Production process and quality control of FPC circuit boards

1、Introduction

Flexible Printed Circuit (FPC) is an indispensable key component in modern electronic devices, with a complex and precise production process involving various raw materials and advanced manufacturing technologies. Strict quality control is crucial to ensure the reliable performance of FPC boards and meet diverse application requirements. This article will delve into the complete production process of FPC boards, from raw material selection to finished product manufacturing, and elaborate on the key points of quality control in each step.

2、Production process flow of FPC circuit board

(1) Preparation of raw materials

Flexible substrate

Polyimide (PI) film is a commonly used flexible substrate, possessing excellent high-temperature resistance, chemical corrosion resistance, and mechanical properties. When selecting PI film, parameters such as thickness, tensile strength, and dielectric constant should be considered. For instance, for FPC used in high-frequency applications, a PI film with a low dielectric constant should be chosen to reduce signal transmission loss.

Other emerging flexible substrates, such as liquid crystal polymer (LCP), have also gradually garnered attention. LCP exhibits lower dielectric loss and superior high-frequency performance, making it suitable for the manufacture of FPCs used in high-speed signal transmission applications like 5G communication. However, its cost is relatively high.

Copper foil

Electrolytic copper foil and rolled copper foil are the two main types. Electrolytic copper foil has a mature production process and low cost, and is commonly used in FPC with general performance requirements; rolled copper foil has better ductility and bending resistance, and is suitable for FPC applications that require frequent bending, such as wearable devices. The thickness of copper foil is usually between 9μm and 35μm, and is selected according to the current carrying capacity and signal transmission requirements of the FPC.

Cover film and adhesive

Cover films are primarily used to protect circuits and provide insulation, typically consisting of PI films and adhesives. The performance of the adhesive directly affects the bonding strength and reliability between the cover film and the copper foil. Common adhesives include acrylic esters and epoxy resins, and the appropriate type and thickness need to be selected based on the usage environment and process requirements of the FPC.

Adhesives are used for bonding between various layers of materials, such as between the substrate and copper foil, and between layers of multilayer boards. Parameters such as bonding strength, curing temperature, and time need to be precisely controlled to ensure strong interlayer bonding of FPC. (II) Circuit Fabrication

Pattern Transfer

First, a layer of photoresist is coated on the surface of copper foil, and then the designed circuit pattern is transferred onto the photoresist through an exposure device. During the exposure process, parameters such as exposure dose, time, and focal length must be precisely controlled to ensure the clarity and accuracy of the circuit pattern. For example, for high-precision FPC with small line widths and line spacings, high-precision laser direct imaging (LDI) technology is required for exposure to avoid errors caused by traditional mask exposure.

After exposure, the photoresist undergoes development processing, during which the unexposed parts are dissolved and removed, revealing the copper foil areas that require etching. The concentration, temperature, and development time of the developer solution must be strictly controlled to prevent over-development or under-development, which can affect the quality of the circuit pattern.

Etching

The exposed copper foil is etched away using a chemical etching solution to form the desired circuit pattern. The composition (such as copper chloride, ammonium chloride, etc.), concentration, temperature, and etching time of the etching solution are key parameters. During the etching process, it is important to ensure uniform etching to avoid issues such as uneven circuit edges, over-etching, or under-etching. For multilayer FPC, attention should also be paid to the interlayer alignment accuracy during the etching process to prevent misalignment of circuits between different layers.

After etching, the remaining photoresist is removed through a stripping process to obtain a clean circuit surface. The selection of stripping solution and stripping conditions must also be carefully controlled to prevent damage to the circuit and substrate.

（3）Drilling and via metallization

Drilling

According to the design requirements of FPC, through-holes or blind holes are drilled at corresponding locations. Drilling methods include mechanical drilling and laser drilling. Mechanical drilling is suitable for larger diameter holes and has the advantage of low cost, but the drilling accuracy is relatively low; laser drilling can achieve micro-hole processing with smaller diameters, high accuracy, and fast speed, but the equipment cost is higher. During the drilling process, it is necessary to control the rotational speed of the drill bit, feed rate, and the use of cutting fluid to ensure the positional accuracy, hole diameter accuracy, and hole wall quality of the drilled holes.

For multi-layer FPC, the interlayer alignment accuracy of drilling is particularly important. High-precision positioning equipment and drilling processes are required to ensure the consistency of hole positions across all layers.

Through-hole metallization

The wall of the drilled hole needs to undergo metallization treatment to achieve electrical connection between layers. Common via metallization processes include electroless copper plating and electroplating copper. First, a thin copper seed layer is formed on the hole wall through electroless copper plating, followed by electroplating copper to achieve the desired thickness of the copper layer on the hole wall. The parameters such as the composition, temperature, and current density of the electroless copper plating solution and electroplating copper solution need to be strictly controlled to ensure the uniformity and reliability of the metallization of the hole wall, and to avoid issues such as hole wall voids or copper layer peeling.

（4）Surface Treatment

Electroless Nickel Immersion Gold

The electroless nickel-gold plating process can form a layer of nickel-gold plating on the surface of FPC circuits, which exhibits good solderability, corrosion resistance, and oxidation resistance. The parameters such as the composition, temperature, and time of the electroless nickel plating solution and gold plating solution need to be precisely controlled. The thickness of the nickel layer is generally 3-5μm, and the thickness of the gold layer is 0.05-0.15μm. The nickel layer serves as a barrier layer to prevent copper diffusion and oxidation, while the gold layer provides good soldering performance, making it suitable for FPC applications that require frequent soldering or long-term use.

