PCB Technical - The function and role of FPC

Discussions on the characteristics, advantages and effects of FPC from various parties in the design, production and application of flexible circuits.

1. The characteristics of flexible circuits

The flexible circuit is small in size and light in weight. The original design of the flexible circuit board is to replace the larger wire harness wire. On current cutting-edge electronic device assembly boards, flexible circuits are usually the only solution to meet the requirements of miniaturization and mobility. Flexible circuits (sometimes called flexible printed circuits) are copper circuits or printed polymer thick film circuits etched on a polymer substrate. For thin, light, compact and complex devices, the design solutions range from single-sided conductive lines to complex multilayer three-dimensional assembly. The total weight and volume of the flexible assembly are 70% less than the traditional round wire harness method. Flexible circuits can also increase their strength by using reinforced materials or liners to achieve additional mechanical stability.

The application of flexible components is increasing rapidly. Strataflex Hudson, President and General Manager of NH Jim Barry, said: "Almost when you pick up any electrical appliance today, you will find a flexible group in it?". Open a 35mm camera and there are 9 to 14 differences in it. The flexible circuit of the camera is becoming smaller and more functional. The only way to reduce the size is to have more filial components, finer lines, tighter pitches, and bendable objects. Pacemakers, medical equipment, videos Cameras, hearing aids, portable computers-almost everything we use today has flexible circuits in them."

2. The advantages and effects of flexible circuits

2.1 Flexibility and reliability of flexible circuits

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

Multi-layer flexible board is to laminate three or more layers of single-sided flexible circuit or double-sided flexible circuit together, through drilling and electroplating to form metallized holes, forming conductive paths between different layers. In this way, there is no need to use a complicated welding process. Al Balzano, vice president and general manager of Basic Electronics in Garden Grove, California, said: "Multilayer circuits have huge functional differences in terms of higher reliability, better thermal conductivity, and easier assembly performance." Although the number of conductive layers designed for this flexible type can be unlimited, in order to ensure ease of assembly when designing the layout, the mutual influence of assembly size, number of layers and flexibility should be considered.

The traditional rigid-flex board is composed of rigid and flexible substrates selectively laminated together. The structure is compact, and the conductive connection is formed with metalized holes. Mario Amalfitano, International Sales Manager of Aero Flexible Ciruitry in Torrance, California, commented: "If your board has components on the front and back sides, rigid-flex board is a good choice. But if all the components are in For one side, it is more economical to choose a double-sided flexible board and laminate a layer of FR-4 reinforced material on its back. FR-4 will not form electrical connections with metallized holes or effective flexible circuits, but only Reinforcement. This not only enhances the reliability, but also reduces the damage in the manufacturing process or the process of installing components, or after installing the components." Amalfitano suggests that considering reliability and price factors, the manufacturer should try to keep the number of layers as small as possible.

The flexible circuit industry is undergoing small but rapid development. The polymer thick film method (PTF) is an efficient and low-cost production process for circuit boards. The process is to selectively screen print conductive polymer inks on a cheap flexible substrate. Its representative flexible substrate is PET. PTF conductors include silk-screened metal fillers or carbon powder fillers. The PTF itself is very clean, using lead-free SMT adhesive, no etching is required. Al Hollenbeck, Technical Director of Poly-Flex in RI Cranstom, said: "Because of its use of additive processes and low-cost substrates, PTF circuits are ten times cheaper than copper Kapton circuits and 2-3 times cheaper than PCBs. PTF is especially It is suitable for the control panel of equipment because of its low cost and easy assembly and replacement under the flat graphic panel. On mobile phones and other portable products, PTF is suitable for converting the components, switches and lighting devices on the PCB motherboard into PTF circuit. It not only saves cost, but also reduces energy consumption.

Whether the structure of these flexible circuits saves costs and whether they are optimally utilized can be evaluated by the convenience of the interconnection design and the total cost. George Serpa is a contract manufacturer of Flextronics International in San Jose, Califonia. As a senior product R&D engineer, I have a good understanding of flexible assemblies. "The overall way of interconnection is different. Mobile phones are in block layout; laptops are in XY orientation; printers are in rigid-flex PCB format. These products are made of different materials with different prices to reduce The cost of connecting leads. Each design must be typologically evaluated to achieve the best performance-to-price ratio."

2.2 The economy of flexible circuits

High-cost raw materials are the main reason for the high price of flexible circuits. Joseph DiPalermo, Application Engineering Manager of Parlex, Mass., in Methuen, said: "The price of raw materials varies greatly. The cost of the polyester flexible circuit with the lowest raw material cost is 1.5 times that of the raw material used; high-performance polyimide. Flexible circuits are as high as 4 times or higher. At the same time, the flexibility of the material makes it difficult to automate processing during the manufacturing process, resulting in a decrease in output; defects are prone to occur in the final assembly process, such as peeling off flexible accessories, Line breaks. This type of situation is more likely to occur when the design is not suitable for the application. Under the high stress caused by bending or forming, it is often necessary to choose reinforcing or reinforcing materials. "Although the raw materials are expensive and the manufacturing is troublesome, DiPalermo still believes, Foldable, bendable, and multi-layer splicing functions will reduce the size of the overall assembly, reduce the materials used, and reduce the total assembly cost.

2.3 The cost of flexible circuits is being further reduced

Despite the above-mentioned cost factors, the price of flexible assembly is declining, becoming close to traditional PCB. This is mainly due to the introduction of newer materials, improved production processes and changes in structure. After the adhesive is removed from the flexible circuit, the flexible circuit is made flame-retardant. This can speed up the UL certification process and further reduce costs. When flexible circuits continue to rapidly develop from the initial military industrial applications to civilian and consumer applications, it is even more important to obtain UL certification. The flexible board solder mask and other surface coatings further reduce the cost of flexible assembly. Barry always believes that in the past ten years, some of these new materials and new processes have greatly reduced costs. At the same time, it is precisely because such products have been widely recognized and demanded that the cost of flexible materials is also falling.

In the next few years, flexible circuits that are more filial, more complex and more expensive to assemble will require more novel assembly, and hybrid flexible circuits will need to be added. The challenge for the flexible circuit industry is to strengthen its technological advantages and keep pace with computers, telecommunication, consumer demand, and active markets. In addition, flexible circuits will play an important role in the lead-free operation.