Rigid-Flex PCB is From Concept to Qualified Production: An Engineering Playbook for Space-Saving, High-Reliability Interconnects
Rigid-Flex in Context — Why It Exists
Traditional “rigid + cable” architectures add weight, volume, and four or more solder joints per interconnect. Rigid-flex PCBs combine copper-clad polyimide flex zones with FR-4 rigid islands inside the same laminated structure, so signals flow across bends without plugs, headers, or harnesses. The result:
| Metric | Cable + Rigid | Rigid-Flex |
| Interconnect joints | ≥ 4 per link | 0 |
| Assembly steps | Board A → Cable → Board B → Screws | Single SMT reflow |
| Z-height | ≥ 11 mm (header stack) | ≤ 1.0 mm folded |
| Dynamic bend life | N/A | > 1 000 000 cycles (RA Cu + PI 25 µm) |
| Field failure points | Connector, harness, cold-joint | Flex hinge only (tested) |
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IPC-6013 / 2223 Rigid-Flex Types (Quick Reference)
| Type | Rigid Layers | Flex Layers | Flex Exposed? | Typical Use |
| Type 1 | 1 rigid | 1 flex | Yes (outer) | Camera hinges, wearables |
| Type 2 | ≥ 2 rigid | 1 flex | Yes | Folded consumer boards |
| Type 3 | 1–N rigid | 2 flex (separate) | Yes | Dynamic cabling replacement |
| Type 4 | ≥ 2 rigid | ≥ 2 flex (sandwiched) | No (internal) | Military, space, HDD heads |
HighPCB regularly builds Type 2 / 3 for IoT & medical and Type 4, Class 3 for aerospace.
Material Toolbox
| Zone | Preferred Material | Key Characteristics |
| Flex core | Adhesiveless polyimide 25–50 µm | Dk 3.4, Df 0.008 @ 1 GHz; no acrylic flow |
| Flex copper | Rolled-annealed (RA) 12/18 µm | Grain-longitudinal ⇒ < 0.3 % elongation |
| Bondply (rigid-flex glue) | No-flow FR-4 106 prepreg or Flex-bond epoxy | Tg ≥ 170 °C, flow 60–90 µm |
| Rigid core | FR-4 Tg 170–200 °C or halogen-free | CTE-Z < 55 ppm/°C |
| Coverlay | PI 12.5 µm + epoxy adhesive 12.5 µm | Laser-defined openings, ±50 µm |
| Stiffener (opt.) | 0.2–0.5 mm FR-4, Al 5052, SUS304 | Stabilises µ-BGA / press-fit pins |
Impedance note: RA copper + adhesiveless PI keeps Df low enough for 10 Gb/s lanes over 150 mm flex.
Stack-Up Examples (Top-Down)
Example A – Type 2, 6-Layer Smartphone Fold Hinge
ENEPIG finish
Solder mask LPI (rigid)
Cu 35 µm — Signal 1
FR-4 Tg 180 °C 0.10 mm
Cu 18 µm — GND Plane
No-flow 1080 PP 0.08 mm
Cu 18 µm — Flex stripline 1 ← exposed
PI core 25 µm
Cu 18 µm — Flex stripline 2 ← exposed
No-flow 1080 PP 0.08 mm
Cu 18 µm — GND Plane
FR-4 Tg 180 °C 0.10 mm
Cu 35 µm — Signal 2
LPI mask
Bend radius safe: 0.25 mm PI × 10 = 2.5 mm dynamic.
Example B – Type 4, 12-Layer Satellite Transceiver, IPC Class 3A
Three sequential laminations, stacked microvia core-fill, inner flex fully buried for EMI.
Process Flow – Detailed Controls & Metrology
| # | Process | Control Spec | Metrology |
| 1 | Flex core etch | 12 µm RA Cu, SAP ±10 µm | Coupon AOI |
| 2 | Coverlay lam. | 190 °C, 1.7 MPa, 60 min | Cross-section void < 1 % |
| 3 | Rigid drilling | 0.10 mm UV-laser, 0.20 mm Mech | IPC-T-50 Δpos ±25 µm |
| 4 | Desmear | Plasma O₂/CF₄ 60 s → KMnO₄ bath | No smear @ 200× optical |
| 5 | Cu plate & via-fill | 25 µm Cu, DC 2.0 A dm⁻² | X-ray void < 5 % |
| 6 | Sequential lam 2/3 | ±40 µm layer register | 3-pin target + vision |
| 7 | Routing/laser skive | Flex edge burr < 25 µm | Stylus profilometer |
| 8 | Dynamic bend test | 180° curl, R = 2 mm, 1 000 cycles | IPC-TM-650 2.4.5A pass |
Design Rule Matrix
| Feature | Min Value | Notes |
| Line / space (rigid) | 75 µm / 75 µm | LDI; 50 µm special |
| Line / space (flex) | 100 µm / 100 µm | SAP 70 µm possible |
| PTH drill | 0.20 mm | 6:1 aspect recommended |
| Microvia (laser) | 0.10 mm | Stack ≤ 2 high |
| Distance rigid-flex seam → pad | ≥ 0.50 mm | Prevent solder wicking |
| Bend count dynamic | > 1 000 000 @ R ≥ 10 × T | RA Cu + adhesiveless PI |
| Differential imp. (90 Ω) | PI 25 µm + Spacing 100 µm | Outer copper 18 µm |
Reliability & Test Suite
| Test | Std | Typical Spec |
| IST (Interconnect Stress) | IPC-TM-650 2.6.26 | ΔR < 5 % after 250 cycles 25↔145 °C |
| Bend Fatigue | IPC-6013 3.4.2 | 1 M cycles R=10T, no crack |
| Thermal Shock | JESD22-A106 | −55 °C/125 °C 500 cycles, no CAF |
| Moisture/Insulation | IPC-TM-650 2.6.3.4 | IR > 1 GΩ @ 85 °C/85 % RH |
| Solder Float | IPC-TM-650 2.4.13 | 288 °C 10 s × 3, no delam |
8 | Cost Model & Optimisation
| Cost Driver | Typical Impact | Mitigation (DFM) |
| Sequential lam cycles | +15 % per cycle | Limit to 3 → combine flex pairs |
| Laser microvia count | +US$ 5–7 /1000 vias | Use dog-bone fan-out for 0.8 mm BGAs |
| Coverlay laser time | +US$ 0.1 /cm² | Gang openings under BGA array |
| Stiffener placement | +US$ 30 /panel setup | Only under fine-pitch BGAs |
| ENEPIG vs ENIG | ×1.25 cost | Use ENIG if no Al wire-bond required |
End-to-End Assembly Considerations
- Panel warpage after lam. < 0.70 mm per 150 mm diagonal → ensures stencil coplanarity.
- Reflow profile: 2-zone; ramp 1.0 °C s⁻¹, peak 245 °C; cool < 3 °C s⁻¹ to avoid copper-grain fatigue.
- Selective-solder fixture prevents masking compound from wicking under coverlay edge.
- Conformal coat: parylene or acrylic; mask flex hinge region to maintain bend life.
- Functional test: 100 % flying-probe → power-on flex while monitoring ΔR for latent crack.
Ordering Checklist
- 3-D mechanical drawing (STEP, *.dxf) with final fold radii.
- Gerber / ODB++ + netlist + IPC-356A test file.
- Stack-up table with material codes, copper weights, dielectric Dk/Df.
- Bend specification – static or dynamic, bend count target.
- Reliability class (IPC-6013 class 2 or 3, space addendum if needed).
- Surface finish, stiffener zones, shield plating, impedance targets.
- Assembly BOM + “no-stuff” flex zones highlighted.
