1. Flexibility and reliability of flexible circuits
At present, there are four kinds of flexible circuits: single-sided, double-sided, multilayer and rigid flexible boards.
① Single sided flexible board is a kind of printed circuit board with low cost and low requirement for electrical performance. In single side wiring, single side flexible board should be selected. The conductive pattern layer on the flexible insulating substrate is calendered copper foil. The insulating substrate can be polyimide, polyethylene terephthalate, aramid fiber ester and polyvinyl chloride.
② The double-sided flexible plate is a conductive pattern etched on both sides of the insulating film. The metallized holes connect the figures on both sides of the insulating material to form a conductive path, so as to meet the design and use functions of the flexibility. The covering film can protect the single and double-sided wires and indicate the location of the components.
③ Multilayer flexible board is a kind of three or more layers of single-sided or double-sided flexible circuit laminated together. Through drilling L and electroplating to form metallized holes, the conductive path is formed between different layers. In this way, complex welding processes are not required. Multilayer circuits have huge functional differences in terms of higher reliability, better thermal conductivity and more convenient assembly performance. In the design of layout, the interaction of assembly size, number of layers and flexibility should be considered.
④ The traditional rigid flexible plate is composed of rigid and flexible base plates with selective formation pressure. The structure is compact and conductive connection is formed by metallization. If a printed circuit board has components on both sides, rigid flexible board is a good choice. But if all the components are on one side, it is more economical to choose double-sided flexible plate and laminate a layer of FR4 reinforcement on its back.
⑤ The hybrid structure flexible circuit is a kind of 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 three different directions of the main board, and each lead is made of different metals. 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 it is the only feasible solution when the electrical performance is relatively harsh at low temperature.
It can be evaluated by the convenience and total cost of the internal design to achieve the best performance price ratio.
2. The economy of flexible circuit
If the circuit design is relatively simple, the total volume is small, and the space is suitable, most of the traditional interconnection methods are much cheaper. If the circuit is complex, processing many signals or has special electrical or mechanical performance requirements, flexible circuit is a better design choice. Flexible assembly is the most economical when the size and performance of the application exceeds the capacity of the rigid circuit. A 12mil pad with 5MIL through hole and a 3mil line and spacing 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 retardant which may be the source of ion drilling pollution. These films may be protective and can be cured at higher temperatures to obtain higher glass transition temperatures. The reason for the cost savings of flexible materials over rigid materials is that connectors are eliminated.
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 raw materials for the lowest cost polyester flexible circuit is 1.5 times that of the rigid circuit, and that of the high-performance polyimide flexible circuit is as high as 4 times or more. At the same time, the flexibility of the material makes it difficult to carry out automatic processing in the manufacturing process, which leads to the decline of production; in the final assembly process, defects are easy to appear, such as peeling off flexible accessories, line fracture. This is more likely to happen when the design is not suitable for application. Under the high stress caused by bending or forming, it is often necessary to select reinforcement materials or reinforcement materials. Although the cost of raw materials is high and the manufacturing is troublesome, the foldable, bendable and multi-layer panel functions will reduce the size of the whole component, reduce the materials used, and reduce the total assembly cost.
Flexible circuit industry is in the process of small scale but rapid development. Polymer thick film process is an efficient and low-cost production process. This process can selectively screen print conductive polymer ink on cheap flexible substrate. The representative flexible substrate is pet. Polymer thick film conductor includes wire screen metal filler or carbon powder filler. The polymer thick film process itself is very clean and uses lead-free SMT adhesive without etching. Because of the addition process and low substrate cost, polymer thick film circuit is 1 / 10 of the price of copper polyimide film circuit and 1 / 2-1 / 3 of that of rigid circuit board. Polymer thick film method is especially suitable for the control panel of equipment. In mobile phones and other portable products, polymer thick film method is suitable for converting components, switches and lighting devices on printed circuit board into polymer thick film circuit. It can save cost and reduce energy consumption.
Generally speaking, the cost of flexible circuit is higher than that of rigid circuit. In many cases, the flexible plate has to face the fact that many parameters are beyond the tolerance range. The difficulty in making flexible circuits lies in the flexibility of materials.
3. Cost of flexible circuits
In spite of the above cost factors, the price of flexible assembly is falling and becoming close to the traditional rigid circuit. The main reason is the introduction of updated materials, the improvement of production process and the change of structure. The current structure makes the thermal stability of the product higher, very high
