Rigid Flex PCB with PI Material, Blind & Buried Vias

Rigid Flex PCB combines rigid FR4 sections and flexible PI polyimide substrate, widely used for devices requiring folding, bending and compact packaging.

Specifications

• Substrate: Rigid part – FR4; Flexible part – Polyimide (PI), 12.5μm / 25μm / 50μm
• Copper Foil: RA Rolled Annealed Copper 9μm /12μm /18μm /35μm
• Stackup: Multi-layer rigid-flex structure
• Via Type: Blind vias, buried vias + optional through vias
• Min Blind Via Diameter: 0.1mm (laser ablation)
• Min Buried Via Hole: 0.15mm
• Min Line Width / Space: 0.075mm / 0.075mm
• Coverlay for Flex Area: PI coverlay, thickness 12.5μm /25μm
• Stiffener: Optional PI / FR4 stiffener for connector mounting zone
• Surface Finish: ENIG, Immersion Silver, OSP, Hard Gold (for gold finger optional)
• Operating Temperature: -40℃ ~ +260℃
• Bend Cycles: ≥100,000 times (design & copper thickness dependent)

Performance Characteristics

1. Hybrid Mechanical Performance: Rigid FR4 zones support component soldering and assembly; PI flex zones enable repeated folding & dynamic bending.
2. High Density Interconnection: Blind and buried vias route signals internally between layers, saving board space, reducing board footprint.
3. Thermal Stability: PI flex material maintains stable property under thermal cycling, low thermal expansion, less risk of delamination.
4. Vibration & Shock Resistance: Reduces separate cable connectors, fewer solder joints, improves reliability in moving/vibration environments.
5. 3D Assembly Capability: Can fold into 3D shape to fit confined device housing.

Design Principle

1. Dual Material Stack-up: Separate rigid FR4 regions for heavy component placement; PI flexible regions reserved for bending movement.
2. Blind & Buried Vias Design Logic
   • Blind via: Connects outer layer to inner layer only, does not penetrate entire board stackup
   • Buried via: Interconnects inner layers only, completely hidden inside stack, no opening on outer surface
3. Flex Zone Layout Rules: Traces in bending area placed at neutral axis; rounded trace corners, avoid sharp angles to prevent copper cracking during flexing.
4. Transition Zone Management: Optimize rigid-flex transition area, reduce stress concentration between FR4 and PI boundary.
5. Layer Planning: Assign high-speed signals on inner layers, use buried vias for inner layer interconnection to shorten signal path.

Solder Mask Properties

• Rigid area: LPI liquid photoimageable solder mask (green/black/red optional)
• Flex PI area: PI coverlay (main protection), optional LPI solder mask for flex section
• Adhesion: Strong bonding on FR4 and PI substrate, no peeling during repeated bending and SMT reflow
• Heat Resistance: Tolerate lead-free reflow up to 260°C, no blistering, delamination
• Insulation & Protection: Block moisture, dust, oxidation; prevent short circuit between adjacent traces.
• Note: Coverlay is the primary protective layer for PI flex area, different from rigid board solder mask.

Signal & Electrical Performance

• Controlled Impedance: Support 50Ω single-ended, 90Ω /100Ω differential impedance; tolerance ±10% standard, ±5% available
• Dielectric Property: PI Dk ~3.2~3.5, low Df, stable dielectric constant under bending and temperature variation
• Insulation Resistance: ≥10⁹ Ω @ DC 500V
• Withstand Voltage: ≥1.5kV AC
• Signal Advantage: Blind & buried vias shorten signal path, reduce parasitic inductance and crosstalk, suitable for high-speed signals
• Current capacity: Determined by copper weight and trace width
• Electrical Test: 100% continuity and isolation test after fabrication

Application Scenarios