section. The bend radius represents the minimum radius at which the flex area can bend without damaging copper traces or the substrate. Designers should determine this value based on the selected material system, copper thickness, and expected bending cycles.
b. Layer Stack-Up
Designers must carefully plan the layer stack-up to balance electrical performance and mechanical reliability. The stack-up defines how rigid and flexible layers are arranged within the structure. A well optimized stack-up supports required interconnections while maintaining structural integrity and controlled impedance. Proper design also helps prevent cracking, delamination, and mechanical stress concentration.
c. Component Placement
Engineers should place components only within rigid areas unless the design specifically allows flex mounted parts. They must also evaluate 3D enclosure constraints and the flex to installation geometry to avoid mechanical interference or stress during assembly.
d. Trace Routing
Routing on rigid-flex PCBs requires special attention in bend areas. Designers should route traces perpendicular to the bend line when possible and avoid sharp angles. They must maintain sufficient spacing between traces and vias to prevent short circuits or copper fatigue during repeated bending.
e. Copper Weight and Plating
Engineers must select appropriate copper thickness and plating parameters based on current carrying requirements, impedance control, and mechanical durability. Excessive copper thickness in flexible regions may reduce bendability, while insufficient copper may compromise electrical reliability.
f. Thermal Management
Designers must implement effective thermal management strategies, especially when high power components generate significant heat. They can use thermal vias, optimized trace widths, copper planes, and heat sinks to improve heat dissipation and prevent localized overheating.
g. Reliability Testing
Engineers should validate the design through rigorous testing and simulation. Typical evaluations include thermal cycling, humidity exposure, vibration testing, and dynamic bend testing to ensure long term reliability under real world conditions.
h. Design for Manufacturability (DFM)
Designers should collaborate closely with the rigid-flex PCB manufacturer, such as QFPCB, during the early design phase. They must consider panelization strategy, alignment features, fiducial placement, minimum trace/space capabilities, and assembly requirements to ensure smooth production.
i. Environmental Considerations
Rigid-flex PCBs often operate in harsh environments, including automotive and aerospace systems. Designers must account for moisture resistance, corrosion protection, vibration, and thermal cycling when selecting materials and defining structural parameters.
j. Material Selection
Engineers must carefully select materials for both rigid and flexible sections. Differences in coefficients of thermal expansion (CTE) between materials can introduce stress during temperature fluctuations. Choosing compatible material systems improves long-term reliability.
k. Signal Integrity and EMI Control
Maintaining signal integrity in flexible sections can be challenging. Designers should carefully manage impedance control, return current paths, and ground reference continuity, particularly near bend regions. They must also implement EMI mitigation techniques such as ground planes and shielding where necessary.
l. Connector Placement
Engineers must strategically position connectors at transitions between rigid and flexible areas. Poor placement can introduce mechanical stress, weaken solder joints, and reduce long term reliability. Proper strain relief design significantly improves durability.
QFPCB Rigid Flex Boards Manufacturing Process
After completing the fabrication of the FPC flexible circuits, QFPCB proceeds with the rigid-flex PCB manufacturing process as follows:
1. Punching
Technicians punch alignment holes in the FR-4 cores and prepreg (PP) sheets. These alignment holes differ from standard through holes and are used to ensure precise layer registration during lamination. After punching, they perform a browning (oxide treatment) process to enhance bonding strength between layers.
2. Riveting and Lay-Up Preparation
Engineers stack the copper clad laminates (CCL), prepreg (PP), and FPC circuits in the required sequence and align them accurately. While the traditional method required step-by-step lamination, QFPCB optimizes the process by completing the stacking in a single operation, improving efficiency and reducing production time.
3. Lamination
During lamination, operators integrate the base materials into a unified rigid-flex structure. They place a copper clad laminate and prepreg at the bottom, position the previously fabricated FPC layer above it, add another prepreg layer, and then place the top copper clad laminate.
After stacking all materials in sequence, they laminate the structure under controlled heat and pressure to form the rigid-flex PCB panel.
