The relentless pursuit of the ultimate technological experience has been an ongoing endeavor, and a notable advancement in this regard is miniaturization, closely tied to the development of flexible circuit technology. In contrast to traditional rigid PCB, flexible PCB made from materials like polyimide, offer compatibility with the internal space of electronic devices. While they bring numerous benefits, it’s crucial to recognize the greater manufacturing complexity of flexible PCB. In this article, TechSparks will delve into each step of the flexible PCB process and introduce advanced FPC manufacturing technologies to better assist you in realizing your projects.
Table of Contents
Step 1: Design and Layout
Design always stands as the initial phase in PCB manufacturing. At this stage, designers can specify materials, thickness, circuit patterns, component layouts, and layering—essentially everything related to manufacturing. Flexible PCB commonly categorized as dynamic and static, share the same underlying principle of meeting three-dimensional spatial layouts, despite their inherent differences. The design should be based on the application’s characteristics and consistently adhere to PCB design rules. It’s crucial to note that constructing multilayer flex pcb is challenging, and manufacturers capable of achieving a 20-layer layout are scarce. Therefore, during the design phase, it’s imperative not to deviate from practical considerations to avoid increased manufacturing costs. Once the design is complete, you need to export the relevant files and send them to the manufacturer for review.
Further Reading: Complete Flexible PCB Design Guide
Step 2: Materials Selection
While various materials play a role in FPC manufacturing, the spotlight often falls on substrate materials when discussing material selection. Substrate materials ensure that the board can withstand various mechanical stresses during application and provide electrical performance. For flexible PCB, substrate materials may include polyimide, polyester film, liquid crystal polymers, and more. The key is to recognize the flexibility, dielectric constant, tensile strength, and other properties of substrate materials, ensuring your choice aligns with your requirements. Additionally, to achieve bendability, coverlay and adhesives also differ.
Further Reading: Which Flexible PCB Material is Best Suited for Your Project
Step 3: Printed Circuit
This stage is used to transfer the designed circuit pattern onto a copper foil to manufacture flexible PCB traces. First, the copper foil needs to be prepared and cleaned, and a dry film is applied to its surface. After exposure and etching, the required circuit pattern is preserved beneath the dry film, while the excess parts are removed, revealing the circuit pattern on the copper foil.
Step 4: Drilling
Drilling is carried out to create holes on the board for assembling and soldering, forming a flexible PCBA board. However, due to the characteristics of flexible boards, bending may occur during the CNC machining process, leading to potential defects. To address this issue, additional fixing measures need to be taken.
For more complex projects, using laser drilling is a more ideal choice. This helps achieve precise control over the position and size of the holes, reducing deformation issues during the manufacturing process. After drilling is completed, it is essential to clean the remaining debris to prevent adverse effects during component installation and soldering.
Further Reading: PCB Drilling Process Guide
Step 5: Plating
In multi-layer PCB, components and circuits are distributed across different layers. Electroplating is employed to deposit a thin layer of copper within the plated through-holes, allowing the flow of current from one layer of the flexible PCB to another, facilitating the connection between layers. This process involves coating the surface of the through-holes with an activator to enhance its reaction with the electroplating solution. Subsequently, the through-holes are immersed in the electroplating bath, and an electric current is applied, causing the metal to deposit on the inner walls of the flexible PCB through-holes.
Step 6: Cover Layer
The cover layer is a polymer film designed to protect the circuit pattern on a flexible PCB from physical damage and oxidation, similar to the function of the solder mask in rigid PCB. Before application, it needs to be cut according to the size of the PCB, then positioned on the flexible PCB and firmly secured to its surface through methods like heating or pressure.
Step 7: Lamination
The lamination process involves placing the stacked materials into a lamination machine, where the internal adhesive melts under pressure and high temperature. After cooling, the layers are tightly bonded together.
Further Reading: Comprehensive Introduction to PCB Lamination Process
Step 8: Surface Finish
PCB surface finish is a process designed to apply a thin film on board, enhancing its performance, corrosion resistance, and soldering characteristics. Various surface finish methods are used for flexible circuits, and the processes for these treatments can differ slightly.
ENIG involves depositing a layer of nickel onto the copper surface, followed by a layer of gold on top of the nickel.
OSP applies an organic protective layer onto the copper surface to improve adhesion and prevent oxidation.
Step 9: Testing
To ensure the quality of manufacturing, necessary PCB tests need to be performed both during and after completion:
- Bending test: The very reason for using a flexible PCB is its ability to withstand repeated folding or bending. To ensure the bending performance, it is crucial to select samples for a bending test.
- Fuse test: The wires and lines of flexible PCB are relatively thin, and subjecting them to excessive circuit can cause wire breakage. Therefore, it is vital to test whether the current load capacity of the manufactured board is up to standard.
Step 10: Cutting
During the production, FPC are often produced in a panel or array, and they need to be separated into individual pieces before they can be shipped to customers. A CNC router is a commonly used tool for this process, which typically involves the following steps:
- Load the full panel onto the cutting machine.
- Align the cutting tool with the cutting line.
- Cut the panel according to the required shape using the mobile router.
- Finish the edges of the board to smooth out any rough edges.
- Inspect the cut PCB to ensure there are no breaks or shorts in the circuitry.
Advanced FPC Manufacturing Technology
In order to achieve precise and efficient production, laser cutting technology is widely used, especially in the multi-layer and high-density flexible PCB manufacturing process. This technology enables manufacturers to cut intricate and irregularly shaped circuit boards from panels, ensuring precise cutting of inner layers in multi-layer boards without causing damage to other layers. Laser cutting is particularly effective in creating slots and holes with high precision. One of its key advantages over traditional methods is the utilization of computer control systems to guide the cutting tool, allowing the laser beam to achieve micron-level accuracy. This level of precision greatly enhances the overall quality and performance of flexible PCB in various applications.
Currently, machining methods, specifically cutting and forming wires and traces using cutters and CNC systems, are predominantly used in PCB manufacturing. However, PCB additive manufacturing, also known as 3D printing, is also being utilized in this field. This technique involves depositing conductive ink layer by layer onto flexible substrates using specialized printers. If you are in the developmental phase of your project, employing this technology for flexible PCB prototyping can expedite the process and offer cost-saving benefits.
Roll-to-roll (R2R) processing is a highly favored method for large-scale projects, offering enhanced efficiency and cost-effectiveness. In this process, lengthy rolls of flexible substrate are unwound and passed through a series of machines where conductive layers are added, adhesives are applied, and other necessary operations are performed. The entire process is continuous, allowing for the production of thousands of feet of circuitry in a single run, depending on the manufacturer’s capacity. However, it is important to note that manufacturing flexible PCB through R2R processing requires strict tolerances and precise component placement. Consequently, circuits with high-density or complex features may pose challenges due to design limitations.
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