What are the steps to manufacture a rigid-flex PCB?

Before designing rigid-flex PCB, it is necessary to clearly understand processes to manufacture rigid-flex PCB. Multilayer rigid-flex boards are basically a combination of rigid boards and flexible boards. However, for circuit board manufacturers to successfully combine the two, they need to have good levels of both rigid and flexible board manufacturing processes.

Material of rigid-flex PCB

Rigid-flex PCBs combine the best of both rigid boards and flexible circuits. Here’s a breakdown of the materials typically used in the construction of rigid-flex PCBs:

Flexible Part Materials

Base Material (Flex Core):

• Polyimide (PI): The most common material due to its good thermal stability, chemical resistance, and flexibility.
• Polyester (PET): Less expensive than PI but with lower thermal resistance.

Conductive Layer:

• Rolled Annealed (RA) Copper: Preferred for its flexibility and fatigue resistance.
• Electrodeposited (ED) Copper: Less expensive but less flexible compared to RA.

Adhesive Layer:

• Acrylic Adhesive: Offers good flexibility and is commonly used.
• Adhesiveless Base Materials: These eliminate the adhesive layer between the copper and the polyimide, reducing the overall thickness and improving the thermal properties.

Coverlay (Cover Film):

• Polyimide Coverlay: Protects the external circuitry of the flex areas.

Stiffeners/Support Materials:

• FR4: Often used to provide rigidity to certain areas of the flex circuit.
• Polyimide Stiffeners: Provide support where needed without significantly increasing thickness.

Rigid Part Materials

Base Material (Rigid Core):

• FR4: The most common substrate material, made of woven fiberglass cloth with an epoxy resin binder that is flame resistant.
• High-Temperature Epoxy and Phenolics: For higher performance applications.

Prepreg (Bonding Sheet):

• Non-Flow Prepreg (NO-FLOW): Used to bond rigid and flexible layers without flowing excessively during the lamination process.

Conductive Layers:

• Copper Foil: Standard material for creating conductive paths in PCBs.

steps to manufacture rigid-flex PCB

1. Material Cutting: Cut the large area of copper-clad laminate into the size required by the design.
2. Soft Board Material Cutting: Cut the original roll material (substrate, pure adhesive, cover film, PI reinforcement, etc.) into the size required by the design.
3. Drilling: Drill through-holes for electrical connections in the circuit.
4. Black Hole: Use a solution to adhere carbon powder to the walls of the holes, providing a good conductive connection.
5. Copper Plating: Plate a layer of copper inside the holes to achieve conductivity.
6. Exposure Alignment: Align the film accurately with the corresponding hole positions on the already dry-film-laminated board to ensure the film pattern correctly overlaps with the board surface, transferring it onto the board dry film by photolithography.
7. Developing: Develop away the unexposed areas of the dry film, leaving the exposed dry film pattern.
8. Etching: Etch away the exposed copper surfaces after the pattern development, leaving behind the parts covered by the dry film.
9. AOI: Detect any open or short circuits in the lines by transferring the image to the processing equipment using the principle of optical reflection.
10. Lamination: Cover the copper foil circuits with a protective film to prevent oxidation or short circuiting, while also serving as insulation and allowing for product flexibility.
11. Pressing: Combine the pre-stacked cover film and reinforcement board by applying high temperature and pressure to fuse them into one piece.
12. Punching: Use molds and a mechanical punching machine to cut the working board into the shipping size required by the customer.
13. Secondary Lamination: Stack the combination of soft and hard boards.
14. Secondary Pressing: Under vacuum conditions, use thermal pressing to fuse the soft board and hard board together.
15. Secondary Drilling: Drill through-holes for the interconnection between the soft board and hard board.
16. Plasma Cleaning: Use plasma to achieve cleaning effects that conventional methods cannot.
17. Copper Deposition (Hard Board): Deposit a layer of copper in the holes to achieve conductivity.
18. Copper Plating (Hard Board): Use electroplating to increase the thickness of the copper in the holes and on the surface.
19. Circuitry (Dry Film): Apply a layer of photosensitive material on the copper-plated board surface to serve as a film for pattern transfer.
20. Etching AOI Tracing: Dissolve away the copper surface outside of the circuit pattern to corrode the required pattern.
21. Solder Mask (Screen Printing): Cover all circuits and copper surfaces, providing protection and insulation.
22. Solder Mask (Exposure): The ink undergoes a photopolymerization reaction, the screen-printed ink remains on the board surface and hardens.
23. Laser De-paneling: Use a laser cutting machine to perform precise laser cutting, removing the hard board portion to reveal the soft board.
24. Assembly: Attach steel pieces or reinforcement in the corresponding areas on the board to serve as adhesive and increase the hardness of the FPC.
25. Testing: Use a probe to test for any open or short circuit defects.
26. Silkscreen: Print symbols on the board for easy assembly and identification in subsequent processes.
27. CNC Milling: Use a CNC machine to mill the shape required by the customer.
28. FQC: Full inspection of the appearance of the finished product according to customer requirements to ensure product quality.
29. Packaging: Package the fully inspected OK boards according to customer requirements, store them in inventory, and ship them out.

