PCB Blog - PCB flexible printed circuit boards

1.、The advantages of PCB flexible printed circuit boards
PCB flexible circuit boards are designed to improve space utilization and product design flexibility, meet the design needs of smaller and higher density installations, and also help reduce assembly processes and enhance reliability. It is the only solution to meet the requirements of miniaturization and mobility of electronic products. Flexible circuit boards are copper circuit boards etched on polymer substrates, or printed polymer thick film circuit boards. Design solutions for thin, lightweight, compact and complex devices range from single-sided conductive traces to complex multi-layer three-dimensional packaging. The total mass and volume of the flexible package can be reduced by 70% compared to the traditional element wire harness method. Flexible circuit boards can also be increased in strength through the use of reinforcing materials or liners for additional mechanical stability. PCB flexible circuit boards can be moved, bent, twisted without damaging the wires, and can have different shapes and special package sizes. Its only limitation is the volume space issue. With the ability to withstand millions of dynamic flexures, flexible circuit boards are well suited for continuous or periodic motion in interconnecting systems as part of the final product functionality. Some products that require electrical signal/power mobility and smaller form factor/package size benefit from flexible circuit boards. PCB flexible circuit boards provide excellent electrical properties. A lower dielectric constant allows for fast transmission of electrical signals; good thermal properties allow components to cool down easily; and a higher glass transition temperature, or melting point, allows components to perform well at higher temperatures.

Flexible circuit boards can provide higher assembly reliability and yield by reducing the hardware required for interconnection, such as solder joints, trunks, backplane lines and cables commonly found on traditional electronic packages. Because the traditional interconnection hardware composed of complex multiple systems is prone to a high component dislocation rate during assembly. With the advent of quality engineering, a thin, flexible system is designed to be assembled in only one way, eliminating the human error typically associated with independent wiring engineering. Early flexible circuit boards were mainly used in the fields of small or thin electronic products and connections between rigid printed boards. In the late 1970s, it was gradually applied to electronic products such as computers, digital cameras, inkjet printers, car audio, CD-ROM drives (see Figure 13-1) and hard disk drives. Turn on a 35mm camera and there are 9-14 different flexible circuit boards inside. The only way to reduce size is to have smaller components, finer lines, tighter pitches, and bendable objects. Pacemakers, medical devices, video cameras, hearing aids, laptops—just about everything used today has flexible circuit boards inside. The double-sided flexible board is a conductive pattern made by etching on both sides of the insulating base film. The metallized hole connects the patterns on both sides of the insulating material to form a conductive path to meet the flexible design and use function. The cover film protects single and double-sided conductors and indicates where the components are placed.

2、The function of PCB flexible circuit board
2.1 Flexibility and reliability of PCB flexible printed circuit boards
At present, there are four types of PCB flexible circuit boards: single-sided, double-sided, multi-layer flexible boards and rigid-flex boards
1) The single-sided flexible board is a cost, when the printed board does not require high electrical performance. When single-sided wiring, a single-sided flexible board 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 rolled copper foil. The insulating substrate can be polyimide, polyethylene terephthalate, aramid and polyvinyl chloride.
2) Double-sided flexible board is a conductive pattern made by etching on both sides of the insulating base film. The metallized hole connects the patterns on both sides of the insulating material to form a conductive path to meet the flexible design and use function. The cover film protects single and double-sided conductors and indicates where the components are placed.
3) Multi-layer flexible board is to laminate one or more layers of single-sided or double-sided flexible circuit boards together, form metallized holes through drilling and electroplating, and form conductive paths between different layers. In this way, there is no need to Using a complex soldering process. Multilayer circuit boards have huge functional differences in terms of higher reliability, better thermal conductivity and easier assembly performance. When designing the layout, the assembly size, number of layers and flexibility should be considered mutual influence.
4) Traditional rigid-flex boards are composed of rigid and flexible substrates selectively laminated together. The structure is compact, with metallized holes forming conductive connections. If a printed board has components on the front and back, rigid-flex boards are a good choice. But if all the components are on one side, it is more economical to use a double-sided flexible board with a layer of FR-4 reinforcement laminated on the back. The PCB flexible circuit board industry is in a small but rapid development. The polymer thick film method is an efficient and low-cost production process. The process selectively screen prints conductive polymer inks on inexpensive flexible substrates. Its representative flexible substrate PET. Polymer thick film conductors include screen-printed metal fillers or carbon powder fillers. The polymer thick film method is inherently clean, uses lead-free SMT adhesives, and does not require etching. Because of the additive process and the low cost of the substrate, the polymer thick film circuit board is 1/10 of the price of the copper polyimide film circuit board; it is 1/2-1/3 of the price of the rigid circuit board. The polymer thick film method is especially suitable for the control panel of the device. On mobile phones and other portable products, the polymer thick film method is suitable for converting components, switches and lighting devices on printed circuit boards into polymer thick film circuit boards. Save costs and reduce energy consumption.
5) The PCB flexible circuit board of mixed structure is a kind of multi-layer board, and the conductive layer is composed of different metals. An 8-layer board uses FR-4 as the inner dielectric and polyimide as the outer dielectric, with leads extending from three different directions of the main board, each of which is made of a different metal. Constantan alloy, copper and gold are used as independent leads. This kind of hybrid structure is mostly used in low temperature situations where the relationship between electrical signal conversion and heat conversion and electrical performance is relatively harsh, and it is the only feasible solution. It can be evaluated by the convenience and total cost of the interconnected design to achieve the best performance-price ratio.

