rigid-flexible board

1.Material Characteristics
Flexible substrate: PI thickness 12.5~50μm, temperature resistance >250℃, CTE≈16 ppm/℃ (close to copper).
Adhesive: low-flow epoxy resin (curing temperature 180℃), peel strength ≥1.0N/mm.
Rigid substrate: FR-4 High Tg (Tg ≥ 170°C), Z-axis CTE ≤ 50 ppm/°C.
2. Design Specifications and Core Challenges
Layout Principle
Bending radius control:
Static bending: minimum radius ≥ 10 x total thickness of flexible zone (e.g. 0.2mm thick flexible zone, radius ≥ 2mm).
Dynamic bending: minimum radius ≥ 20 x thickness, avoid arranging perforations or components in the bending area.
Transition zone design:
The junction of rigid and flexible zones uses a gradual copper thickness (e.g., 1 oz→0.5 oz) or tear-drop shaped alignment to reduce stress concentration.
Example: Add 5mm copper-free zone at the junction and fix it with reinforcement sheet (stainless steel sheet).
3. Manufacturing process key points
Core process steps
Layered processing:
Rigid and flexible areas are etched with graphics, drilled holes and pre-applied with a cover film (Coverlay).
Lamination integration:
Adopt step-by-step pressing: Cure the rigid zone first, then bond the flexible zone by low temperature adhesive (e.g. acrylic) to avoid high temperature deformation of PI.
Profile cutting:
Laser cut the contour of the flexible zone and mechanically mill the rigid zone to ensure smooth edges (Ra <10μm) in the transition zone.
4. Typical application scenarios and design cases
Folding screen cell phone hinge circuit
Flexible zone: 2-layer PI substrate (total thickness 0.15mm), calendered copper alignment (0.1mm line width/spacing).
Rigid zone: 4 layers of FR-4 (0.4mm thick), carrying motherboard connectors and chips.
Transition zone: circular copper foil alignment with 3mm bend radius, covered with bend-resistant adhesive (hardness 60 Shore A).
Medical endoscope imaging module
Requirements: disinfectant corrosion resistance, 360° bending > 50,000 times, signal transmission rate > 5Gbps.
Design:
Flexible zone: 6-layer LCP substrate (Dk=2.9@10GHz), differential line impedance 100Ω±5%.
Rigid Zone: High frequency FR-4 (RO4350B) with integrated CMOS sensor and LED driver.
Protective layer: Parylene (2μm) coated on the surface of flexible area, chemical and moisture resistant.