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Rigid flex boards combine the structural stability and component-carrying capacity of rigid PCBs with the flexibility and conformability of flexible printed circuits (FPCs). Capable of adapting to complex spatial installations and dynamic bending environments, they are widely used in consumer electronics, automotive equipment, medical devices, smart wearables and other fields. However, the composite nature of the structure significantly increases the complexity of design and routing, particularly in terms of area layout, routing rules, copper foil processes, electromagnetic compatibility and reliability design, where there are significant differences compared to pure FPC flexible boards.
Layout Design Specifications
The layout determines the board’s structural stability, bending reliability and component lifespan. Due to the structural boundary between rigid and flexible zones, which results in uneven stress distribution, the layout constraints for Rigid flex boards are far stricter than those for pure FPC boards. The core principles are the separation of functional zones and the avoidance of bending stress damage.
1. Rules for Component Zoning
Rigid flex boards: The flexible zone serves solely to conduct electrical signals and provide circuit transitions; the placement of active components (chips, resistors, capacitors, diodes, transistors, etc.) and SMT pads is strictly prohibited. All active devices and precision components must be placed in the rigid zone. This is because the flexible zone undergoes continuous bending during equipment operation; rigid solder joints cannot adapt to this deformation, leading over time to pad peeling, cracking and detachment, much like a brittle biscuit shattering under external force. The flexible zone should retain only pure transmission traces to minimise structural load in the bending area.
FPC (Flexible Printed Circuit): Consisting entirely of a flexible substrate with no rigid zone restrictions, this allows small surface-mount devices to be placed in the flexible zone, provided that dual reinforcement is employed to ensure long-term reliability under repeated bending: ① Reinforcement of component pad areas through gold-plated fingers to enhance conductivity and structural strength; ② A polyimide (PI) reinforcement film is laminated to thicken the local substrate and disperse bending stress. Following this dual reinforcement, the component area can stably withstand over 100,000 standard bends, meeting the requirements of most dynamic operating scenarios.
2. Design of Safety Clearances Between Zones
Rigid flex boards: The interface between rigid and flexible zones is a stress-concentration weak point, where lamination and bending processes generate sustained interlaminar and shear stresses. Therefore, a blank area (stress buffer zone) of ≥1 mm must be reserved around all components, pads and structural solder joints at the Rigid flex interface. This effectively disperses concentrated stresses, preventing pad tearing, circuit breakage or delamination.
FPC Flexible Boards: As there is no Rigid flex structural boundary and stress is distributed uniformly across the board, there are no fixed boundary safety margin requirements. However, in areas subject to frequent dynamic bending, component placement must comply with a bending radius of ≥20 times the board thickness to prevent excessive stretching of the copper foil or component lead breakage caused by an insufficient bending radius.
Routing Design Specifications
1. Requirements for Trace Orientation
Rigid flex boards: All traces in the flexible section must be at a 90° angle to the board’s bending axis. If traces run parallel to the bending axis, repeated bending will subject the copper foil to unidirectional tensile and compressive stresses, leading to metal fatigue, cracking and even fracture; perpendicular routing allows stresses to be distributed evenly across the entire trace, significantly improving bend resistance and service life.
FPC Flexible Boards: In conventional static or low-speed bending areas, 45° angled routing is permitted to optimise routing density. However, in dynamic bending areas subject to high-frequency repetitive motion, serpentine routing must be adopted. The undulating structure cushions bending stresses, prevents localised stress accumulation, and accommodates high-frequency bending conditions.
2. Control of Trace Width Consistency
Rigid flex Boards: Trace widths in the flexible section must remain constant throughout, with a tolerance of ≤±10%. Sudden changes in trace width lead to uneven impedance (causing signal reflection and attenuation) and stress concentration (making narrow traces prone to breakage), severely affecting signal integrity and reliability.
FPC Flexible Boards: Step-like gradual changes in trace width are permitted to meet impedance matching or routing space requirements; however, abrupt right-angle changes are prohibited. Teardrop-shaped transition structures must be added at all transition points to eliminate stress peaks through smooth arcs, thereby preventing trace cracking or delamination.
3. Via Design Process
Rigid flex boards: Mechanical drilling is prohibited in the flexible areas (as it compromises substrate integrity and causes micro-cracks). Multi-layer interconnections must utilise laser-drilled blind vias with a diameter ≤0.2 mm and a ring spacing ≥0.3 mm to minimise substrate damage and evenly distribute bending stress.
