In the manufacturing of Flexible Printed Circuits (FPC), Rigid-Flex PCBs, and standard Printed Circuit Boards (PCB), the alkaline stripping process—a core step to remove cured dry film and expose conductive copper traces—directly determines the integrity of fine-pitch circuits and the long-term reliability of electronic devices. For industries relying on high-performance FPCs (e.g., consumer electronics, automotive, medical devices), mastering this process is non-negotiable.

As a global FPC ODM/OEM manufacturer and PCB rigid-flex solutions provider, Shenzhen Huaruixin Electronics Co., Ltd. leverages advanced technology and industry expertise to optimize the FPC dry film stripping workflow. This blog breaks down Huaruixin’s unique approaches to the stripping process, key quality control measures, and why it stands out as a trusted partner for custom FPC manufacturing.

1. Huaruixin’s Unique Solutions for FPC & PCB Stripping Process

The standard stripping process uses a 3-5% sodium hydroxide (NaOH) solution, operated at 50-60℃ for 30-60 seconds. However, flexible circuits—with their thin polyimide substrates and ultra-fine copper traces (down to 0.05mm line width)—demand more precision. Huaruixin’s tailored strategies address these FPC-specific challenges:

1.1 Custom Alkaline Stripping Formulas for FPC Substrate Compatibility

Unlike generic NaOH solutions, Huaruixin develops substrate-specific stripping chemistries to protect delicate FPC materials while ensuring complete dry film removal:

For polyimide (PI) FPCs (used in high-temperature applications like automotive electronics), the solution includes a low-volatility corrosion inhibitor (0.15% benzotriazole) to prevent PI substrate degradation and copper trace pitting.

For PET-based FPCs (common in wearable devices), a milder alkaline blend (4% NaOH + 0.5% ethylene glycol) reduces processing temperature to 48-52℃, avoiding PET warpage.

For rigid-flex PCBs with mixed FR-4 and PI layers, a dual-solution system targets the cured dry film on both substrates without compromising either material’s structural integrity.

1.2 Automated Process Control for Fine-Pitch FPC Traces

Fine-pitch FPC circuits (≤0.1mm line spacing) are highly sensitive to temperature and time fluctuations. Huaruixin’s intelligent stripping line eliminates human error with:

Real-time PID temperature control: Each stripping tank is equipped with a precision sensor (±0.3℃ accuracy) linked to an automated heating system. If temperature deviates by >0.5℃, the system adjusts instantly—critical for preventing over-etching of FPC copper traces or incomplete dry film removal.

Product-specific conveyor timing: Using CAD data from customer FPC designs, the system calculates exact residence time (30-60 seconds) for each batch. For example, double-sided FPCs with blind vias require a 55-second cycle to ensure stripping in via holes, while single-layer FPCs use a 32-second cycle to minimize copper loss.

Ultrasonic agitation for FPC blind vias: For FPCs with micro-vias (≤0.1mm diameter), ultrasonic transducers (40kHz) in the stripping tank create micro-bubbles that dislodge dry film residues from via walls— a common pain point in standard stripping processes.

1.3 Post-Stripping Cleaning & Drying for FPC Reliability

Residual alkaline solution or dry film fragments on FPCs can cause electrical shorts or poor solderability. Huaruixin’s 3-stage post-stripping process ensures FPC cleanliness:

High-pressure deionized (DI) water rinsing: A 0.4MPa DI water spray (filtered to 0.2μm) removes loosened dry film, with nozzles angled to target FPC flex folds and via openings.

Neutralization bath: A 0.5% citric acid solution neutralizes any remaining alkaline residues, preventing long-term copper corrosion on FPC traces.

Hot-air knife drying: A temperature-controlled (60-70℃) hot-air knife removes moisture from FPC surfaces and vias—critical for avoiding water spots on polyimide FPC substrates.