2.2 The economy of PCB flexible circuit board
If the circuit board design is relatively simple, the total volume is not large, and the space is suitable, the traditional interconnection method is mostly much cheaper. If the circuit is complex, handles many signals or has special electrical or mechanical performance requirements, a flexible circuit board is a better design choice. Flexible assembly is economical when the size and performance of the application exceeds the capabilities of rigid circuit boards. A flexible circuit board with 12mil pads with 5mil through holes and 3mil lines and spaces can be made on a single 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 ionic contamination. These films may be protective and cure at higher temperatures, resulting in higher glass transition temperatures. The reason for the cost savings of flexible materials over rigid materials is the elimination of connectors. The high cost of raw materials is the main reason for the high price of flexible circuit boards. The price of raw materials varies greatly. The cost of raw materials used in polyester flexible circuit boards is 1.5 times that of rigid circuit boards; high-performance polyimide flexible circuit boards are as high as 4 times or more. At the same time, the flexibility of the material makes it difficult to automate processing during the manufacturing process, resulting in lower yields; defects are prone to occur during the assembly process, including peeling off flexible attachments 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 or reinforcing materials. Although the cost of raw materials is high and the manufacturing is troublesome, the foldable, bendable and multi-layer paneling functions will reduce the size of the overall assembly, and the materials used will be reduced, so that the total assembly cost will be reduced. Generally speaking, PCB flexible circuit boards are indeed more expensive and costly than rigid circuit boards. In the manufacture of flexible boards, many cases have to face the fact that many parameters are out of tolerance. The difficulty in manufacturing flexible circuit boards is the flexibility of the material.

3、The cost of PCB flexible circuit board
Despite the above-mentioned cost factors, the price of flexible assemblies is falling and is becoming close to that of traditional rigid circuit boards. The main reasons for this are the introduction of newer materials, improved production processes and altered structures. The current structure makes the product more thermally stable, with very few material mismatches. Some newer materials allow for more precise lines due to thinner copper layers, making components lighter and more suitable for fitting into small spaces. In the past, copper foils were attached to adhesive-coated media using a rolling process. Today, copper foils can be produced directly on the media without the use of adhesives. These techniques can obtain copper layers that are several microns thick, and precise lines with widths of 3 mil or even narrower. The flexible circuit board has flame retardant properties after some adhesives have been removed. This can speed up the UL process and further reduce costs. Flexible circuit board solder masks and other surface coatings further reduce flexible assembly costs. In the coming years, smaller, more complex and more expensive to assemble flexible circuit boards will require newer methods of assembly and the increase of hybrid flexible circuit boards. The challenge for the flexible circuit board industry is to take advantage of its technology to keep pace with computing, telecommunication, consumer demands and dynamic markets. In addition, flexible printed circuit boards will play an important role in the lead-free movement.