Organic solderability preservative (OSP) treatment

OSP treatment involves forming an organic protective film on the copper surface to prevent oxidation before soldering. During soldering, the protective film decomposes due to heat, enabling the copper surface to bond well with the solder. The concentration, temperature, and immersion time of the OSP solution must be strictly controlled to ensure that the thickness of the protective film is uniform and within an appropriate range (typically 0.2-0.5μm). OSP treatment is relatively low-cost and is commonly used for FPC products where soldering performance is not particularly critical.

（5）Lamination and Molding

lamination

For multi-layer FPC, the materials for each layer with pre-fabricated circuits (including substrate, copper foil, adhesive, etc.) are laminated according to design requirements. During the lamination process, parameters such as lamination temperature, pressure, and time must be controlled to ensure tight bonding between layers without defects such as bubbles and delamination. The precision and stability of the lamination equipment also have a significant impact on the quality of lamination, and regular maintenance and calibration are required.

Forming

Based on the application shape of FPC, the laminated FPC is processed into the desired shape through methods such as punching, laser cutting, or mold forming. During the forming process, attention should be paid to the parameter settings of the cutting tool or laser to prevent damage to the FPC circuits and substrate, ensuring that the dimensional accuracy and edge quality of the formed FPC meet the requirements.

3、Key points of quality control in FPC circuit board production

（1）Raw material inspection

Incoming Inspection

Each batch of incoming raw materials, such as flexible substrates, copper foil, cover films, adhesives, etc., undergoes rigorous inspection. The inspection items include visual inspection (for any scratches, wrinkles, impurities, etc.), dimensional measurement (to ensure thickness, width, etc. meet specifications), and performance testing (such as tensile strength of substrates, resistivity of copper foil, bonding strength of adhesives, etc.). Only qualified raw materials can be used in production to prevent defects in FPC finished products caused by raw material quality issues.

Establish a quality assessment system for raw material suppliers, regularly audit and evaluate suppliers to ensure the stability and reliability of the raw material quality they provide. For suppliers experiencing quality issues, take timely measures, such as requiring rectification or switching to a different supplier.

（2）Process quality control

Online detection

During the production process, multiple online inspection points are set up to conduct real-time monitoring of product quality at each process. For example, after pattern transfer, the accuracy and integrity of the circuit pattern are inspected through a microscope; after etching, the line width, line spacing, and etching uniformity of the circuit are inspected; after via metallization, the quality of the metallization on the via wall is inspected. Once quality issues are discovered, timely adjustments or rework are carried out to prevent problematic products from flowing into the next process.

Automated testing equipment, such as automatic optical inspection (AOI) systems and X-ray inspection equipment, is employed to enhance testing efficiency and accuracy. The AOI system can swiftly detect defects on the surface of circuit boards, including open circuits, short circuits, and foreign objects. X-ray inspection equipment, on the other hand, is utilized to assess issues such as interlayer alignment and via quality within multilayer boards.

Process parameter monitoring

Key process parameters in the production process, such as exposure dose, etching solution concentration, electroplating current density, and lamination temperature, are monitored and recorded in real-time. Advanced sensors and data acquisition systems are employed to ensure that these process parameters fluctuate within the set range. Once the process parameters exceed the allowable range, an automatic alarm is triggered and corresponding adjustment measures are taken to ensure the stability and consistency of the production process.

Regularly conduct statistical analysis on process parameters, optimize process parameter settings based on product quality data feedback, and improve the production quality and efficiency of FPC circuit boards.

（3）Finished Product Inspection

Electrical performance test

Conduct comprehensive electrical performance tests on finished FPC circuit boards, including continuity testing (checking whether the lines are conductive and whether there are open circuits), insulation resistance testing (detecting the insulation performance between lines and between lines and the substrate), impedance testing (ensuring that the impedance of the signal transmission lines meets design requirements), and capacitance testing (checking the parasitic capacitance of the lines, etc.). Utilize professional electrical testing equipment, such as multimeters, insulation resistance testers, vector network analyzers, etc., to ensure that the electrical performance of the FPC circuit board meets application requirements.

Reliability test

Conduct reliability tests to simulate various scenarios of FPC in actual usage environments, such as high temperature and high humidity testing, temperature cycling testing, bending testing, vibration testing, etc. Through reliability tests, assess the long-term stability and reliability of FPC circuit boards and identify potential quality issues. For example, test the insulation performance of FPC and the corrosion resistance of metal plating in high temperature and high humidity environments; assess the bending resistance of FPC and the performance changes of the circuit during bending through bending tests. Only finished products that pass reliability tests can be delivered to customers for use.

4、Conclusion

The production process of FPC circuit boards encompasses multiple complex steps, ranging from raw material preparation to finished product manufacturing, with each step requiring precise control and rigorous quality management. By reasonably selecting raw materials, optimizing production processes, strengthening process quality control, and conducting strict finished product inspections, high-quality production of FPC circuit boards can be ensured, meeting the requirements of modern electronic devices for performance, reliability, and stability. With the continuous development of electronic technology, the production technology of FPC circuit boards will also continue to innovate and progress, and the quality control system will become more comprehensive, providing strong support for the development of the electronics industry.