4. Trimming
Operators remove excess material from the panel edges where no circuitry is required. They then measure dimensional stability to evaluate material expansion and contraction. Because polyimide (PI) used in flexible circuits exhibits dimensional movement, careful control at this stage ensures overall alignment accuracy.
5. Drilling
Technicians drill through holes, vias, and other required holes according to the PCB design specifications. They strictly control drilling parameters to ensure positional accuracy and hole quality.
6. Desmear and Plasma Cleaning
After drilling, operators remove resin smear generated during the drilling process. They then use plasma treatment to clean the hole walls and board surface, improving metallization quality and adhesion.
7. Electroless Copper (Immersion Copper)
Technicians deposit a thin layer of copper onto the hole walls through electroless copper plating. This step establishes electrical conductivity between layers and enables through hole metallization.
8. Panel Plating
Operators electroplate additional copper onto the panel surface and inside the holes to increase copper thickness to the required specification
9. Outer Layer Imaging (Dry Film Application)
Technicians apply dry film photoresist to the panel surface and transfer the outer layer circuit pattern onto the copper. After development, they inspect the pattern before proceeding.
10. Pattern Plating
Operators perform pattern electroplating to build up copper thickness on the defined circuit traces and through hole areas. They control plating current density, time, and thickness according to design requirements.
11. Alkaline Etching
They remove unwanted copper using an alkaline etching process, leaving only the required circuit patterns.
12. Solder Mask Printing
Technicians apply solder mask (commonly green) to protect the circuit and prevent solder bridging. After printing, they inspect the panel for coverage quality and defects.
13. Coverlay Opening (Rigid-Flex Exposure)
Operators use laser cutting to open the coverlay in the designated flexible areas. This process exposes the FPC section while maintaining rigidity in the required regions.
14. Curing
They cure the solder mask and related coatings through controlled baking to ensure proper adhesion and durability.
15. Surface Finishing
At this stage, the rigid-flex PCB structure is complete. Technicians apply a surface finish such as ENIG, OSP, or other specified treatments to protect the copper from oxidation and mechanical wear while ensuring good solderability.
16. Legend (Silkscreen) Printing
Operators print component markings, reference designators, and product information on the rigid sections of the board.
17. Electrical Testing
Engineers conduct electrical testing to verify circuit integrity. Tests typically include open/short testing and impedance verification, based on customer requirements.
18. Final Inspection
Quality control personnel perform a comprehensive visual and dimensional inspection to ensure the product meets all specifications.
19. Packaging and Shipping
QFPCB packages the rigid-flex PCBs individually in protective bags and uses vacuum packaging to prevent moisture absorption and oxidation before shipment.
Rigid Flex PCB Cost Factors
Rigid-flex PCBs tend to cost more than standard rigid PCBs due to the specialized materials, processes, and lower fabrication volumes. Here are some of the key factors that influence rigid-flex PCB pricing:
1. Layer Count
Adding more conductive layers increases material costs, lamination complexity, and fabrication difficulty. High layer count rigid-flex PCBs cost exponentially more than 2-4 layer versions.
2. Panel Utilization
Rigid-flex PCB panels often have lower utilization due to complex board geometries. Less PCB area per panel drives up cost. Tight panel layout Optimization is critical.
3. Finishing and Coatings
The specialized solder mask, coverlay, and surface finish add cost compared to baseline FR-4 finishing. Thick copper, buried vias, and other techniques also increase cost.
4. Flexible Material Type
The flexible dielectric material choices like polyimide, LCP, PEN drive cost. More durable and heat resistant flex materials are more expensive.
5. Registration Accuracy
The precision alignment of layers and drilling/routing accuracy requirements affect cost. Tighter tolerances require advanced equipment and processes.
6. Design Complexity
Dense routing, high component counts, HDI features, and impedance control requirements increase fabrication difficulty and cost.
7. Low Volume
The overall smaller market for rigid-flex PCBs prevents economies of scale. Shorter fabrication runs increase cost per board.
