Flexible Printed Circuits (FPC) are widely used in electronic products due to their light weight, high wiring density, thin profile, and flexibility.
1. FPC Introduction
Flexible Printed Circuits (FPC) are widely used in electronic products due to their light weight, high wiring density, thin profile, and flexibility.
The surface of FPC is covered with a resin film (also called PI coverlay, mainly made of Polyimide (PI)), which provides circuit protection and solder masking. PI is a high-temperature-resistant polymer with excellent dielectric, mechanical, radiation-resistant, and wear-resistant properties at high temperatures, suitable for precision electronics such as aviation and electronic appliances.
FPC substrates are divided into PI (Polyimide) and PET (Polyester):
- PI: Higher cost, good flame resistance, preferred for soldering applications; thickness ranges 12.5–125 μm; dielectric constant (Dk): ~3.5 (with adhesive), ~3.3 (adhesive-free).
- PET: Lower cost, poor heat resistance; thickness ranges 25–125 μm; Dk ~3.4.
Coverlay is composed of substrate + adhesive, matching the copper foil substrate material. Adhesives are mainly Acrylic and Epoxy (most commonly used, thickness 0.4–1 mil, typically 1 mil).
2. Stack-Up Design
The design adopts 4 layers for the rigid region and 2 layers for the flexible region:
- Flexible part thickness: 0.24 ± 0.05 mm
- Total board thickness: 0.8 ± 0.1 mm
The stack-up includes base copper, inner copper, PI substrate, coverlay adhesive, and coverlay PI with specified thicknesses.
3. Material Performance & Signal Integrity Simulation Basics
PI material datasheets only provide Dk/Df at 1 MHz, not at 10 GHz. Given the low Dk/Df values and referring to M6 material differences between 1 GHz and 10 GHz, the high-frequency loss of this PI material is expected to be low.
Key performance indicators (per IPC TM-650 test methods):
- Peel strength, solder heat resistance (315°C/260°C, >120 sec), UL-94 V-0 flammability
- Dk ~3.2–3.3 (1 MHz), Df ~0.002 (1 MHz); Dk ~3.61, Df ~0.004 at 10 GHz
- Volume resistivity ~10¹⁶–10¹⁷ Ω·cm, low water absorption, high tensile strength
4. Impedance Calculation
Impedance is calculated using Polar SI9000; FPC impedance calculation must account for double-sided coverlays:
- Manufacturer method (Model 1B): Ignore coverlays and subtract fixed values (single-ended −4 Ω, differential −10 Ω) based on factory experience.
- Alternative model: 1B2A Embedded Microstrip model for independent impedance evaluation.
5. Test Board Design & Measurement
Test Board Design
Route standard impedances: 50 Ω single-ended, 85 Ω/95 Ω/100 Ω differential. FPC fabrication lead time is longer than rigid PCBs (approx. 8–10 days longer).
Test Results
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Impedance
50 Ω single-ended and 85 Ω differential: Flexible region impedance ~5 Ω higher; rigid region ~3 Ω lower. Rigid-flex combined boards impose high requirements on manufacturer impedance control.
Grid copper design leads to higher impedance (manufacturers often ignore grid copper in calculations).
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Loss
Use
AFR de-embedding to measure the loss of a 2-inch pure flexible segment. Loss per inch is equivalent to M6 rigid PCB material.
6. FPC Simulation & Simulation-Test Correlation
- Directly using datasheet Dk/Df (Df = 0.004) leads to over-optimistic simulation results.
- Simulation-test fitting is required to derive equivalent Dk/Df (test-fitted Df = 0.009, more than double the datasheet value).
- A closed-loop workflow: Test → De-embed → Fit → Simulate → Re-verify ensures simulation matches measurement for the same material.
- For rates below 25 Gbps, copper foil roughness can be neglected; fitting parameters suffice for accurate simulation.
7. FPC High-Speed Signal Application
With proper material selection, FPC achieves low loss comparable to M6 material, supporting 10–25 Gbps high-speed signals.
Application example:
PCIe 3.0 riser card with ~6–7 inches of flexible trace; the measured waveform meets expectations and operates normally.
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