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Flexible printed circuits (FPCs) are core components of smart hardware. Using polyimide (PI) film as the substrate, they are widely used in mobile phones, wearable devices and medical equipment due to their lightweight, flexible and high-density wiring characteristics, enabling flexible three-dimensional connections within devices. Compared to conventional rigid boards, the production and soldering processes for FPCs are more complex; processes such as substrate treatment, circuit formation, lamination and pad protection directly determine the quality of the finished product and the yield rate.
Flexible Printed Circuit (FPC) Production Process
1. Substrate Pre-treatment and Material Preparation
The core substrate for FPCs is polyimide (PI) film. This material possesses high heat resistance, high toughness, excellent flexibility and superior insulation properties, making it the optimal substrate for flexible circuits. The surface of raw PI film is relatively smooth when it leaves the factory; direct lamination of copper foil is prone to issues such as insufficient adhesion and delamination. Therefore, preliminary surface pre-treatment is essential. Through precision cleaning, surface dust, oil and impurities are removed, followed by physical roughening or plasma activation processes to enhance the surface roughness and reactivity of the substrate, thereby significantly improving the bonding strength between the subsequent copper foil, adhesive layer and the PI substrate.
The mainstream substrate for mass production is copper-clad PI film, which is divided into two processes: adhesive-coated lamination and adhesive-free lamination. The adhesive-coated process is more cost-effective and suitable for standard products; the adhesive-free process offers better heat dissipation, more stable impedance and superior flex resistance, and is predominantly used for high-end precision FPCs. Depending on the product’s current-carrying capacity and impedance design requirements, rolled copper foil or electrolytic copper foil of various thicknesses, such as 12μm or 18μm, can be selected to lay the foundation for subsequent fine-line fabrication.
2. Precision Circuit Formation Process
Circuit formation is the core process in FPC production, directly determining circuit conductivity accuracy and signal integrity. The entire process utilises photolithography to form micro-scale circuits, comprising five major steps: film application, exposure, development, etching and film removal.
The process begins with the application of the dry film. In a cleanroom environment, high-quality photosensitive dry film (photoresist) is uniformly applied to the surface of the copper-clad substrate. Temperature-controlled hot pressing ensures the dry film lies flat, free from bubbles and creases, and adheres completely to the copper foil surface. Subsequently, a UV exposure unit is used to precisely expose the film according to the pre-set circuit pattern on the film, causing the dry film in the light-transmitting areas to undergo photosensitive curing and form a protective layer that matches the design.
Once exposure is complete, the process moves to the development stage. A specialised alkaline developer is used to wash the board surface, completely dissolving and removing the unexposed, uncured dry film from the blank areas, thereby exposing the copper foil to be etched whilst retaining the cured dry film protective layer over the circuit patterns. Next, an acidic etching solution (ferric chloride and hydrochloric acid etching system) is applied to uniformly etch the exposed copper foil, precisely removing excess copper material and retaining only the conductive circuit patterns protected by the dry film. Once etching is complete, a stripping agent is used to remove the residual cured dry film from the board surface, fully exposing the complete, clean pure copper circuit, thereby completing the formation of the single-layer circuit.
3. Interlayer Lamination and Interconnection Process for Multi-layer FPCs
For double-layer and multi-layer flexible printed circuits, the single-layer circuit fabrication process must be repeated to prepare the top-layer, inner-layer and bottom-layer circuit patterns respectively. Once the single-layer panels are fabricated, the process proceeds to the multi-layer alignment and lamination stage. High-precision alignment equipment ensures the precise alignment of circuit patterns across all layers. Using specialised flexible bonding adhesives or adhesive-free lamination technology, the multi-layer flexible boards are laminated into a single unit under high temperature and pressure conditions. At the same time, positions for interlayer vias are pre-reserved to ensure the accuracy of subsequent electrical interconnections.
Following lamination, drilling operations are carried out. Depending on the required hole diameter precision, either mechanical drilling or laser micro-drilling processes are selected. Laser drilling offers higher precision, smooth hole walls and no burrs, making it suitable for high-end FPC products requiring micro-diameter holes and high-density interconnections. Following drilling, hole metallisation is essential. Through chemical copper plating and copper plating thickening processes, a uniform and dense conductive copper layer is deposited on the inner walls of the insulated vias, enabling electrical interconnection between upper and lower layers and establishing a complete three-dimensional conductive network.
4. Production of Cover Layers and Solder Mask Protective Layers
Once the circuit has been formed, insulation and protection treatment is required to prevent the exposed copper foil from oxidising, absorbing moisture or short-circuiting, whilst simultaneously enhancing the product’s resistance to bending and friction. The mainstream processes are divided into two methods: cover film lamination and liquid solder mask ink.
