Rigid-flex PCBs, which integrate both rigid and flexible circuit substrates into a single board, are widely used in high-end electronics such as wearable devices, medical instruments, aerospace systems, and automotive electronics. Thanks to their compact structure, bendability, and fatigue resistance, they offer unmatched design flexibility. However, their assembly process is more complex than standard PCBs and requires precise techniques and strict quality control. This article provides a detailed step-by-step overview of the rigid-flex PCB assembly process.
1.Pre-Assembly Preparation
1.1 Material Verification
Before starting the actual assembly, thorough material preparation is essential to ensure product quality and manufacturing efficiency.all materials including bare rigid-flex boards, components, and auxiliary materials. The flexible areas should be free of creases or damage, while the rigid sections must have intact copper layers and solder masks. Reinforcement materials (e.g., PI or FR4) should meet thickness and adhesion requirements.
First, the rigid-flex PCB itself must be fully fabricated and tested. These boards are usually composed of multiple layers combining rigid FR4 with flexible PI (polyimide) materials in a stepped configuration. Only boards that pass electrical tests and visual inspections can proceed to assembly.
Next, all components—both SMT (Surface Mount Technology) and THT (Through-Hole Technology)—must be procured and sorted based on the BOM (Bill of Materials). Common components include ICs, resistors, capacitors, connectors, and various modules.
In addition, auxiliary materials such as solder paste, flux, high-temperature tape, stiffeners, EMI shielding materials, thermal adhesives, and backing adhesives must be prepared. All incoming materials must pass IQC (Incoming Quality Control) before entering production.
1.2 Environmental Control
Due to the sensitivity of rigid-flex PCBs to temperature and humidity, operations should be conducted in a controlled environment (25±3°C, <60% RH) to prevent moisture absorption, which could lead to soldering or lamination defects. Additionally, anti-static measures (e.g., ESD mats, ionizers) must be implemented to avoid electrostatic damage to flexible sections.
2. SMT Assembly: Precision is Critical
The assembly process typically begins with SMT.
Solder paste is first applied via stencil printing on the rigid sections of the board. Care must be taken to prevent solder paste from spilling into flexible regions.
Next, high-speed pick-and-place machines mount components onto the solder pads. Accurate programming of placement coordinates and careful handling are essential to avoid damaging the flexible areas or misplacing parts.
Reflow soldering follows, where the boards pass through a controlled temperature reflow oven to melt the solder and secure components. For rigid-flex boards, the reflow profile must be carefully optimized to ensure reliable solder joints without overheating or warping the flexible sections.
2.1 SMT Assembly (Rigid Sections)
Board Fixturing
Flexible PCBs are prone to warping during SMT processes. Use specialized fixtures or high-temperature tape to secure the board and prevent misalignment on the pick-and-place conveyor.
Reflow Profile Optimization
Flexible materials have lower heat resistance, so the reflow profile must be carefully adjusted. The peak temperature should be maintained between 230–245°C to prevent blistering or delamination of the flexible substrate.
2.2. Flexible Section Processing
Reinforcement Attachment
Flexible areas often require stiffeners (e.g., PI, stainless steel) for mechanical support. Lamination should be performed using a hot press, with temperature (160–180°C) and pressure evenly distributed to avoid air bubbles or warping.
Gold Finger Protection
The gold fingers (connector ends) on flexible sections are susceptible to oxidation or contamination. Apply high-temperature tape for protection until final assembly.
2.3. Rigid-Flex Bonding
Lamination Process
The bonding of rigid and flexible sections is critical. A vacuum laminator is recommended to eliminate air pockets. Typical parameters: 180–200°C, 15–20 kg/cm² pressure, and 30–60 minutes duration.
Post-Lamination Inspection
After lamination, conduct X-ray or ultrasonic testing to verify the absence of delamination, cracks, or misalignment.
3. THT Insertion and Wave Soldering: For Through-Hole Components
If the board design includes through-hole components such as large connectors or transformers, they are inserted manually or using automated equipment after SMT.
Wave soldering is then used to solder these components. It’s important to control the wave height and solder contact time to avoid affecting the flexible areas. In sensitive zones or for small batches, manual soldering may be preferred to prevent thermal stress or board warping.
4. Stiffener Bonding: Reinforcing Flexible Areas
Flexible sections of the board are prone to mechanical stress and fatigue, so stiffeners are commonly bonded to reinforce these areas. Materials such as PI or thin FR4 sheets are used for reinforcement.
Stiffeners are applied using hot-press equipment with adhesive films. Accurate alignment using positioning fixtures is required to avoid misalignment, bubbles, or delamination. Surface cleaning before bonding and proper temperature/pressure settings are key to achieving a strong and reliable attachment.
5. Module Assembly and Press-Fit: Integrating with Housing Structures
In some applications—such as camera modules or smartwatches—the rigid-flex PCB needs to be folded or pressed into a housing or mechanical module.
This step involves shaping and fixing the flex section using hot or cold press molds. To avoid cracking or damage, the bend radius should be carefully controlled and repeated stress should be minimized. The board may be further fixed in place with screws, adhesives, or mechanical clips, depending on the product design.
6. Connector Soldering and External Interfaces
In the final assembly stages, connections are made between the flex section and external devices or connectors (e.g., FPC connectors, board-to-board (BTB) connectors). These often require fine-pitch precision soldering under a microscope.
For high-reliability joints, such as image sensor attachments or module-to-board connections, laser welding or ultrasonic welding techniques may be used to ensure electrical and mechanical integrity.
7. Protection and Cleaning: Safeguarding the Final Product
After all structural and electrical assembly steps are complete, the board may undergo protection treatments. These may include conformal coating, insulation film application, EMI shielding cover installation, or potting—depending on the environment and usage conditions.
Additionally, thorough cleaning using ionic cleaners or ultrasonic systems is essential to remove residual flux or contaminants. This prevents issues like corrosion or electrochemical migration during long-term use.
8. Testing and Inspection: Ensuring Quality Before Delivery
Before shipping, rigid-flex PCBs must undergo a series of inspections and tests.
Visual inspection checks for soldering defects, component misalignment, scratches, or contamination. Electrical tests verify continuity, shorts, and basic functionality.
For high-reliability applications, environmental and mechanical tests may be performed—including thermal shock, bending fatigue, vibration tests, and high-temperature/humidity aging—to ensure stability under extreme conditions.
9. Conclusion: Assembly is a Systematic Effort
Rigid-flex PCB assembly is a comprehensive and detail-oriented process. From material preparation, SMT, soldering, stiffener bonding, to final press-fit and testing, every step must be carefully planned and executed to accommodate the board’s hybrid structure.
To ensure high yield and reliable mass production, close collaboration among design, process, and manufacturing teams is crucial from the early design stage. DFM (Design for Manufacturability) evaluation should be integrated early in the product cycle. Additionally, working with experienced manufacturers specializing in rigid-flex assembly is essential for ensuring delivery quality and timeline.