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Product Description
What is Rigid Flex PCB?
Rigid-Flex PCB (Rigid-Flex Printed Circuit Board) is a hybrid circuit board that combines the characteristics of rigid PCBs and flexible PCBs (flex PCBs) into a single integrated structure. It integrates rigid sections (for stable mounting of components) and flexible sections (for bending, folding, or dynamic movement) without the need for additional connectors, making it widely used in electronic devices that require compact design, reliability, and space optimization.
Core Components & Structure
A typical rigid-flex PCB consists of three key parts, layered and bonded together through specialized processes:
Key Advantages
Compared to standalone rigid PCBs or flex PCBs, rigid-flex PCBs offer distinct benefits:
Common Applications
Rigid-flex PCBs are indispensable in industries where space, reliability, and flexibility are paramount:
The Types of Rigid Flex PCB
The classification dimensions of Rigid-Flex PCB are numerous. The core classification can be based on structural form, flexible area characteristics, number of layers, and adaptability to application scenarios. Different categories correspond to different process difficulties and usage requirements.
Classified by Structural Form
This is the most commonly used classification method. The core basis is the connection mode and layout characteristics of the rigid and flexible areas, which directly determine the installation compatibility of the PCB.
The entire board has only one continuous flexible zone, with both ends or one end connected to the rigid board. The flexible zone is mainly used to achieve "folding" or "turning" functions, such as the flat cable connecting the laptop screen to the main unit. It has a simple structure and low cost, and is the most common basic model in consumer electronics.
Contains two or more independent flexible zones and forms a complex structure through the series connection of multiple rigid boards. Suitable for equipment that requires multi-directional bending and decentralized layout, such as industrial sensor modules and lens module circuit boards for multi-camera mobile phones.
The flexible area can achieve bending and twisting in three-dimensional space, while the rigid area is distributed in different spatial planes. Mostly used in equipment with extremely compact space and irregular shapes, such as UAV fuselage control boards and car instrument panel integrated circuit boards.
Classified by Flexible Region Characteristics
The main distinction is whether the flexible part can be repeatedly bent to adapt to static installation or dynamic working scenarios.
The flexible area is only bent and shaped once during assembly, maintaining a fixed form during subsequent use and not subject to repeated deformation. The core requirement is structural stability after shaping.
The flexible area needs to withstand long-term and repeated bending, twisting and other dynamic actions. Typical applications include printer paper feed mechanism circuit boards and wearable device opening/closing components, which need to meet the requirement of no failure after tens of thousands of bends.
Classified by Number of Line Layers
Similar to ordinary PCBs, classified by the number of layers of conductive circuits. The more layers there are, the higher the integration degree and the more complex the process.
Contains only one layer of conductive copper foil. Simplest structure and lowest cost, suitable for low-complexity circuits such as simple toy control boards and small remote control connection lines.
Consists of two layers of conductive copper foil, with interlayer conduction achieved through vias. Taking into account both cost and practicality, widely used as a peripheral module for smartphones and tablet computers.
The number of layers is ≥4, usually composed of alternating superposition of "rigid layer – flexible layer – rigid layer". Suitable for high-end electronic devices such as satellite circuit boards in aerospace and precision detection mainboards in medical equipment.
Classified by Special Function Adaptation
Specialized types designed for extreme environments or special needs, with additional customized requirements for materials and processes.
Adopts flexible substrates with low dielectric constant and low loss (such as PTFE-modified polyimide), suitable for 5G equipment, radar systems, and satellite communication equipment.
Can operate stably within a temperature range of -55℃ to 200℃ or even wider. Widely used in peripheral equipment of aero engines and industrial high-temperature sensors.
By using micro-vias and fine line width and line spacing technologies, it enhances the line density and reduces the board size, suitable for miniaturized devices such as smartwatches and implantable medical devices (e.g., auxiliary circuit boards for pacemakers).
How to Design Rigid Flex PCB?
The design and production of rigid-flex combined PCBs need to take into account both the structural stability of rigid PCBs and the bending adaptability of flexible PCBs. During the design stage, potential risks in subsequent production and use should be avoided.
Structural and Layout Design
- Clear rigid/flex boundary: A transition zone (usually 0.5–1mm) should be reserved at the boundary between the rigid zone and the flexible zone to prevent components, vias, and copper foils from crossing the boundary and to avoid cracking due to material differences during bending. Edges of the flexible zone need to be rounded (radius ≥1mm) to reduce stress concentration.
- Flexible zone bending planning: The flexible zone should avoid high-stress areas. In dynamic bending scenarios, the bending radius should meet the requirement that "the minimum bending radius of the inner copper foil is ≥10 times the thickness of the flexible substrate, and that of the outer copper foil is ≥15 times". Avoid overlapping and bending in the flexible zone to prevent long-term compression from damaging the circuit.