Most high-end FPCs utilise a PI cover film lamination process, in which an adhesive-backed polyimide cover film is precisely laminated onto the circuit surface, leaving only openings for areas requiring electrical connection, such as pads and contacts. Through high-temperature vacuum hot-press curing, the cover film is bonded tightly to the board surface, ensuring no bubbles or delamination, and providing comprehensive protection for the circuit. Some lightweight and standard FPC products utilise the liquid solder mask ink process. This involves a sequence of ink application, pre-baking, exposure, development and curing to precisely expose the pads, forming a thin insulating protective layer.
5. Anti-oxidation Treatment for Pad Surfaces
Exposed pads on FPCs are highly susceptible to oxidation and sulphidation when in direct contact with air, leading to poor soldering, increased contact resistance and signal instability. It is therefore essential to apply a specialist surface treatment to the exposed pads. Depending on the product grade, application scenario and cost requirements, there are three main processes.
Electroless Gold Plating: This process deposits a nickel-gold alloy layer onto the pad surface. The plating is uniform, offers exceptional oxidation resistance, high surface flatness and excellent soldering reliability, making it suitable for precision chips, high-frequency signals and high-end end-user products. Tin or Silver Plating: Offering better value for money and excellent electrical conductivity, this process meets standard soldering and electrical continuity requirements for general applications and is commonly used in standard consumer FPCs. OSP (Organic Solderable Paste) Process: This creates an ultra-thin organic protective film on the copper surface. It does not affect soldering performance and is extremely cost-effective, making it suitable for short-term storage and standard low-frequency circuit products.
6. Precision Contour Cutting and Forming
Following the protective coating process, precision contour finishing is carried out in accordance with the product structural drawings to remove process edges, scrap material and excess substrate. Two methods are commonly used in mass production: die-cutting offers high efficiency and is suitable for standardised products in large volumes; laser cutting provides high precision, is stress-free and produces no burrs, making it suitable for irregular shapes, high-precision and ultra-thin flexible boards. Following forming, the board surface undergoes edge and corner finishing to ensure dimensional tolerances, flatness and visual quality meet specifications.
7. Local Reinforcement Process (On-demand processing)
As FPCs are inherently flexible, areas such as connector solder zones, plug-in contacts and mounting points are subject to frequent stress and are prone to deformation and fracture; therefore, targeted reinforcement is required. By laminating rigid materials such as FR4 reinforcement boards or stainless steel reinforcement sheets onto designated areas, and curing them via hot-pressing with specialised high-temperature adhesives, the board’s hardness, compressive strength and flexural resistance are locally enhanced. This effectively resolves issues of insufficient local load-bearing capacity and susceptibility to fracture in flexible boards, significantly improving product lifespan and assembly stability.
8. Comprehensive Final Inspection
At the conclusion of production, a comprehensive quality inspection must be carried out, comprising two main sections: electrical performance testing and visual structural inspection. Electrical testing includes continuity testing, impedance testing, insulation resistance testing and withstand voltage testing to identify electrical defects such as open circuits, short circuits and abnormal impedance. Visual inspection utilises high-definition microscopes to detect issues such as circuit misalignment, notches, burrs, bubbles in the cover layer, delamination, reinforcement misalignment and poor plating. Products may only be stored or dispatched once all criteria have been met.
Standardised Operating Procedure for FPC Soldering
1. Pre-soldering Preparation
Prior to formal soldering, the FPC pads must undergo pre-treatment cleaning. Wipe the board surface with lint-free alcohol swabs to remove oxidation, dust and grease. Apply an appropriate amount of neutral flux evenly to the pads to improve solder wettability and prevent cold solder joints or solder lift. At the same time, calibrate the soldering iron temperature in advance. For precision flexible circuit board soldering, set the iron temperature to a stable level of 300°C or higher to ensure rapid soldering, minimise the substrate’s exposure to high temperatures, and prevent scorching or deformation of the PI substrate.
2. Alignment and Fixation of QFP Precision Chips
Gently grasp the PQFP chip with anti-static tweezers and place it steadily onto the corresponding pad area of the FPC. Align all pins precisely with the pads and strictly verify the chip’s orientation to prevent issues such as reverse mounting or misalignment. Once alignment is confirmed, gently press down to secure the chip. Dip the soldering iron tip in a small amount of solder and solder the two opposite pins first. This locks the chip in place at two points, preventing it from shifting during the soldering process. After securing the chip, re-check the alignment accuracy. If any deviation is found, desolder and realign immediately to ensure the pins are fully in contact with the pads.
3. Soldering the Entire Row of Pins
Once the diagonal pins are secured, apply a layer of flux to all chip pin locations to keep the pins moist and improve solder flow. Dip the soldering iron tip evenly into an appropriate amount of solder, keep the tip parallel to the pins, and slide it at a steady speed to heat the area, allowing the solder to evenly wet the pins and pads and spread naturally into shape. Strictly control the amount of solder used during the process to avoid excessive build-up causing solder bridges or short circuits, whilst ensuring there are no cold joints, missed joints or voids.
