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Knowledge of Flexible Printed Circuit Board Processing Technology

Ⅰ. Flexible printed circuit board (FPC) overview

Printed circuit boards(PCBs) are the necessary products of the electronic industry.

They are widely used in electronic products such as communication equipment, computers, automotive electronics, industrial equipment, and various household appliances. Their main functions are to support circuit components and interconnect circuit components. FPC(Flexible Printed Circuit Board) is a broad category of the PCBs, as shown in Figure 1 and Figure 2. According to the structure of FPC, it can be divided into a single-layer, double-sided, and multilayer flexible PCB according to the number of conductor layers.

FPC-Flexible Printed Circuit Board
Figure 1 FPC-Flexible Printed Circuit Board
Assembled Flexible Circuit Board
Figure 2 Flexible PCB Assembly

1.1 Flexure and reliability of flexible circuit board

At present, the FPC flexible circuit board has four kinds: single side, double side, multi-layer flexible board, and rigid-flexible board.

① The Single-sided flexible circuit board is the lowest cost printed circuit board that does not require high electrical performance. For a single-sided layout, a single-sided FPC should be used. It has a layer of chemically etched conductive patterns, and the conductive pattern layer on the surface of the flexible insulating substrate is a rolled copper foil. The insulating substrate can be polyimide, polyethylene terephthalate, aramid cellulose ester, and polyvinyl chloride.

② The double-sided flexible circuit board is a conductive pattern made by etching on both sides of the insulating base film. The metalized hole connects the models on both sides of the insulating material to form a conductive path to meet the design and use the function of flexibility. The cover film can protect single and double-sided wires and indicate where the components are placed.

③ The Multi-layer flexible circuit board is to laminate 3 or more layers of single-sided or double-sided FPC flexible circuit boards together and form metalized holes by drilling and electroplating to form conductive paths between different layers. In this way, no complicated welding process is required. Multilayer circuits have vast functional differences in terms of higher reliability, better thermal conductivity, and more convenient assembly performance. When designing the layout, the mutual influence of assembly size, the number of layers, and flexibility should be considered.

④The traditional rigid-flex PCB board is composed of rigid and flexible substrates selectively laminated together. The structure is compact, and the metalized hole forms a conductive connection. If there are components on the front and back of a printed board, a rigid-flex PCB is the right choice. But if all the parts are on one side, it will be more economical to choose a double-sided flexible PCB and laminate a layer of FR4 reinforced material on its back.

⑤The flexible circuit board with a mixed structure is a multilayer board, and the conductive layer is made of different metals. An 8-layer board uses FR-4 as the inner layer medium and polyimide as the outer layer medium. Leads extend from three different directions of the mainboard, and each point is made of a different metal. Copper and gold are used as independent leads. This kind of hybrid structure is mostly used in the relationship between electrical signal conversion and heat conversion and under low-temperature conditions where electrical performance is relatively harsh and is the only feasible solution.

1.2. The economy of FPC

FPC flexible circuit board is a better design choice.

If the circuit design is relatively simple, the total volume is not large, and the space is suitable, most of the traditional internal connection methods are much cheaper. If the circuit is complex, processes many signals, or has special electrical or mechanical performance requirements, FPCs are a better design choice. When the size and performance of the application exceed the capacity of the rigid circuit, the flexible assembly method is the most economical. A 12mil pad with 5mil through holes and a flexible circuit board with 3mil lines and spacing can be made on a film. Therefore, it is more reliable to mount the chip directly on the film. Because it does not contain flame retardants that may be a source of ion drilling pollution, these films may be protective and cure at higher temperatures to obtain a more upper glass transition temperature. The reason why flexible materials save costs compared to rigid materials is the elimination of connectors.

High-cost raw materials are the main reason for the high price of FPC flexible circuit boards.

The amount of raw materials varies greatly. The cost of raw materials used in the lowest-cost polyester FPC flexible circuit board is 1.5 times that of the raw material used in rigid circuits; the high-performance polyimide FPC flexible circuit board is 4 times or higher. At the same time, the flexibility of the material makes it challenging to automate processing during the manufacturing process, which leads to a decrease in output; defects are likely to occur in the final assembly process, such as peeling off flexible accessories and breaking lines. This type of situation is more likely to happen when the design is not suitable for the application. Under high stresses caused by bending or forming, it is often necessary to select reinforcing materials or reinforcing materials. Although the raw material cost is high and the manufacturing is troublesome, the foldable, bendable, and multi-layer jigsaw function will reduce the size of the overall assembly, reduce the materials used, and reduce the total assembly cost.