Challenges of manufacturing rigid-flex PCB

Flex PCB

PCB production equipment is used to make soft boards. Because the material of soft boards is soft and thin, all horizontal lines must be passed using a traction plate to avoid board jamming and scrapping.

Lamination of PI cover film. It’s important to apply the PI cover film locally and pay attention to the lamination parameters. During lamination, the pressure must reach 2.45 MPa to be flat and compact, without any issues such as bubbles or voids.

Rigid PCB

Hard boards use depth-controlled milling to create windows, and NO-FLOW PP must be used for the PP to prevent excessive overflow of adhesive during lamination.

Expansion and contraction control during the lamination of combined soft and hard boards. Since the expansion and contraction stability of soft board materials is relatively poor, it is necessary to complete the manufacturing of soft boards and the lamination of the PI cover film first, and then manufacture the hard board parts according to their expansion coefficient.

Precautions for Manufacturing Rigid-Flex PCBs

Manufacturing rigid-flex PCBs demands meticulous attention to various precautions throughout the process to ensure reliability and high performance. Here are key considerations divided into four critical areas:

1. Design and Material Considerations

Designing a robust rigid-flex PCB begins with balancing the needs of flexibility and rigidity. Carefully select materials such as Polyimide for flexibility in the flex parts and FR4 for structural support in rigid areas. Define appropriate bend radii to prevent stress and potential breakage during operation. Ensuring the design meets these criteria minimizes risks during manufacturing and enhances the durability of the final product.

2. Process Control and Quality Assurance

Maintain stringent process control measures to uphold quality standards. Monitor lamination parameters closely, including temperature, pressure (typically around 2.45 MPa), and time, to achieve consistent bonding without voids or delamination. Implement thorough cleaning procedures, such as plasma cleaning, to eliminate contaminants that could compromise adhesion. Conduct regular inspections throughout production, such as AOI, to detect and rectify defects promptly, ensuring each PCB meets specifications before progressing to the next stage.

3. Environmental and Handling Conditions

Control environmental factors rigorously within the manufacturing environment. Maintain stable temperature and humidity levels to prevent fluctuations that could alter material properties or affect process outcomes. Minimize dust and airborne particles that might compromise circuit patterns or lamination quality. Handle flexible circuits with care to avoid mechanical stress and ensure rigid sections do not have sharp edges that could damage the flexible parts during assembly or handling.

4. Testing and Validation Procedures

Prioritize comprehensive testing and validation protocols to guarantee the functionality and reliability of rigid-flex PCBs. Perform thorough electrical tests to verify circuit continuity, insulation integrity, and overall functionality under simulated operating conditions. Conduct mechanical tests, such as bend testing, to assess the durability and resilience of the PCB design. Maintain meticulous documentation of test results, materials used, and process parameters for traceability and continuous improvement.

By adhering to these precautions throughout the manufacturing process, PCB manufacturers can enhance product quality, reduce production risks, and deliver rigid-flex PCBs that meet the stringent demands of modern electronics applications.