Multilayer Rigid-Flex PCB Production Process: Step-by-Step Guide & Industry Insights

Multilayer rigid-flex PCBs represent a hybrid innovation in electronics, combining the structural stability of rigid PCBs with the flexibility of flex circuits. This unique design enables devices to bend, fold, or conform to tight spaces—critical for modern applications like foldable smartphones, automotive sensors, and medical implants—while supporting dense, multi-layered circuitry. However, their production process is far more complex than that of traditional rigid or flex-only PCBs, requiring specialized materials, precision lamination, and careful handling of flexible segments.

This guide demystifies the production process of multilayer rigid-flex PCBs, from material selection to final testing. It includes detailed steps, comparative data against other PCB types, and critical best practices to ensure reliability. Whether you’re an engineer designing for miniaturization or a manufacturer scaling production, understanding this process will help you leverage the full potential of multilayer rigid-flex technology.

What Are Multilayer Rigid-Flex PCBs?
Before diving into production, it’s essential to define multilayer rigid-flex PCBs and their unique value:

  1.Structure: They consist of alternating rigid layers (typically FR-4) and flexible layers (e.g., polyimide), connected via plated vias to form a single, integrated circuit.
  2.Key Advantage: Unlike rigid PCBs (fixed shape) or flex-only PCBs (limited layer count), multilayer rigid-flex designs support 4–20 layers of circuitry while enabling bending in specific areas (e.g., a foldable phone’s hinge).
  3.Common Uses: Foldable electronics, automotive ADAS modules, wearable medical devices, and aerospace sensors—applications where space, weight, and durability are non-negotiable.

Their production process must balance two conflicting needs: the precision required for multilayer circuitry and the flexibility to avoid damaging flex layers during manufacturing.

Step 1: Material Selection – The Foundation of Reliable Rigid-Flex PCBs
Material choice is make-or-break for multilayer rigid-flex PCBs, as each component must withstand lamination heat, bending cycles, and end-use environments. Below is a breakdown of critical materials and their specifications:

Critical Material Considerations
  1.Flex-Rigid Compatibility: Adhesives must match the CTE (coefficient of thermal expansion) of both flex and rigid substrates to avoid warpage during lamination. For example, polyimide flex cores pair best with epoxy adhesives (CTE ~20 ppm/°C) to minimize stress.
  2.Flex Layer Durability: Use rolled-annealed (RA) copper for flex traces—its ductility withstands 10,000+ bending cycles, vs. 1,000–2,000 cycles for electrodeposited (ED) copper.
  3.High-Temp Applications: For automotive or aerospace use, select LCP (liquid crystal polymer) flex substrates, which maintain flexibility at 200°C+ and resist chemicals.

Step 2: Step-by-Step Multilayer Rigid-Flex Production Process
The production process integrates rigid PCB manufacturing (lamination, drilling) with flex PCB techniques (handling delicate substrates, avoiding creases). Below is a detailed, sequential breakdown:

Phase 1: Pre-Production & Material Preparation
Before circuit patterning, materials are prepped to ensure uniformity and adhesion:

1.Flex Core Preparation:
  a.Flexible substrates (e.g., 50μm polyimide) are cleaned with isopropyl alcohol to remove oils and dust—contaminants that cause   b.adhesive failure.
Copper foil (12–35μm RA copper) is laminated to both sides of the flex core using heat (180°C) and pressure (300 psi), forming a “flex copper-clad laminate (CCL).”
2.Rigid Core Preparation:
  a.Rigid substrates (e.g., 1.6mm FR-4) are cut to panel size (typically 18”x24”) and deburred to remove sharp edges.
  b.Copper foil (35–70μm ED copper) is bonded to the rigid core via thermal lamination, creating the base for rigid circuit layers.

Phase 2: Circuit Patterning (Flex & Rigid Layers)
Patterning creates conductive traces on both flex and rigid layers, using photolithography and etching:

1.Photoresist Application:
  a.A photosensitive resist (liquid or dry film) is applied to the copper-clad flex and rigid laminates. For flex layers, a flexible resist is used to avoid cracking during handling.
2.Exposure & Development:
  a.The resist is exposed to UV light through a photomask (with the circuit pattern). Unexposed resist is washed away with a developer solution, leaving the copper traces to be etched exposed.
3.Etching:
  a.Flex layers: Immersed in a mild etchant (ammonium persulfate) to remove unwanted copper—etch time is reduced by 20% vs. rigid layers to avoid damaging the polyimide substrate.
  b.Rigid layers: Etched with ferric chloride or cupric chloride, standard for FR-4.
4.Resist Stripping:
  a.Remaining photoresist is stripped with a solvent (e.g., sodium hydroxide), revealing the final circuit pattern on both flex and rigid layers.