8. Testing
Rigid-flex PCBs require extensive inspection and electrical testing to validate quality. This adds cost compared to basic PCB qualification.
In addition to fabrication costs, there are engineering costs associated with specialized rigid-flex design, simulation, prototyping, documentation, and qualification.
Commonly Used Materials in Rigid-Flex PCB Manufacturing
Manufacturers build rigid-flex PCBs by combining rigid and flexible materials, with each material playing a critical role in achieving the required performance, reliability, and long term durability. The following materials are commonly used in rigid-flex PCB manufacturing:
1. Rigid PCB Materials
FR-4 (Flame Retardant-4):
- A widely used material for the rigid sections of rigid-flex PCBs.
- Made from woven fiberglass and epoxy resin, it offers high mechanical strength, excellent electrical insulation, and cost effectiveness.
- Suitable for multi-layer designs due to its excellent heat resistance and dimensional stability.
Polyimide (PI):
- Used for enhanced thermal and mechanical properties in high performance applications.
- Offers superior heat resistance compared to standard FR-4, making it ideal for demanding environments.
2. Flexible PCB Materials
Polyimide (PI):
- The most common substrate material for the flexible sections.
- Known for its flexibility, thermal stability, and excellent electrical properties.
- Thin and lightweight, making it suitable for compact and foldable designs.
Glued material: copper foil + adhesive layer + substrate:
Adhesiveless Copper-Clad Laminates:
- A flexible copper layer directly laminated onto the polyimide substrate without using adhesive.
- Provides better flexibility, reduces overall thickness, and improves reliability in high frequency applications.
Glueless material: copper foil + base material:
SF202 Double Side Adhesiveless FCCL:
3. Adhesive Systems
Acrylic Adhesive:
- Used for bonding rigid and flexible layers.
- Offers good bonding strength and flexibility, suitable for medium performance requirements.
Epoxy Adhesive:
- Provides higher heat resistance and dimensional stability compared to acrylic.
- Often used in high-performance or high temperature applications.
Adhesiveless Construction:
- Some advanced rigid-flex PCBs use adhesive free bonding for improved flexibility, thinner profiles, and enhanced electrical performance.
4. Conductive Materials
Manufacturers primarily use copper as the conductor material in rigid-flex PCB fabrication because it offers excellent electrical performance, high ductility, and good processability. For circuit formation, they typically select two types of copper foil: electro deposited (ED) copper foil and rolled annealed (RA) copper foil. Each type is available in various thicknesses and copper weights to meet different design requirements.
Before using these foils in rigid-flex PCB production, manufacturers apply surface treatments to improve adhesion between the copper and dielectric materials. These treatments also enhance bond strength, reduce the risk of bond degradation, and provide oxidation resistance.
In specialized applications, manufacturers sometimes use constantan foil for constantan-based flexible circuits (constantan FPC) when stable resistance characteristics are required.
Electro Deposited (ED) copper
Electro Deposited (ED) copper is manufactured by depositing copper foil using the electrolysis method. Electricity is passed through solutions containing copper compounds through cathode and anode. The copper from the anode gets oxidized and pure copper metal gets collected on the cathode. This pure copper from the cathode is then placed onto a rolling cylindrical metallic rod to form a thin layer of copper foil. This Thin layer of copper is brittle and has a rough surface which makes it rigid. Moreover, the rough surface of ED copper also causes a relatively high insertion loss when compared with RA copper, ED copper is most often used in rigid PCB boards.
Rolled Annealed (RA) copper
Rolled Annealed (RA) copper foil is made by subjecting a copper strip through a rolling mill until it reaches its desired thickness. RA copper has a smoother surface which makes them ideal for use in flexible circuitry. The smoothness of the surface will cause low insertion loss of high frequency signals. Insertion loss is kept to a minimum, to obtain the best performance.
RA Copper or Rolled Annealed copper is a popular choice for rigid-flex PCBs, flex circuit manufacturing, and bendable boards. RA Copper foil has excellent extensibility/flexibility up to 20% to 45% making it ideal for flexible PCBs. However, they are considerably more expensive than ED Copper.
