Flexible circuit board three main functions introduced
1, flexible circuit board flexibility and reliability
At present, flexible circuits are: single-sided, double-sided, multi-layer flexible board and rigid flexible board four.
① single-sided flexible board is the lowest cost, when the performance requirements of the printed circuit board is not high. In single-sided wiring, you should use single-sided flexible board. Flex PCB Which has a layer of 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 and polyvinyl chloride.
② double-sided flexible plate is in the insulating base film on both sides of each layer has a layer of conductive pattern. The metallized hole forms a conductive path on both sides of the insulating material to meet the flexibility of the design and use of the function. The cover film protects the single and double sided wires and indicates the position where the components are placed.
③ multi-layer flexible plate is 3 or more layers of single or double-sided flexible circuit laminated together, through the drilling larvae L, plating to form a metal hole, in different layers to form a conductive path. This eliminates the need for complex welding processes. Multilayer circuits have significant functional differences in terms of higher reliability, better thermal conductivity, and more convenient assembly performance. Flex PCB In the design of the layout, should take into account the assembly size, layers and flexibility of mutual influence.
The traditional rigid and flexible plates are composed of rigid and flexible substrates that are selectively laminated together. The structure is compact and forms a conductive connection with a metalized larvae. If a printed circuit board is positive and negative components, flexible circuit board is a good choice. Flex PCB But if all the components are on the side, then use double-sided flexible board, and in the back of a layer of FR4 reinforced material, will be more economical.
⑤ mixed structure of the flexible circuit is a multi-layer, conductive layer by the different metals. An 8-layer board uses FR-4 as the inner layer of the media, using polyimide as the outer layer of the media, extending from the motherboard in three different directions, each lead made of different metal. Comon alloy, copper and gold, respectively, as a separate lead. This hybrid structure is mostly used in the electrical signal conversion and heat conversion relationship and electrical performance is relatively harsh low temperature case, Flex PCB is the only feasible solution.
Can be designed through the convenience of the interior and the total cost of evaluation, in order to achieve the best cost performance.
2, the flexibility of flexible circuit boards
If the circuit design is relatively simple, the total volume is not large, and the space is appropriate, most of the traditional way to connect much cheaper. Flexible circuits are a better design option if the lines are complex, handle many signals, or have special electrical or mechanical performance requirements. Flexible assembly is the most economical when the size and performance of the application exceeds the capabilities of rigid circuits. Flex PCB In a film can be made with 5mil through the hole within the 12mil pad and 3mil lines and spacing of the flexible circuit. Therefore, the direct placement of the chip on the film is more reliable. Because it does not contain flame retardants that may be ion drilling sources. These films may be protective and cured at higher temperatures to achieve a higher glass transition temperature. The reason why the flexible material is cost-effective compared to the rigid material is that the connector is removed.
High cost of raw materials is the main reason for the high price of flexible circuits. The cost of raw materials is relatively large, the cost of the lowest cost of polyester flexible circuit raw materials used in the cost of raw materials is 1.5 times the rigid circuit; high-performance polyimide flexible circuit is up to 4 times or higher. At the same time, the flexibility of the material makes it difficult to automate the processing in the manufacturing process, resulting in a decrease in production; defects in the final assembly process, including the removal of flexible accessories, line breaks. This is more likely to occur when the design is not suitable for the application. In the bending or molding caused by high stress, often need to select the reinforcement material or reinforcement material. Despite the high cost of raw materials and the trouble of being made, the foldable, bendable and multi-ply features reduce the overall component size and reduce the amount of material used to reduce overall assembly costs.
Flexible circuit industry is in small scale but rapid development. Polymer thick film method is an efficient, low-cost production process. The process selectively chooses conductive polymer inks on inexpensive flexible substrates. Its representative flexible substrate is PET. Polymeric thick film conductors include screened metal fillers or toner fillers. The polymer thick film process itself is very clean, using lead-free SMT adhesives, without etching. Because of its use of the addition process and low cost of the substrate, the polymer thick film circuit is the copper polyimide film circuit price of 1/10; is the price of rigid circuit board 1/2 to 1/3. Polymer thick film method is particularly suitable for the control panel of equipment. In mobile phones and other portable products, the polymer thick film method is suitable for the printed circuit board components, switches and lighting devices into polymer thick film circuit. Both cost savings and reduced energy consumption.
In general, the flexible circuit is indeed more expensive than the rigid circuit, the higher the cost. Flexible circuit boards have, in many cases, have to face the fact that many parameters are outside the tolerance range. The difficulty of making flexible circuits lies in the flexibility of the material.
3, the cost of flexible circuits
Despite the above-mentioned cost factors, the price of flexible assembly is declining and becomes close to the traditional rigid circuit. The main reason is the introduction of updated materials, improved production processes and changed the structure. The current structure makes the product more thermally stable, with few materials that do not match. Some of the updated materials because of the copper layer is thinner and can produce more sophisticated lines, so that components more compact, more suitable for loading into a small space. In the past, Flex PCB copper foil was adhered to the adhesive-coated medium by the roll process, and now, copper foil can be directly formed on the medium without using an adhesive. These techniques yielded a few microns of copper layer, resulting in 3m. 1 even narrower width of the lines. Removal of certain adhesives after the flexible circuit has a flame retardant performance. This will speed up the uL certification process and further reduce costs. Flexible circuit board solder masks and other surface coatings make flexible assembly costs further reduced.
In the next few years, smaller, more complex and assembled costly higher flexible circuits will require a more innovative method of assembly, and the need to add hybrid flexible circuits. The challenge for the flexible circuit industry is to take advantage of its technological advantages to keep pace with computers, telematics, consumer demand, and active marketing. In addition, flexible circuits will play an important role in lead-free operations.