The flexible circuit board industry is undergoing small but rapid development.

The polymer thick film method is an efficient and low-cost production process. This process selectively screen prints conductive polymer inks on inexpensive, flexible substrates. Its representative flexible substrate is PET. Polymer thick film conductors include silk screen metal fillers or carbon powder fillers. The polymer thick film method itself is immaculate, using lead-free SMT adhesive, no etching is required. Because of its use of additive technology and low base material cost, polymer thick film circuit is 1/10 of the price of copper polyimide film circuit; it is 1/2 to 1/3 of the amount of rigid circuit board. The polymer adhesive film method is particularly suitable for the control panel of the device. In mobile phones and other portable products, the polymer thick film method is ideal for converting the components, switches, and lighting devices on the printed circuit board into polymer dense film circuits. It saves costs and reduces energy consumption.

Generally speaking, flexible circuit boards are indeed more expensive than rigid circuits.

Ⅱ. Main raw materials (physical and chemical materials) for the manufacture of FPC flexible printed circuit boards

(1) Flexible copper clad laminate (FCCL): According to the difference between one side covered with copper foil or both sides covered with copper foil, it is called single-sided copper-clad laminate and double-sided copper-clad laminate; according to the difference between the copper foil and the base film whether there is an adhesive, It is called glued copper-clad laminate and glueless copper clad laminate. The structure of the flexible copper clad laminate is shown in Figure 3. The substrate films of flexible copper clad laminates are commonly used polymer films such as PI, PET, PEN, and LCP. The copper-clad laminate is mainly responsible for the functions of conduction, insulation, and support on the entire printed circuit board. The performance, quality, processability in the manufacturing process, manufacturing cost, and manufacturing level of printed circuit boards depend to no small extent on the production of copper-clad laminates.

Flexible copper-clad (FCCL) structure
Figure 3 Flexible copper-clad (FCCL) structure

(2) Cover film: It is composed of organic film and adhesive, as shown in Figure 4. The function of the cover film is to protect the completed flexible circuit conductor part. Adhesive films have different substrate film and adhesive types and thickness specifications. Some flexible printed circuit boards do not use cover film on the surface but use solder resist to reduce costs.

Covering Film Structure
Figure 4 Covering Film Structure

(3) Adhesive film: It is formed by pouring adhesive on both sides or one side of a substrate film. There are also adhesive films with a transparent adhesive layer without substrate, as shown in Figure 5. The sticky film has different adhesive types and thickness specifications. The sticky film is used for interlayer adhesion and insulation of multilayer boards.

Bond Film Structure
Figure 5 Bond Film Structure

(4) Copper foil: There are electrolytic copper foil and rolled copper foil, as well as different copper foil thickness specifications. Copper foil is used for the two surface conductor layers in the production of multilayer printed circuit boards.

Besides, some FPC flexible printed circuit boards use stiffener materials, such as metal sheets, plastic sheets, resin films, and epoxy glass substrates. The role of the reinforcing material is to reinforce a part of the flexible circuit board for support and fixation, as shown in Figure 6.

FPC flexible printed circuit board with stiffeners
Figure 6 FPC flexible printed circuit board with stiffeners

Ⅲ. FPC flexible printed circuit board processing process

Flexible printed circuit board processing uses the same process and similar equipment conditions as rigid PCB boards. In the form of processing, there are sheet-by-sheet processing, which is identical to the intermittent and step-by-step processing of rigid boards, or Roll to Roll, which is the continuous processing of a roll of the substrate.

(1) Single-sided flexible printed circuit board manufacturing process (Figure 7)

Single-sided flexible printed circuit board manufacturing process
Figure 7 Single-sided flexible printed circuit board manufacturing process

(2) Double-sided flexible printed circuit board manufacturing process (Figure 8)

Double-sided flexible printed circuit board manufacturing process
Figure 8 Double-sided flexible printed circuit board manufacturing process

(3) The manufacturing process of multilayer flexible printed circuit board (Figure 9)

Manufacturing process of multilayer flexible printed circuit board
Figure 9 Manufacturing process of multilayer flexible printed circuit board
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