Phase 3: Lamination – Bonding Flex & Rigid Layers
Lamination is the most critical step in rigid-flex production, as it must bond layers without creasing flex segments or damaging circuits:

1.Adhesive Cutting:
  a.Adhesive sheets (e.g., epoxy-based) are laser-cut to match the panel size, with openings for vias and flex areas (to avoid bonding flexible segments to rigid layers).
2.Layer Stackup:
  a.Layers are aligned using fiducial marks (1mm copper circles) to ensure via and trace registration (tolerance ±0.02mm). The stackup typically follows: Rigid Layer → Adhesive → Flex Layer → Adhesive → Rigid Layer.
3.Controlled Lamination:
  a.The stack is pressed in a vacuum laminator at 160–180°C and 400–500 psi for 30–60 minutes. Vacuum removes air bubbles, while gradual pressure prevents flex layer creasing.
  b.For high-layer designs (10+ layers), sequential lamination is used: layers are added one at a time, with intermediate curing to maintain alignment.

Phase 4: Drilling – Creating Vias for Layer Connectivity
Vias (holes connecting layers) are drilled after lamination, with techniques tailored to flex and rigid areas:

1.Drill Planning:
  a.Gerber files specify via locations: Through-holes (connect all layers), Blind vias (connect outer to inner layers), and Buried vias (connect inner layers only). Flex areas use smaller vias (0.1–0.2mm) to avoid cracking.
2.Drilling Methods:
  a.Mechanical Drilling: Used for rigid layers (via diameter ≥0.2mm) with carbide drills (30,000 RPM) to ensure clean holes.
  b.Laser Drilling: Used for flex layers and microvias (≤0.15mm) with UV lasers—minimizes heat damage to polyimide substrates.
3.Deburring & Desmearing:
  a.Flex layers: Plasma etching removes resin smears from via walls (avoids short circuits) without abrading the delicate substrate.
  b.Rigid layers: Chemical desmearing (using potassium permanganate) cleans via walls for plating.

Phase 5: Plating – Ensuring Electrical Connectivity
Plating coats via walls with copper to connect layers and adds surface finishes for solderability:

1.Electroless Copper Plating:
  a.A thin copper layer (0.5–1μm) is deposited on via walls and circuit traces via a chemical reaction (no electricity), creating a base for electroplating.
2.Electroplating:
  a.The panel is immersed in a copper sulfate bath, with an electric current (2–4 A/dm²) building copper thickness to 15–25μm—critical for low-resistance via connections. Flex areas use lower current density (1.5–2 A/dm²) to avoid copper cracking.
3.Surface Finish Application:
  a.ENIG (Electroless Nickel Immersion Gold): Preferred for flex areas—gold’s ductility withstands bending; nickel prevents copper diffusion.
  b.HASL (Hot Air Solder Leveling): Used for rigid areas (cost-effective, good solderability).
  c.OSP (Organic Solderability Preservative): Ideal for high-volume consumer electronics (low cost, flat surface).

Phase 6: Solder Mask & Silkscreen
Solder mask protects traces, while silkscreen adds component labels—both must accommodate flex areas:

1.Solder Mask Application:
   a.Liquid photoimageable (LPI) polyimide solder mask is screen-printed onto the panel. Flex areas use a more flexible mask formulation (elongation ≥100%) to avoid cracking during bending.
   b.UV exposure and development define openings for pads and vias; the mask is cured at 150°C for 60 minutes.
2.Silkscreen Printing:
   a.Polyurethane-based ink is printed onto rigid areas (flex areas avoid silkscreen, as ink cracks during bending). Text size is ≥0.8mm x 0.4mm for readability, with 0.1mm clearance from pads.

Phase 7: Routing & Singulation – Separating Individual PCBs
Routing cuts the panel into individual rigid-flex PCBs, with special care for flex segments:

1.Panel Fixturing:
  a.The panel is mounted on a rigid frame to stabilize flex areas during routing, preventing tearing.
2.CNC Routing:
  a.A CNC router with a 0.8mm end mill cuts around the PCB perimeter. Flex areas are routed with a slower feed rate (50mm/min vs. 100mm/min for rigid) to avoid fraying.
3.Singulation:
  a.For high-volume production, laser routing is used for flex areas—creates clean edges without mechanical stress. V-scoring is avoided (it weakens flex-rigid boundaries).

Phase 8: Testing & Quality Control
Rigid-flex PCBs undergo rigorous testing to ensure electrical and mechanical reliability:

Multilayer Rigid-Flex vs. Other PCB Types: A Comparative Analysis
To understand why rigid-flex is chosen for specific applications, compare its production and performance to alternatives:

Critical Production Challenges & Solutions
Multilayer rigid-flex production faces unique hurdles—addressed by specialized techniques:

1.Flex Layer Creasing During Lamination
  a.Challenge: Uneven pressure causes flex segments to fold, damaging traces.
  b.Solution: Use vacuum laminators with programmable pressure ramps (gradual increase from 100 to 500 psi) and silicone pads to distribute pressure evenly.
2.Via Plating Uniformity in Flex Areas
  a.Challenge: Small vias (≤0.15mm) in flex layers suffer from thin plating.
  b.Solution: Increase electroless copper bath temperature to 45°C (vs. 40°C for rigid) and add surfactants to improve solution flow into small vias.
3.Delamination at Flex-Rigid Boundaries
  a.Challenge: Adhesive failure between flex and rigid layers due to CTE mismatch.
  b.Solution: Use acrylic-epoxy hybrid adhesives (CTE ~18 ppm/°C) and pre-cure flex layers at 120°C before final lamination.
4.Trace Cracking During Bending
  a.Challenge: Copper traces in flex areas crack after repeated bending.
  b.Solution: Use RA copper (ductile) and design trace angles of 45° (not 90°) to distribute stress; add “stress relief” loops in flex segments.

