1. Introduction: The Emergence and Value of Rigid-Flex PCBs

2. Structural Composition and Core Characteristics of Rigid-Flex PCBs

 Structural Composition

• Rigid Regions: Made of traditional rigid PCB substrates, such as FR-4 epoxy glass cloth substrates, with a thickness typically ranging from 0.4 to 2.0mm. This region serves as the main carrier for components (e.g., chips, connectors, resistors, capacitors), providing stable mechanical support and good heat dissipation performance.
• Flexible Regions: Using polyimide (PI) as the core substrate, with a thickness generally between 0.1 and 0.3mm. Copper is coated on the surface to form conductive traces. Flexible regions can be repeatedly bent, folded, or twisted, acting as a "bridge" connecting different rigid regions. Their bending radius can usually reach 5-10 times their own thickness (depending on materials and design).
• Transition Regions: The connection part between rigid and flexible regions. A gradient transition structure must be adopted in the design to avoid fracture caused by stress concentration. The copper foil in the transition region is usually reinforced, such as adding a cover layer or using a special etching process.
• Conductive Interconnection Layers: Metallized through-holes (PTH) are used to achieve electrical connections between rigid and flexible regions, as well as between different layers. The design of through-holes must consider the bending characteristics of flexible regions to prevent through-hole cracking from affecting conductivity.
• Protective Layers: Solder mask (green oil) is used to protect rigid regions; flexible regions use polyimide coverlay or flexible solder mask to prevent trace oxidation and mechanical damage while maintaining flexibility.

 Core Characteristics

3. Classification of Rigid-Flex PCBs

 Classification by Number of Layers

• Double-Layer Rigid-Flex PCBs: Composed of two conductive layers (top and bottom). The rigid regions use FR-4 substrates, and the flexible regions use PI substrates. With a relatively simple structure and low cost, they are suitable for medium-low power, simple interconnection scenarios, such as mobile phone camera modules and small sensors.
• Multi-Layer Rigid-Flex PCBs: Contain 3 or more conductive layers, with rigid and flexible substrates alternately combined through lamination. They can realize complex signal routing and power distribution, suitable for high-end electronic devices such as laptops and aerospace electronic components. The number of layers is usually 4-12, and can reach more than 20 in special scenarios.

 Classification by Characteristics of Flexible Regions

• Single-Segment Flexible Rigid-Flex PCBs: Contain only one flexible segment connecting two rigid regions, such as the circuit board connecting the dial and strap sensor in smart watches.
• Multi-Segment Flexible Rigid-Flex PCBs: Contain multiple flexible segments, enabling complex connections between multiple rigid regions, such as the multi-layer rigid-flex PCB connecting the screen, motherboard, and battery in foldable phones.
• Foldable Rigid-Flex PCBs: The flexible regions are specially designed to withstand thousands of folds without damage, serving as core components in foldable screen phones and wearable devices.

4. Manufacturing Process of Rigid-Flex PCBs

1. Substrate Preparation: Prepare rigid substrates (FR-4) and flexible substrates (PI) separately, and cut them into corresponding sizes according to design requirements. Flexible substrates need surface cleaning and roughening treatment to enhance the bonding force with adhesives.
2. Inner Layer Fabrication: Perform photolithography and etching on the inner copper foil of rigid and flexible substrates to form inner conductive patterns. A temporary support film must be attached to the surface of flexible inner layers to prevent deformation during subsequent processing.
3. Lamination Assembly: Alternately stack rigid substrates, flexible substrates, and prepreg (PP) according to the designed lamination structure, and place them in a laminator for pressing under high temperature and pressure. The pressing process requires precise control of temperature (180-220℃), pressure (20-40kg/cm²), and time (60-90min) to ensure tight bonding of all layers without damaging the flexible regions.
4. Drilling and Metallization: Use laser or mechanical drilling equipment to drill through-holes in the circuit board, then metallize the through-holes through electroless copper plating and electrolytic copper plating to achieve electrical connections between layers. Through-holes in flexible regions need reinforcement, such as resin filling or adding copper rings.
5. Outer Layer Fabrication: Apply photoresist on the outer copper foil, then perform exposure, development, and etching to form outer conductive patterns, followed by removing the photoresist.
6. Protective Layer Coating: Apply solder mask on rigid regions and cure it; attach polyimide coverlay or apply flexible solder mask on flexible regions, and achieve bonding through hot pressing or UV curing.
7. Shape Processing: Use CNC punching or laser cutting equipment to process the circuit board into the desired shape, separate rigid and flexible regions, and polish the edges.
8. Testing and Inspection: Conduct electrical performance tests (such as continuity, insulation, and impedance tests) and mechanical performance tests (such as bending life test of flexible regions), and check for circuit defects and appearance quality through AOI (Automated Optical Inspection).

5. Application Fields of Rigid-Flex PCBs

 Consumer Electronics

 Automotive Electronics

 Aerospace and Defense

 Medical Devices

 Industrial and IoT