- Reasonable component layout: Heat-generating components should be concentrated in the rigid area and far away from the flexible area. Heavy-duty components need to be fixed in rigid areas, with reserved heat dissipation space and fixing holes.
Line and Layering Design
- Line direction adapted to bending: In the flexible zone, the line direction is preferred to be horizontal (parallel to the bending direction) to reduce the tensile/compressive damage to the copper foil during bending. The line width and spacing should be slightly larger than those in the rigid area (minimum line width/spacing ≥0.1mm recommended).
- Stacking structure optimization: For multi-layer rigid-flex PCBs, the flexible layer should adopt a symmetrical structure to prevent the board from warping. Rigid core materials are prohibited in the flexible zone. The adhesive should be made of flexible materials (such as acrylic) to ensure bending performance.
- Via and grounding design: Metallized vias are prohibited in the flexible zone. All vias must remain in the rigid zone and be at least 1mm away from the boundary of the flexible zone. In high-frequency scenarios, "microstrip line" or "ribbon line" structures can be adopted to optimize signal transmission.
Material Selection
- Flexible substrate: For dynamic bending scenarios, choose polyimide (PI) with good fatigue resistance; for static scenarios, polyester (PET) with lower cost can be selected. High-frequency equipment should select PI substrates with low dielectric constants (such as PTFE-modified PI).
- Copper foil and covering film: For flexible areas, rolled copper foil (with good ductility and bending resistance) is preferred, while for rigid areas, electrolytic copper foil can be used. The surface of the flexible zone should be covered with a flexible covering film (PI film), with the edge extending 0.2–0.3mm beyond the copper foil for protection and insulation.
Key Points of Manufacturing
The production of rigid-flexible combined PCBs integrates the processes of rigid and flexible PCBs, and requires additional control over the connection links between rigidity and flexibility. The core points are as follows:
Substrate Pretreatment and Lamination Process
- Substrate cleaning: The surface of the flexible PI substrate is prone to adsorbing impurities. It is necessary to remove oil stains and oxide layers through plasma cleaning or chemical treatment to enhance the adhesion between the copper foil and the substrate. Rigid FR-4 substrates need to be dried and dehumidified to prevent air bubbles from forming during lamination.
- Precise lamination: During lamination, temperature, pressure and time need to be controlled. The stepwise lamination method is adopted — first the flexible layer is laminated, then it is laminated with the rigid layer to ensure a tight interlayer bond.
Circuit Fabrication and Etching Control
- High-precision exposure: Flexible substrates are prone to stretching. Special fixtures should be used for fixation during exposure to prevent line deviation. The ultraviolet parallel light exposure machine is adopted to ensure line width error in the flexible zone is within ±0.02mm.
- Etching process optimization: Alkaline etching solution is used to treat the copper foil in the flexible zone. The etching speed is slightly lower than that in the rigid zone to prevent excessive etching from causing line breakage. After etching, it is necessary to clean it in time to avoid residual liquid from corroding the substrate.
Cutting and Forming Processes
- Flexible zone contour cutting: The flexible zone contour is processed by laser cutting (CO₂ laser or ultraviolet laser), with a smooth cut and no burrs, avoiding the tearing of the base material caused by mechanical cutting. The laser power needs to be controlled to prevent damage to internal circuits.
- Rigid zone forming: The rigid zone can be formed by CNC milling, and the edges need to be deburred and chamfered. The boundary between the rigid zone and the flexible zone needs to be ground a second time to ensure a smooth transition and reduce stress concentration during bending.
Post-Processing and Quality Inspection
- Surface treatment: Rigid areas can undergo surface treatments such as tin spraying, gold plating, and OSP to enhance soldering performance. The surface of the flexible zone is only protected by a covering film. High-temperature treatments such as tin spraying are prohibited to prevent deformation of the base material.
- Full-process inspection: During production, interlayer adhesion (through peel test) and line conductivity (flying probe test) need to be inspected. Finished products need to undergo bending tests (dynamic bending requires simulating tens of thousands of cycles) and high/low temperature environment tests to ensure stability and reliability.
| RIGID FLEXIBLE PCB CAPABILITY | |||||
|---|---|---|---|---|---|
| Category | Parameter | Standard | Advanced | ||
| RIGID FLEX PCB | Flex Layers | Lamination structure ≤4L；Book structure ≤10L | Lamination structure ≤4L；Book structure ≤12L | ||
| Max. Panel Size (shipping) | 210MM × 480MM | 460MM × 480MM | |||
| FPC Dielectric Thickness | 2MIL、3MIL、4MIL | 1MIL | |||
| PTH Rigid-Flex Transition Area | ≤1.2MM | ≤1.0MM | |||
| Pattern Spacing, Rigid Area to Rigid-Flex Transition Area | ≥1.2MM | ≥0.9MM | |||
| Rigid-Flex Structure | Typical structure、Flying tail structure、Multi-flex book structure、Two-flex lamination structure、Single-sided asymmetric structure、Asymmetric structure etc. | ||||
