FPC circuit board: a complete analysis from materials to processes
04/03/2025 在: Industry News /通过: administratorIn the electronics manufacturing industry, flexible printed circuit boards (FPCs) play a vital role. With the rapid development of science and technology, higher requirements are placed on the processing technology of FPCs. In order to meet market demand and improve production efficiency, we need to continuously innovate and optimize FPC processing technology. In this article, we will make a comprehensive analysis of FPC from materials to processing technology to help everyone better understand flexible circuit boards.
FPC concept
FPC, the full name of Flexible Printed Circuit, is a flexible printed circuit board, or soft board for short. It uses photo-imaging pattern transfer and etching technology on a flexible substrate to construct a conductor circuit, realize the electrical interconnection of the inner and outer layers of double-sided and multi-layer circuit boards, and protect and insulate through PI and glue layers. FPC is known for its high wiring density, light weight and thin design, and is widely used in many electronic products such as mobile phones, laptops, PDAs, digital cameras and LCMs.
Main raw materials of FPC
The main raw materials of FPC include: substrate, cover film, reinforcement material and other auxiliary materials. These materials play a vital role in the manufacturing process of FPC and together constitute the basis of flexible printed circuit boards.
1. Substrate:
As the supporting core of FPC, it determines the basic performance of the product. There are many types of substrates, and the selection should be based on the specific application scenarios and needs.
1.1 Glue substrate
Glue substrate, mainly composed of copper foil, glue and PI materials, is divided into single-sided substrate and double-sided substrate. Single-sided substrate is covered with copper foil only on one side, while double-sided substrate is covered with copper foil on both sides.
1.2 Glue-free substrate
Glue-free substrate, that is, substrate without glue layer, has a simpler structure than ordinary glue substrate, and is composed of only two parts: copper foil and PI. The advantages of this substrate are its thinner characteristics, excellent dimensional stability, excellent heat resistance, bending resistance and excellent chemical resistance. For this reason, glue-free substrate has been widely accepted and applied in various fields today.
In terms of copper foil, the common thickness specifications on the market include 1OZ, 1/2OZ and 1/3OZ. Recently, thinner 1/4OZ copper foil has been introduced. Although such materials have been used in China, their advantages are more obvious when making products with ultra-fine lines (line width and line spacing of 0.05MM and below). With the increasing demand of customers, this specification of copper foil is expected to be more widely used in the future.
2. Covering film
The covering film is mainly composed of release paper, glue layer and PI. During the production process, the release paper plays a role in protecting the glue layer to prevent it from being contaminated with foreign matter. But in the end, the release paper will be torn off, and the glue layer and PI together constitute an important part of the product.
3. Reinforcement material
Reinforcement material is specially designed for FPC to enhance the support strength of specific parts of the product, thereby improving the overly “soft” characteristics of FPC. There are many types of common reinforcement materials on the market.
1) FR4 reinforcement: It is mainly made of glass fiber cloth and epoxy resin glue, which is exactly the same as the FR4 material used in PCB.
2) Steel sheet reinforcement: This reinforcement material is mainly composed of steel, which is not only outstanding in hardness, but also has strong supporting strength.
3) PI reinforcement: It is similar to the cover film, consisting of PI and adhesive release paper, but the special thing is that its PI layer thickness can be customized from 2MIL to 9MIL.
Pure glue: This thermosetting acrylic adhesive film consists of protective paper/release film and a layer of glue. It is mainly used to bond layered boards, soft-hard boards, and FR-4 and steel sheet reinforcement boards.
Electromagnetic protective film: It is designed to be attached to the board surface to play a shielding role.
Pure copper foil: This material is composed only of copper foil and is a key material in the production process of hollow boards.
Unique advantages of flexible circuit boards
Flexible printed circuit boards, with their flexible insulating substrate as their feature, create many superior properties that rigid printed circuit boards do not have:
1. Flexibility: Flexible circuit boards can bend, roll and fold freely, fully adapting to the needs of spatial layout, while achieving easy movement and stretching in three-dimensional space, thus efficiently integrating component assembly with wire connection.
2. Size and weight advantages: With the help of flexible circuit boards, the volume and weight of electronic products can be significantly reduced, perfectly matching the trend of electronic products towards high density, miniaturization and high reliability. For this reason, flexible circuit boards have been widely used in aerospace, military, mobile communications, laptops, computer peripherals, PDAs, digital cameras and other fields or products.
3. Excellent characteristics: Flexible circuit boards not only have good heat dissipation and solderability, but also are easy to install and connect, and the overall cost is relatively low. Its soft and hard combination design makes up for the slight lack of flexible substrate in component carrying capacity to a certain extent.
Types of FPC
There are many types of flexible circuit boards, including single-sided flexible, double-sided flexible and multi-layer flexible. Among them, the cover layer of the single-sided flexible PCB is bonded to the single-sided FPC core without adhesive, while the double-sided flexible PCB is a double-sided FPC core without adhesive with cover layers bonded on both sides and plated through holes. Multi-layer flexible PCB contains three or more conductive layers with plated through holes, and its manufacturing capacity can reach more than 12 layers. In addition, there are special types of flexible circuit boards such as hollow boards, layered boards, and soft-rigid boards.
Detailed explanation of FPC production process
Single-sided board process flow:
Cutting: First, cut out the board of suitable size according to the design requirements.
Baking: Preheat the board to increase its machinability.
Dry film: Paste a layer of dry film on the board as a protective layer for subsequent processes.
Exposure: Transfer the circuit pattern to the dry film through the exposure machine.
Development: Wash off the unexposed dry film with chemical solution to expose the circuit pattern.
Etching: Etch off the part not covered by the dry film with etching liquid to form a circuit.
Demolding: Peel off the dry film from the board.
Pretreatment: Clean and activate the board to enhance surface adhesion.
Coating film: Paste a layer of covering film on the board to protect the circuit.
Lamination: Lay the covering film and the board together to form a circuit layer.
Curing: Cure the lamination layer by heating and pressurizing.
Surface treatment: Surface treatment of the circuit to enhance its corrosion resistance and conductivity.
Electrical measurement: Detect the connectivity and performance of the circuit through electrical measurement equipment.
Assembly: Assemble the circuit board with other components.
Pressing: Press the circuit board again to ensure that the connection between the components is firm.
Curing: Heat and pressurize again to cure the assembly layer.
Text: Print logos and instructions on the circuit board.
Shape: Cut the shape of the circuit board according to the design requirements.
Final inspection: Perform a final inspection on the circuit board to ensure that its quality and performance meet the requirements.
Packaging and shipment: Qualified circuit boards are packaged and then shipped.
Summary
Flexible PCB manufacturing requires full-link coordination from material selection, process parameters to testing standards. In the future, as AIoT devices develop towards high frequency and miniaturization, flexible PCBs will evolve towards ultra-thick copper layers (>3oz), embedded components, self-healing materials, etc., becoming the core technology supporting the innovation of smart hardware.