Complete Material Guide: Polyimide Flex, Polyester Flex, and Emerging Polycarbonate Flexible PCB
Why “Flex”?
Flexible printed-circuit boards (FPCs) replace bulky harnesses and connectors with ultra-thin copper conductors laminated to plastic films. They:
- Shrink Z-height to < 0.2 mm.
- Bend or fold 360 °—up to 10 million cycles with rolled-annealed copper (RA).
- Survive vibration & thermal shock better than wired assemblies.
- Enable dynamic 3-D packaging inside wearables, cameras, cars, medical implants, and satellites.
Flexible PCB Fabrication
Flexible PCB Manufacturer
Polycarbonate PCB
Polyester PCB
Polyimide Flexible PCB
Why Order Flexible PCBs from HighPCB?
Material portfolio
adhesiveless polyimide, PET, transparent polycarbonate, PEN, plus copper or silver-ink options.
48-h DFM + mechanical fold simulation
delivered with every quote.
Dual process lines
roll-to-roll printing for PET volume, sheet-fed LDI for fine-pitch PI.
Fast turnaround
5-day PI proto (1-layer), 7-day double-sided PI; 10-day PET roll orders.
Value-added services
stiffener lamination, selective ENEPIG, hot-bar flex attachment, 100 % dynamic bend testing.
Material Landscape at a Glance
| Property | Polyimide (PI) | Polyester (PET) | Polycarbonate (PC) |
| Tg (°C) | > 360 | ~ 80 | ~ 150 |
| Max service temp (°C) | 150 (short-term 200) | 105 | 120 |
| Dielectric constant (1 MHz) | 3.4 | 3.2 | 2.9 |
| Loss tangent (1 MHz) | 0.008 | 0.018 | 0.010 |
| Tensile elongation (%) | 60 | 120 | 100 |
| Transparency | Amber | Clear | Optically clear |
| Typical copper | RA 12 / 18 µm | ED 12 µm | RA 9 µm |
| Relative laminate cost | 1.0 × | 0.5 × | 1.3 × |
Polyimide Flexible PCBs (The Industry Work-Horse)
1 Core Benefits
- Thermal endurance—passes 6 × 260 °C Pb-free reflow with zero delamination.
- Dynamic flex life—> 10 million bends @ R = 10 × t with RA copper.
- Signal integrity—Df 0.008 supports 10 Gb/s over 200 mm.
- Chemical resistance to flux, solvents, sterilisation gases.
2 Typical Applications
- Folded camera modules, 5G antennas, surgical tools, high-density rigid-flex (Type 4), space avionics harness replacement.
3 Fabrication Key Points
| Step | Control | Spec |
| Etch | SAP / subtractive, ±10 µm | Line/space ≥ 70 µm |
| Coverlay | PI 12.5 µm + epoxy | Void < 1 % AOI |
| Stiffener | FR-4 / SUS / Al | Flatness < 0.08 mm under BGA |
| Surface finish | ENIG / ENEPIG / Imm Ag | Au 0.05 µm ± 0.02 |
Design tip: Keep flex copper ≤ 18 µm; avoid plated through-holes in dynamic zones—use staggered microvias instead.
Polyester Flexible PCBs (Cost-Effective Consumer Choice)
1 Core Benefits
- Lowest raw-material cost—≈ 50 % of polyimide.
- Excellent dimensional stability for adhesive keyboards, membrane switches.
- Transparent options enable LED back-lighting and capacitive touch overlays.
2 Limitations
- Heat sensitivity—peak 105 °C; not reflow compatible.
- Lower bond strength—acrylic adhesive layer can cold-flow; limit wire-bonding.
3 Typical Applications
- Disposable medical sensors, membrane keypads, low-power LED strings, RFID antennas, greeting-card audio boards.
4 Fabrication Highlights
- Roll-to-roll screen printing of silver paste common for ultra-low cost.
- Etched ED copper 12 µm available for 0.25 mm pitch ICs; finish with OSP or low-temp Sn Bi.
Design tip: Keep bend radius ≥ 15 × t; place stiffeners under all SMT parts if PET film ≥ 125 µm.
Polycarbonate Flexible PCBs (Specialty Transparent Flex)
1 Why Polycarbonate?
- Optical clarity—transmittance > 85 % in visible range.
- Mid-range thermal window—higher than PET, lower than PI.
- Formability—vacuum-form or thermoform into complex shapes.
2 Niche Use-Cases
- Transparent touch/key surfaces with embedded LEDs.
- Illuminated automotive interior trim.
- Smart-glass antenna and heater grids.
3 Process Considerations
| Step | Adjustment |
| Lamination | Low-pressure (< 1 MPa) to prevent crazing |
| Surface prep | O₂ plasma to increase Cu adhesion |
| Finish | Imm Ag or OSP only (avoid ENIG heat) |
| Max panel | 400 × 500 mm due to thermoform tooling |
Design tip: Keep copper density balanced; differential shrink of PC vs. copper can warp after thermoforming.
Flexible PCB Design Rule Summary
| Feature | Polyimide | Polyester | Polycarbonate |
| Min line / gap | 70 / 70 µm | 100 / 100 µm | 90 / 90 µm |
| Min PTH drill | 0.15 mm laser | Avoid (jumpers) | 0.20 mm laser (low-temp Cu plate) |
| Bend radius (static) | ≥ 6 × t | ≥ 10 × t | ≥ 8 × t |
| Bend radius (dynamic) | ≥ 10 × t | Not recommended | N/A static only |
| Max Cu thickness | 35 µm (rigid areas) | 12 µm | 18 µm |
| Coverlay window relief | +100 µm | +150 µm | +120 µm |
Reliability & Test Matrix
| Test | Polyimide | Polyester | Polycarbonate |
| Dynamic bend (R = 10 t, 25 °C) | > 1 M cycles | 50 k cycles | 10 k (static use) |
| Solder float 260 °C / 10 s | Pass | Fail | Fail |
| Moisture, 85 °C / 85 % RH, 168 h | ΔR < 2 % | ΔR < 5 % | ΔR < 4 % |
| UV ageing, 96 h | Slight yellowing | Clear | Clear |
Cost Drivers & Optimisation
| Driver | Impact | Mitigation |
| Adhesiveless PI core | +15 % | Use adhesive PI if no impedance or reflow |
| Laser cut coverlay | +US$ 0.10 / cm² | Combine openings; gang-punch on PET |
| Double-sided PI flex | +30 % | Switch to singe-sided + jumper if layer count < 2 |
| ENEPIG finish | ×1.3 | Use ENIG unless Al wire-bonding |
| VIPPO on PI | +US$ 5 / 100 vias | Dog-bone fan-out for 0.65 mm BGAs |
Support All Types of PCB Fabrication
Biggest PCB Manufacturers
FAQ
Q1. Can PET flex circuits handle reflow soldering?
No. PET softens around 110 °C. Use low-temperature Sn Bi solder waves (< 180 °C) or crimp connectors.
Q2. How many dynamic bends can a polyimide flex survive?
With RA copper 12 µm and PI 25 µm, > 1 million cycles at radius 10 × thickness.
Q3. Is polycarbonate flex UL-rated?
UL 94 V-2 grades are available; specify in your stack-up.
Q4. Maximum flexible circuit length from HighPCB?
Roll-to-roll PET: 1.2 m continuous; panel PI/PC: 510 × 610 mm.