FPC Flexible Boards: The use of 0.3 mm mechanical vias is permitted to reduce costs, but back-drilling must be performed to remove burrs from the via walls and excess copper foil, thereby preventing stress concentration or short circuits. Vias in dynamic bending areas must also be filled with conductive adhesive to seal and reinforce the via walls, preventing cracking or delamination.
Design of Copper Foil, Cover Film and Reinforcement Processes
1. Cover Film Window Cutting Process
Rigid flex boards: For pad window cutting in the flexible area, teardrop-shaped compensation windows are used (rather than right-angled rectangles), and the distance between the window edge and the pad edge must be ≥0.5mm. This ensures a smooth stress transition, preventing the cover film from cracking, curling or delaminating.
FPC Flexible Boards: Full-board lamination with a cover film may be used to protect the copper foil; however, the gold finger areas require secondary laser cutting for precision finishing (accuracy ±0.05 mm) to ensure the contact surfaces are flat and clean, thereby guaranteeing reliable electrical continuity during insertion and removal.
2. Logic for Reinforcement Plate Layout
Rigid flex boards: A 0.2 mm thick PI reinforcement plate must be laminated at the interface between rigid and flexible sections to cushion concentrated stresses caused by lamination and bending, balance the difference in stiffness between rigid and flexible areas, and prevent delamination or circuit breakage.
FPC Flexible Boards: The connector routing area must utilise 3M reinforcement steel sheets (thickness 0.2–0.5 mm, Shore hardness ≥70) to enhance local rigidity, resist insertion/removal forces and installation stresses, and prevent connector misalignment, cold solder joints or detachment.
Electromagnetic Compatibility and Impedance Control Design
1. Impedance Control Parameters
Rigid flex boards: The dielectric constant of the substrate in the flexible region varies with bending and temperature, rendering standard impedance parameters unsuitable. For 50Ω differential signal lines, a corrected line width-to-spacing ratio of 5/7 mil (line width/spacing) must be adopted to compensate for changes in the dielectric constant, ensuring impedance matching and eliminating signal reflection and crosstalk.
FPC Flexible Boards: As the substrate exhibits good uniformity, a standard 4/6 mil line width-to-spacing ratio is sufficient for conventional high-speed differential lines. However, the temperature drift characteristics of PI substrates must be taken into account, and an impedance tolerance margin must be provided to ensure that impedance deviation is ≤5% at high temperatures, thereby guaranteeing signal stability across the entire temperature range.
2. Ground Plane Treatment Methods
Rigid flex Boards: The rigid section employs a solid, continuous ground plane to shield against electromagnetic interference and reduce ground impedance; in the flexible section, solid copper plating is strictly prohibited (due to poor ductility and susceptibility to cracking), and a grid-patterned copper layout must be adopted to enhance flexibility and deformation capacity whilst maintaining shielding performance.
FPC Flexible Boards: Ground plane design is flexible, allowing for local cut-outs to facilitate heat dissipation. However, a solid copper plane must be maintained beneath all high-speed signal layers to form a coplanar waveguide transmission structure, shielding against signal crosstalk and electromagnetic radiation.
Reliability Verification and Failure Analysis Standards
1. Bending and Environmental Testing
Rigid flex boards: Must pass dynamic bending tests (bending radius ≥ 10 times board thickness, 100,000 cycles without failure) and thermal shock tests (-65°C ↔ 125°C, 100 cycles, with no delamination, cracking or blistering), comprehensively verifying mechanical and environmental performance.
FPC Flexible Boards: Emphasis is placed on dynamic bending reliability; in accordance with IEC 60335 standards, the number of bending cycles must be ≥200,000, suitable for high-frequency, long-term deformation operating scenarios.
2. Failure Analysis Testing Methods
Rigid flex Boards: Failures often manifest as hidden defects such as interlayer delamination, micro-cracks in via walls, and delamination at the Rigid flex interface. Non-destructive testing using CT scanning and acoustic microscopy is employed to precisely locate internal hidden defects.
FPC Flexible Boards: Failures are concentrated in surface/shallow-layer defects such as copper foil fatigue cracks, cover film delamination, and pad delamination. Metallographic sectioning and SEM (Scanning Electron Microscopy) are used to observe copper foil grain deformation, crack propagation, and delamination interfaces, providing data support for process optimisation.
Rigid flex boards, leveraging the structural load-bearing capacity of the rigid sections and the bending capability of the flexible sections, are suitable for complex modules requiring localised support whilst accommodating deformation (e.g. camera modules, medical endoscopes); whereas FPC flexible boards excel in their fully flexible substrate, high bending endurance and flexible routing, making them better suited to dynamic scenarios involving high frequency and long travel (e.g. mobile phone hinges, wearable devices).