Benefits of Multilayer Rigid-Flex PCBs (Driven by Production Process)
The specialized production process delivers unique advantages over traditional PCBs:

  a.Space Savings: Integrates multiple rigid PCBs into one design, reducing connector count by 50–70% (e.g., a foldable phone’s hinge uses 1 rigid-flex PCB vs. 3 separate rigid PCBs).
  b.Weight Reduction: 30–40% lighter than equivalent rigid PCBs, critical for aerospace and wearable devices.
  c.Increased Reliability: Fewer connectors mean fewer failure points—field failure rates are 60% lower than rigid PCBs with wired connections, per IPC data.
  d.Design Freedom: Enables 3D packaging (e.g., wrapping around a motor) and foldable form factors impossible with rigid PCBs.

Industry Applications of Multilayer Rigid-Flex PCBs
The production process is tailored to meet the needs of key sectors:
1. Consumer Electronics
   a.Foldable Phones (e.g., Samsung Galaxy Z Fold): Multilayer rigid-flex PCBs in hinges support 20+ layers of circuitry, enabling 200,000+ bending cycles.
   b.Wearables (e.g., Apple Watch): Thin (0.5mm) rigid-flex designs conform to wrists while housing 6–8 layers of sensors and processors.

2. Automotive
  a.ADAS Sensors: Rigid-flex PCBs bend around vehicle frames, connecting cameras, radar, and LiDAR—withstanding -40°C to 125°C temperatures.
  b.EV Battery Management Systems (BMS): Flexible segments route power between battery cells, reducing weight by 35% vs. rigid PCBs.

3. Medical Devices
  a.Implantable Pacemakers: Biocompatible polyimide flex layers and 4–6 layers of circuitry fit in 1cm³ volumes, withstanding bodily fluids.
  b.Portable Ultrasound Probes: Rigid-flex PCBs bend to match probe shapes while maintaining signal integrity for high-resolution imaging.

4. Aerospace & Defense
  a.Satellite Antennas: Lightweight rigid-flex PCBs (30g per board) fold into launch vehicles and deploy in space, withstanding radiation and extreme cold.
  b.Military Headsets: Flexible segments conform to user ears, while rigid layers house communication chips—meeting MIL-STD-883 vibration standards.

FAQ
Q: What’s the maximum number of layers in a multilayer rigid-flex PCB?
A: Most manufacturers produce 4–12 layer designs, but advanced processes (sequential lamination) can achieve 20+ layers for aerospace and medical applications.

Q: How long does it take to produce multilayer rigid-flex PCBs?
A: Prototypes take 2–3 weeks (due to specialized lamination and testing); high-volume production (10k+ units) takes 4–6 weeks.

Q: Can rigid-flex PCBs use surface-mount components (SMDs) on flex areas?
A: Yes, but components must be “flexible-friendly” (e.g., chip resistors ≤0603, no large ICs) to avoid cracking during bending. Solder paste volume is reduced by 30% on flex areas to prevent joint stress.

Q: What’s the minimum bend radius for a multilayer rigid-flex PCB?
A: Typically 5–10x the flex layer thickness (e.g., a 50μm polyimide layer has a minimum bend radius of 250–500μm). Tighter radii risk trace cracking.

Q: Are multilayer rigid-flex PCBs RoHS compliant?
A: Yes—materials like lead-free solder, halogen-free adhesives, and RoHS-compliant polyimide are used. Manufacturers provide DoC (Declaration of Conformity) documents to verify compliance.

Conclusion
The production process of multilayer rigid-flex PCBs is a technical marvel, balancing the precision of multilayer rigid manufacturing with the delicacy of flex circuit handling. From material selection (polyimide for flex, FR-4 for rigid) to controlled lamination and laser routing, each step is optimized to create boards that are compact, durable, and versatile.

While production costs are higher than traditional PCBs, the benefits—space savings, weight reduction, and increased reliability—make multilayer rigid-flex PCBs indispensable for innovation in foldables, automotive, medical, and aerospace industries. For manufacturers, partnering with specialists experienced in rigid-flex production (and following strict quality control) is key to unlocking these benefits.

As devices continue to shrink and demand more functionality, the role of multilayer rigid-flex PCBs will only grow—driven by advances in production techniques that reduce cost and improve performance.