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Three main characteristics of flexible circuit board (FPC)
1. Flexibility and reliability of flexible circuits

Current flexible circuits include: single-sided, double-sided, multi-layer flexible boards and rigid flexible boards.

1 Single-sided flexible board is the lowest cost printed board when the electrical performance is not high. In single-sided wiring, one-sided flexible board should be used. It has a layer of a chemically etched conductive pattern, and the conductive pattern layer on the surface of the flexible insulating substrate is a rolled copper foil. The insulating substrate may be polyimide, polyethylene terephthalate, aramid fiber ester, and polyvinyl chloride.

2 double-sided flexible board is a conductive pattern made by etching on both sides of the insulating base film. The metallization holes connect the patterns on both sides of the insulating material to form conductive paths to meet the design and use functions of flexibility. The cover film protects single and double conductors and indicates where the components are placed.

3 Multi-layer flexible boards laminate three or more layers of single-sided or double-sided flexible circuits together, forming metallization holes through drill collars L, electroplating, and forming conductive paths between different layers. This eliminates the need for complex welding processes. Multilayer circuits have huge functional differences in terms of higher reliability, better thermal conductivity and more convenient assembly performance. When designing the layout, consideration should be given to the interplay between assembly dimensions, layers, and flexibility.

4 Conventional rigid-flex panels consist of rigid and flexible substrates that are selectively laminated together. The structure is tight, with the metallization L forming a conductive connection. If there is a component on the front and back of a printed board, a rigid flexible board is a good choice. However, if all the components are on one side, it is more economical to use a double-sided flexible board and laminate a layer of FR4 reinforcement on the back.

5 The flexible circuit of the hybrid structure is a multilayer board, and the conductive layer is composed of different metals. An 8-layer board uses FR-4 as the inner layer medium and polyimide as the outer layer medium. The leads extend from the board in three different directions. Each lead is made of a different metal. Constantan alloys, copper and gold are separate leads. This hybrid structure is mostly used in the low-temperature situations where the electrical signal conversion and heat conversion are critical and the electrical properties are severe. This is the only feasible solution.

It can be evaluated through the convenience and total cost of the internal design to achieve the best performance-cost ratio.

2. Economics of flexible circuits

If the circuit design is relatively simple, the total volume is small, and the space is suitable, the traditional internal connection methods are mostly much cheaper. If the lines are complex, deal with many signals or have special electrical or mechanical performance requirements, flexible circuits are a good design choice. When the size and performance of applications exceed the capabilities of rigid circuits, flexible assembly is the most economical. A 12 mil pad with 5 mil through holes and a 3 mil line and pitch flexible circuit can be made on one film. Therefore, it is more reliable to mount the chip directly on the film. Because it does not contain flame retardants that may be sources of ion drilling. These films may be protective and cure at higher temperatures to give higher glass transition temperatures. The reason for the cost savings of flexible materials over rigid materials is to eliminate connectors.

The high cost of raw materials is the main reason for the high price of flexible circuits. The price of raw materials varies greatly, the cost of the raw materials used for polyester flexible circuits with the lowest cost is 1.5 times that of raw materials for rigid circuits, and the high-performance polyimide flexible circuits are up to 4 times or more. At the same time, the flexibility of the material makes it difficult to perform automated processing during the manufacturing process, which results in a decrease in yield; defects occur in the final assembly process. These defects include the removal of flexible attachments and broken lines. This kind of situation is more likely to occur when the design is not suitable for the application. In the high stress caused by bending or forming, it is often necessary to choose reinforcing materials or reinforcing materials. Despite the high cost of raw materials and manufacturing difficulties, the foldable, bendable, and multi-layered panels function will reduce the overall assembly size, use less material, and reduce overall assembly costs.

The flexible circuit industry is in a small but rapid development. Polymer thick film method is an efficient, low-cost production process. This process selectively screens conductive polymer inks on inexpensive flexible substrates. A representative flexible substrate is PET. Polymer thick film conductors include screen printed metal fillers or carbon powder fillers. The polymer thick film process itself is very clean, using lead-free SMT adhesives that do not require etching. Due to its use of additive processes and low substrate cost, polymer thick film circuits are 1/10th the price of copper polyimide thin film circuits; they are 1/2 to 1/3 of rigid circuit board prices. The polymer thick 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 suitable for converting components, switches, and lighting devices on a printed circuit board into polymer thick film circuits. This will not only save costs but also reduce energy consumption.

In general, flexible circuits do cost more than rigid circuits and have higher costs. When a flexible board is manufactured, in many cases it has to face the fact that many parameters are out of tolerance. The difficulty in making flexible circuits lies in the flexibility of the material.

3. Flex circuit cost

Despite the above-mentioned cost factors, the price of flexible assembly is declining and becomes close to the traditional rigid circuit. The main reasons for this are the introduction of newer materials, improved production processes, and changed structures. The current structure makes the product more thermally stable and there is little material mismatch. Some newer materials can produce more precise lines due to thinner copper layers, making the components lighter and more suitable for small spaces. In the past, the copper foil was adhered to the adhesive-coated media by a roll press process. Today, copper foil can be directly formed on the media without using an adhesive. These techniques can get a few microns thick copper layer, get 3m. 1 Even more narrow precision lines. The flex circuit after removing some of the adhesive has flame retardant properties. This will both accelerate the uL certification process and further reduce costs. Flexible circuit solder masks and other surface coatings further reduce the cost of flexible assembly.

In the next few years, smaller, more complex, and more costly assembly of flexible circuits will require newer methods of assembly and require the addition of hybrid flex circuits. The challenge for the flexible circuit industry is to take advantage of its technology and keep pace with computers, telecommunications, consumer demand, and an active market. In addition, flexible circuits will play an important role in the lead-free action.