5.Rigid-Flex PCB Surface: Coverlay vs. Solder Mask
The surface of rigid-flex PCBs is comprehensively coated using protective films, which is called protective coating. This helps rigid-flex PCB resisting chemicals, oils, hydrocarbon solutions, dust, and other contaminations. The protective coating is selected after understanding the types of materials used in the rigid-flex PCB assembly, compatibility of PCB components with the coating material, and most importantly the application areas. The most commonly used forms of coating are:
Coverlay: When a flexible film like polyester or polyimide is combined with a suitable adhesive, the resulting product is a cover layer (coverlay). Coverlay has three major roles to play in a rigid-flex PCB assembly: (1) to provide comprehensive protection to the entire assembly. (2) to access circuitry areas like circuit pads for further processing. (3) to augment the reliability and resilience of the circuitry.
Solder Mask: Unlike coverlay method, a thin coating of liquid acrylated epoxy and acrylated polyurethane solder mask ink is applied onto the circuitry surface. The liquid coating is applied using several methods, one of such is screen printing. The coating is then thermally cured. In some complex rigid-flex boards, the coverlay openings on flex parts are so dense, we suggest to use flexible solder mask.
6. Protective Coatings
Surface Finishes:
- Common finishes include ENIG (Electroless Nickel Immersion Gold), OSP (Organic Solderability Preservative), and Immersion Silver.
- These finishes enhance solderability, protect against oxidation, and improve signal performance.
Material Selection Factors:
When selecting materials for rigid-flex PCBs, several factors need to be considered:
- Thermal Requirements: Materials should withstand high temperatures during assembly and operation.
- Mechanical Durability: Especially in flexible sections, the material should tolerate bending, folding, and dynamic stress.
- Electrical Performance: Low dielectric constant (Dk) and low loss factor (Df) are crucial for high-frequency applications.
- Environmental Resistance: The materials should resist moisture, chemicals, and extreme environmental conditions.
- Cost Considerations: Balancing performance with cost efficiency for the intended application.
The material used greatly determines the quality and overall functioning of the rigid-flex boards. As mentioned earlier, PCB materials must be carefully chosen after analyzing several criterial including cost, shelf life, and electrical requirements of the printed circuit board, among others. This helps produce rigid-flex PCBs that provide many years of reliable and trouble free service.
IPC Standards for Rigid and Flexible PCBs
The list of IPC standards below applies to rigid PCBs and flex circuits. Take note that this list is not exhaustive, and additional IPC standards may need to be considered. You should consult the ipc.org website for a full list of available IPC standards.
- IPC-2221A, Generic Standard on Printed Board Design
- IPC-2223, Sectional Design Standard for Flexible Printed Boards
- IPC-4101, Specification for Base Materials for Rigid and Multilayer Printed Boards
- IPC-4202, Flexible Base Dielectrics for Use in Flexible Printed Circuitry
- IPC-4203, Adhesive Coated Dielectric Films for Use as Cover Sheets for Flexible Printed Circuitry and Flexible Adhesive Bonding Films
- IPC-4204, Flexible Metal-Clad Dielectrics for Use in Fabrication of Flexible Printed Circuitry
- IPC-6013, Qualification and Performance Specification for Flexible Printed Wiring
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QFPCB Rigid Flex PCB Stack-Up Reference
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4 Layer Rigid Flex PCB Stackup from QFPCB
4 Layer Rigid-Flex 1st-Order HDI Printed Circuit Board Stackup
6 Layer Rigid Flex Circuit Board with a 4-layers flex stack-up
6-Layer Rigid-Flex PCBs with a 2+2 Layers Flex stackup
QFPCB factory specializes in providing turnkey service of customized PCB & PCBA solutions to meet the diverse needs of our clients. Our factory is committed to providing one-stop PCB & SMT assemble solutions to drive technological advancement and innovation.If you have any further questions, please feel free to leave a comment below or contact QFPCB by email ([email protected]).