4. Cleaning, Defect Removal and Re-inspection
Once all pins have been soldered, reapply flux to clean the soldered surface. Use desoldering strips or a desoldering pump to remove excess solder, thoroughly eliminating defects such as solder bridges and short circuits. Once the board has cooled naturally, use tweezers to inspect the soldering condition of each pin individually, checking for mis-soldered, cold solder joints or detached solder joints. Finally, use a brush dipped in anhydrous alcohol to gently wipe the board surface along the direction of the pins, thoroughly removing any residual flux, solder dross and dust to ensure the board surface is clean and tidy.
5. A Simple Method for Soldering SMD Resistors and Capacitors
Small SMD components such as surface-mount resistors and capacitors have a simple structure and are very compact, making the soldering process even simpler. First, apply a small amount of solder to one side of the pad. Use tweezers to hold the component and position it precisely. Once in place, solder one side of the leads first. After confirming that the component is aligned, flat and free of misalignment, solder the other side of the leads. Control the amount of solder used throughout the process to ensure the solder joints are full, secure and free of short circuits, meeting the fine-line soldering requirements of FPCs.
Core Advantages of FPC Flexible Circuit Boards
Exceptional spatial adaptability: FPCs can be freely bent, folded, twisted and coiled, allowing circuits to be arranged arbitrarily to suit irregular internal spaces and confined structures within products. They support three-dimensional routing, completely resolving the limitations of traditional rigid board routing. This achieves integration of component assembly, circuit connection and structural adaptation, significantly optimising internal product design.
Significant Advantages in Lightweight and Miniaturised Design: Compared to rigid PCBs, FPCs are thinner, extremely lightweight and feature higher circuit density. They can substantially reduce the volume of electronic products and lower the overall weight of the device, perfectly aligning with the high-density and miniaturisation trends in smartphones, wearable devices, ultraportable laptops and micro-smart devices.
Excellent overall performance: FPC substrates offer good heat dissipation, uniform circuit layout and rapid heat dispersion; they possess excellent solderability, suitable for the soldering of various precision surface-mount components; the assembly process is streamlined, significantly simplifying the overall assembly procedure and enhancing production efficiency. At the same time, the design of rigid-flex boards effectively addresses the shortcomings of purely flexible boards, such as weak load-bearing capacity and susceptibility to deformation, balancing flexibility with structural stability.
Wide range of applications: FPCs are now widely used across numerous sectors, including mobile communications, consumer electronics, computer peripherals, smart wearables, medical devices, aerospace, military equipment and automotive electronics, making them an indispensable core component in high-end smart hardware.
Inherent disadvantages of FPCs
Due to limitations in material properties, manufacturing processes and structural characteristics, FPCs also have significant drawbacks, presenting certain constraints in mass production, post-production maintenance and application scenarios.
Firstly, the costs of initial R&D and moulding are high. As FPCs are customised products, circuit design, film production, process development and mould customisation must be carried out individually according to the product structure, resulting in R&D investment far exceeding that of standard rigid PCBs. Consequently, FPCs offer extremely poor value for money in scenarios involving small batches or simple circuits, and are unsuitable for small-scale mass production applications.
Secondly, modifications and repairs are extremely difficult. Once the FPC circuit has been formed, laminated and coated, the structure becomes highly integrated, with the circuit completely enclosed by a protective layer; once the design is finalised, it cannot be modified at will. In the event of circuit faults or soldering defects, the protective film must be peeled off, the circuit repaired and the protective layer reapplied; this operation is complex and highly prone to damaging the board surface, resulting in high repair costs and a low success rate.
Thirdly, there are production limitations regarding product dimensions. Due to the specifications of FPC production equipment—such as lamination tables, exposure machines and cutting equipment—conventional mass production cannot produce excessively long or wide flexible circuit boards. Such large-scale circuit applications are difficult to accommodate and can only be realised through segmented splicing, which to some extent affects overall stability.
Fourthly, the materials are fragile and the operational threshold is high. FPC substrates are soft, the circuits are fine, and the pads are thin; during manual assembly, soldering and rework, issues such as circuit breakage, pad detachment, substrate cracking and thermal damage are highly likely to occur. This places extremely high demands on the technical proficiency of operators, who must undergo professional training before working with them; rework costs and defect rates are higher than for standard PCBs.
With their unique bendable properties and high-density interconnection capabilities, flexible printed circuits are driving the continuous evolution of electronic products towards lighter, thinner and more compact designs. As the demand for flexible interconnections continues to grow in 5G, wearable devices and medical electronics, mastering the full process of FPC manufacturing has become a practical skill in the hardware sector.