1.4 AI-Powered AOI Inspection for FPC Defect Detection

Manual inspection fails to catch micro-defects in fine-pitch FPCs. Huaruixin’s AI-enhanced Automated Optical Inspection (AOI) system sets a new standard:

2D/3D imaging for FPC traces: The AOI captures 10μm-resolution images of FPC surfaces, detecting under-etching (residual copper slag), over-etching (thinned traces), and dry film residues with 99.8% accuracy.

FPC-specific defect training: The AI model is trained on 15,000+ FPC defect cases (e.g., trace pinholes, via blockages) to distinguish critical flaws from non-consequential blemishes—reducing false rejects by 35% compared to generic AOI systems.

Real-time SPC reporting: Inspection data is fed into a Statistical Process Control (SPC) system, which alerts operators to trends (e.g., increasing over-etching) before they impact production—ensuring consistent FPC quality control.

2. Critical Considerations for FPC & PCB Stripping Process

Even with advanced technology, certain challenges can compromise FPC performance. Huaruixin prioritizes these key issues to deliver reliable results:

2.1 Copper Trace Protection in FPC Stripping

Alkaline solutions can etch FPC copper traces if not controlled. Huaruixin mitigates this by:

Limiting maximum stripping time to 60 seconds (even for complex rigid-flex PCBs) and conducting copper thickness testing (using a micro-eddy current gauge) every 10 batches—ensuring no more than 3% copper loss.

Using low-etch alkaline additives (e.g., sodium gluconate) that target cured dry film polymers without reacting with copper, ideal for high-density FPCs with 0.08mm traces.

2.2 Uniform Stripping for Large-Area FPCs

Large-format FPCs (e.g., 500mm × 300mm for industrial displays) often suffer from uneven stripping. Huaruixin solves this with:

Rotary nozzle arrays: Stripping tanks feature 360° rotating nozzles that distribute alkaline solution evenly across FPC surfaces, eliminating "dead zones" where dry film remains.

Conveyor tension control: For roll-to-roll FPC manufacturing (used in high-volume production), the conveyor belt’s tension is adjusted in real time to keep the FPC flat—preventing solution pooling in flex folds.

2.3 Compatibility with FPC Surface Finishes

FPCs with surface finishes like ENIG (Electroless Nickel Immersion Gold) or OSP (Organic Solderability Preservative) require gentle stripping. Huaruixin adapts by:

Using a low-temperature stripping process (45-50℃) for ENIG-coated FPCs to avoid nickel layer oxidation.

Adding a non-ionic surfactant to the stripping solution for OSP-coated FPCs, ensuring OSP layers remain intact post-stripping.

2.4 Environmental Compliance in FPC Manufacturing

Huaruixin adheres to global environmental standards (e.g., RoHS, REACH) in its stripping process:

Alkaline solution recycling: Used stripping solutions are filtered and reconditioned (via ion exchange) for reuse, reducing waste by 60%.

Wastewater treatment: Post-stripping rinse water is treated to remove copper ions (to <0.5ppm) before discharge, meeting China’s GB 21900-2008 and EU’s Directive 91/271/EEC standards.

3. Partner with Huaruixin for Custom FPC & PCB Solutions

Whether you need high-frequency FPCs for 5G devices, high-temperature rigid-flex PCBs for automotive under-the-hood applications, or medical-grade FPCs (ISO), Huaruixin’s optimized stripping process and end-to-end manufacturing capabilities ensure your products meet the strictest standards.

As a leading FPC ODM/OEM manufacturer with a 10,000㎡ production base, Huaruixin offers:

Custom FPC design support (from prototype to mass production).

Strict quality control (ISO 9001).

Fast turnaround times (7-10 days for FPC prototypes, 2-3 weeks for mass production).

Contact Huaruixin Today

Learn more: Visit www.hrxfpc.com for detailed insights into FPC manufacturing processes, PCB stripping technology, and case studies.

Inquire now: Email sales@hrxfpc.com to discuss your custom FPC/PCB project—our engineers will provide a tailored solution and free quote within 24 hours.

For reliable, high-precision FPC and PCB stripping processes, trust Shenzhen Huaruixin Electronics—your global partner for flexible circuit solutions.