Rigid-Flex PCBs\FPC
Flexible Printed Circuit (FPC) and Rigid-Flex PCB: Core Characteristics, Manufacturing Challenges, Applications, and Trends
1. Core Characteristics and Material Innovations
- FPC Structure:
- Substrates: The core substrates include PI films (with a temperature resistance of up to 260°C) or PET (for cost-effectiveness). Copper foils are categorized into rolled annealed copper (RA, for high flexibility) and electrodeposited copper (ED, for low cost).
- Lightweight Design: FPCs can be as thin as 0.1 mm, being over 70% lighter than rigid PCBs, and support dynamic bending for at least 200,000 cycles (e.g., in foldable screen hinges).
- Rigid-Flex PCB Design:
- Laminate Integration: The rigid regions employ FR-4 high-Tg materials, while the flexible regions utilize adhesive-free PI substrates. These are laminated using low-flow prepregs (Low-Flow PP) to prevent delamination caused by Z-axis thermal expansion differences.
- Via Technologies: Laser microvias (with diameters ≤ 0.1 mm) and buried vias in the rigid-flex transition zones are employed to reduce exposed solder joints and enhance shock resistance (critical in automotive vibration scenarios).
2. Manufacturing Process Challenges
- FPC Manufacturing:
- Coverlay Lamination: The coverlay (CVL) protective film requires precise window opening alignment (with a tolerance ≤ ±25 μm) to prevent pad contamination [citation:22].
- Bending Reliability: Dynamic regions avoid the use of vias (prone to fracture) and adopt "arc-shaped traces" to distribute stress.
- Rigid-Flex PCB Manufacturing:
- Layered Lamination: Independent drilling of rigid and flexible regions is required before lamination. The pressing temperature gradient must be controlled ≤ 5°C/min to prevent PI substrate deformation.
- Material Compatibility: An epoxy resin transition layer is added to mitigate delamination between PI (CTE 40 ppm/°C) and FR-4 (CTE 14 ppm/°C).
3. Application Scenarios and Performance Advantages
| Category | Typical Applications | Core Advantages |
|---|---|---|
| FPC | ▶️ Flexible cables for phone screens (e.g., OLED foldable displays) ▶️ Wiring in TWS earbud cavities ▶️ Sensor wiring for drone flight control systems |
3D wiring saves space; withstands temperature variations from -55°C to 150°C (e.g., in automotive engine compartments) |
| Rigid-Flex PCB | ▶️ Battery management systems (BMS) in new energy vehicles ▶️ Camera modules for endoscopes ▶️ Deployment mechanisms for satellites |
Reduces connector solder joints by 70%; passes CAF (conductive anodic filament) leakage resistance certification |
4. Technology Trends and Challenges
- High-Density Integration: FPCs are evolving towards 10+ layer Any-layer HDI designs, with line widths/spaces reaching 0.05 mm (to accommodate AI chip packaging).
- Material Upgrades:
- Nanocellulose Substrates: Enhance bendability by 30%, offering a domestic alternative to imported DuPont Kapton®.
- LCP (Liquid Crystal Polymer): Used in 5G millimeter-wave FPCs, with a dielectric constant (Dk) ≤ 2.9 (reducing signal loss).
- Cost Barriers: Rigid-Flex PCBs have a yield rate of only 65%–75% (requiring multilayer alignment tolerances ≤ ±30 μm), relying on automated optical inspection (AOI) and electrical testing (ET) for efficiency improvements.
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FPC\Rigid-Flex PCB Technics Capacity No. Item Description DataSheet 1 FPC Main material Brand Taiflex、Graceth、SY 2 PCB Material Brand SY 、ITEQ、KB 3 Texture Texture Brand PI、 PET 4 Cover Film Brand Taiflex、Graceth、SY 5 Max Layers LAYER 1-24layers（sample）、1-12layers(manufacture） 6 Finish Board thickness mm 0.25-6.0mm(samplel） Rigid - Flex PCB 0.25-6.0mm 7 Min Pattern Width / Spacing mm 0.05mm/0.05mm 8 Max finish board size mm 230*450mm 9 Finish Board thickness tolerance mm ±50.05mm 10 PP Thickness um 12.5um 、 25um 、 50um 11 Copper thickness um 12um 、 18um 、 36um 、 70um 12 Stiffener materail Variety FR4/PI/PET/SUS/PSA 13 Surface treatment Variety ENIG、Immersion tin、OSP、immersion silver、plating gold 14 Min hole size mm Mechanical Hole ：0.15mm 、Laser Hole0.1mm 15 Hole tolerance mm NPTH:±0.05mm 、PTH:±0.075mm 16 Cover film color Variety Yellow 、 Black 17 PI thickness mil 0.5mil、0.7mil、0.8mil、1mil、2mil 18 Max number of layers of FPCB 1-8layers 19 Min finished size mm 5mm*8mm 20 Min pad mm inner layer（5mil）、outer layer（4mil） 21 Stiffener min size mm 4mm×5mm 22 Stiffener max size mm 32mm×32mm 23 Stifferner alignment accuracy mm ±0.075mm 24 Cover minimum openning size mm 0.6×0.6mm(steeling tooling)、0.5×0.5mm(Precision tooling) 25 Minimum openning spacing for covering film mm 0.5mm(Precision tooling) 、0.2mm（Laser Routing）、 0.15mm(Normal drilling) 26 Coating film overflow amount（unilateral） mm Normal0.08-0.12mm 、 Limitation0.03mm 27 Min diameter of gold finger semicircle hole mm 0.25mm ，Normal value0.3mm 28 Rigid-Flex PCB：Peel-off strength N 1.4N 29 Rigid-Flex PCB：Planeness um Less than 15um before baking; less than 30um after baking 30 Rigid-Flex PCB：thermal shock ℃ 288℃（3 times within 10 seconds） 31 Rigid-Flex PCB：W/B gold wire pull g > 6g 32 Rigid-Flex PCB：min.board thickness mm FPCB 0.1mm、4Layers 0.3mm、6Layers 0.5mm、8Layers 0.6mm、
10Layers0.8mm
