Features and Benefits 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 reduced by 70% compared to 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 flexible circuit can be moved, bent, twisted without damaging the wire, and can comply with different shapes and special package sizes. The only limitation is the volume space issue. Because it can withstand millions of times of dynamic bending, the flexible circuit can be well adapted to continuous motion or periodic motion interconnection system, and become a part of the final product function. The solder joints on rigid PCBs are subjected to thermal mechanical stress and fail after hundreds of cycles. Randy Lia, product manager at Shel-dahl, Northfield, Minn, said: "Certain products that require electrical signals/power to move, and smaller form factors/package sizes benefit from flexible circuits."
Flexible circuit has excellent electrical properties, dielectric properties, and heat resistance. Don Friedman, CEO of Endicott, Inter-national Flex TEchnologies in New York said. "The lower dielectric constant allows the rapid transmission of electrical signals; the good thermal performance makes the components easy to cool down; the higher glass transition temperature or melting point makes the components run well at higher temperatures."
Flexible circuits have higher assembly reliability and output. The flexible circuit reduces the hardware required for internal connections, such as solder joints, relay lines, backplane lines and cables commonly used in traditional electronic packaging, so that the flexible circuit can provide higher assembly reliability and output. Because the traditional interconnected hardware composed of multiple complicated systems is prone to high component misalignment rate when assembling. Mike Giesler, Marketing Manager of 3M Electronic Products Division, Austinm, Texas, said: "Flexible circuits have low rigidity and small size. It is precisely because of the small size of flexible circuit board components that they use less materials." With the quality With the advent of engineering, a thin flexible system is designed to be assembled in only one way, thereby eliminating many human errors usually associated with independent wiring projects.
The application of flexible components is increasing rapidly. Jim Barry, President and General Manager of Strataflex Hudson NH, said: "Almost when you pick up any electrical appliance today, you will find a flexible group in it?". Open a 35mm camera, there are 9 to 14 different Flexible circuits, because cameras are becoming smaller and have more functions. The only way to reduce the size is to have more filial components, finer lines, tighter pitches, and bendable objects. Pacemakers, medical equipment, video cameras , Hearing aids, portable computers-there are flexible circuits in almost everything we use today."
2. The advantages and effects of flexible circuits
2.1 Flexibility and reliability of flexible circuits
Four types of flexible circuits are currently popular: single-sided, double-sided, multilayer and rigid-flex combination. Friedman said: "The cost of a single-sided flexible board is the lowest. When the electrical performance is not high and single-sided wiring can be used, a single-sided flexible board should be used." This most common form has been commercialized. Such as the inkjet cartridge of the printer, the memory of the computer. The single-sided flexible board 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 used for flexible assembly can be polyimide (Kapton), polyethylene terephthalate (PET), aramid fiber paper (Nomex) and polyvinyl chloride (PVC).
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, no complicated welding process is required. 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 tight, 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 the same side, it is necessary to use a double-sided flexible board. It will be more economical to laminate a layer of FR-4 reinforced material on the back of the board. FR-4 will not form electrical connections with metallized holes or effective flexible circuits, but Play a reinforcement role. This not only enhances the reliability, but also reduces the manufacturing process or the process of installing components, or the damage after the installation of the components." Amalfitano suggests that considering the 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. 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 costs, but also reduces energy consumption."
There is also a flexible circuit with a hybrid structure, which is also a multilayer board, but the conductive layer of the multilayer board is composed of different metals. Jack Lexin, an application engineer at LEFlex Circuits in Carlsbad, California, said: "An 8-layer board uses FR-4 as the inner layer and kapton as the outer layer, extending from three different directions of the motherboard. Leads, each lead 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 the relationship between electrical signal conversion and heat conversion, and the electrical performance is relatively harsh at low temperatures. in the case of. In this case, flexible hybrid circuits are the only feasible solution.
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 research and development engineer, I have a good understanding of flexible assemblies. "The overall method of internal connection is different. Mobile phones are in a block layout; portable computers are in XY orientation; the printer is in a rigid-flex PCB format. These products are made of different materials with different prices to reduce cost. The cost of connecting leads. Each design must be typologically evaluated to achieve the best performance-to-price ratio."
2.2 The economics of flexible circuits
If the circuit design is relatively simple, the total volume is not large, and the space is suitable, most of the traditional interconnection methods are more cost-effective. If the circuit is complicated, processes many signals, or has special electrical or mechanical performance requirements, flexible circuits are a better design choice. Tim Patterson, design and development manager of Smartflex in Tustin, Califonia, said: "PCB should be preferred if possible. Multilayer is especially cheap. When the size and performance of the application exceeds the capabilities of the PCB, flexible assembly is the most economical choice. A flexible circuit with 5 mil aperture, 3 mil lines and pitch in 12 mil pads can be made on a piece of film.Therefore, it is more reliable to mount chips directly on films (such as polyimide films) because they do not contain flame retardants that may be sources of ion contamination. These films may be protective and cure at higher temperatures, resulting in higher glass transition temperatures. "Compared with rigid materials, flexible materials have a potential cost saving reason, which is the elimination of connectors.
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 the cost of the raw material used in the PCB; high-performance polyimide Flexible circuits are up to 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 and breaking lines. This type of situation is more likely to occur 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 its raw materials are more expensive and it is troublesome to manufacture, DiPalermo still believes that 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.
Amalfitano commented: "Generally speaking, flexible circuits are indeed more expensive than rigid, and have always been more expensive. Compared with rigid boards, flexible boards have to face the fact that in many cases when they are manufactured: The parameters are outside the tolerance range. The difficulty in manufacturing flexible circuits lies in the flexibility of the material. On the rigid board, you are processing a 15mil FR-4 glass cloth board, and you punch a hole in the glass cloth board or perform all the processing In the process, when you come back, the hole is still in the exact position. On the flexible material, when you come back, the hole has moved 5mil. This is the number one reason why the flexible board is expensive."
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. There is an example of eliminating the use of acrylic adhesives on rigid-flex board assemblies with many layers. "If you build a 12 or 14-layer rigid-flex circuit board, and you use acrylic in it, there will be Z-axis expansion and metallized holes failure." Barry said, "The current structure makes the product hot The stability is higher, and there are few material mismatches. The output is increased, and the cost is reduced. Now some newer materials can produce finer lines due to the thinner copper layer. Thinner copper layer promotes more and more components Lightweight, lighter and thinner assembly makes flexible components more suitable for packing in smaller spaces. In the past, we used a rolling process to adhere the copper foil to the adhesive-coated medium. Today, it is not necessary to use it. The adhesive directly generates copper foil on the medium. The copper layer of several micrometers can be obtained by these technologies, which enables the industry to obtain fine lines of 3 mil or even narrower width. "After removing the adhesive from the flexible circuit, it makes flexible The circuit has flame retardant properties. 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 with more filial piety, more complexity and higher assembly cost 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, telecommunications, consumer demand, and active markets. In addition, flexible circuits will play an important role in the lead-free operation.
