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1. Flexibility and reliability of flexible circuits
At present, there are four types of flexible circuits: single-sided, double-sided, multilayer flexible boards and rigid flexible boards.
1. Single-sided flexible board is the lowest cost printed board that does not require high electrical performance. In 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 a rolled copper foil. The insulating substrate can be polyimide, polyethylene terephthalate, aramid cellulose ester and polyvinyl chloride.
2. 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 design and use function of flexibility. The cover film can protect single and double-sided wires and indicate where the components are placed.
3. Multi-layer flexible board is to laminate 3 or more layers of single-sided or double-sided flexible circuits together, and form metallized holes by drilling and electroplating to form conductive paths between different layers. In this way, there is no need to use a complicated welding process. Multilayer circuits have huge functional differences in terms of higher reliability, better thermal conductivity and more convenient assembly performance. When designing the layout, the mutual influence of assembly size, number of layers and flexibility should be considered.
4. The traditional rigid-flex board is composed of rigid and flexible substrates selectively laminated together. The structure is compact, and the metallization hole L forms a conductive connection. If there are components on the front and back of a printed board, a rigid-flex board is a good choice. But if all the components are on one side, it will be more economical to choose a double-sided flexible board and laminate a layer of FR4 reinforced material on its back.
5. The flexible circuit of mixed structure is a kind of multilayer board, and the conductive layer is made of different metals. An 8-layer board uses FR-4 as the inner layer medium and polyimide as the outer layer medium. Leads extend from three different directions of the main board, and 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 low temperature conditions where the relationship between electrical signal conversion and heat conversion and the electrical performance are relatively harsh, and it is the only feasible solution.
It can be evaluated by the convenience of the internal connection design and the total cost to achieve the best performance-price ratio.
2. The economy 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 internal connection methods are much cheaper. If the circuit is complicated, processes many signals, or has special electrical or mechanical performance requirements, flexible circuits are a better design choice. When the size and performance of the application exceed the capacity of the rigid circuit, the flexible assembly method is the most economical. A 12mil pad with 5mil through holes and a flexible circuit with 3mil lines and spacing can be made on a 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 ion drilling pollution. These films may be protective and cure at a higher temperature to obtain a higher glass transition temperature. The reason why flexible materials save costs compared to rigid materials is the elimination of connectors.
High-cost raw materials are the main reason for the high price of flexible circuits. The price of raw materials varies greatly. The cost of the raw materials used in the lowest-cost polyester flexible circuit is 1.5 times that of the raw materials used in the rigid circuit; the high-performance polyimide flexible circuit is 4 times or higher. At the same time, the flexibility of the material makes it difficult to automate processing during the manufacturing process, which leads to a decrease in output; defects are likely 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 material cost is high and manufacturing is troublesome, the foldable, bendable, and multi-layer splicing function will reduce the overall component size, reduce the materials used, and reduce the total assembly cost.
The flexible circuit industry is undergoing small but rapid development. The polymer thick film method is an efficient and low-cost production process. This process selectively screen-prints conductive polymer inks on inexpensive flexible substrates. Its representative flexible substrate is PET. Polymer thick film conductors include silk-screened metal fillers or carbon powder fillers. The polymer thick film method itself is very clean, uses lead-free SMT adhesive, and does not need to be etched. Because of its use of additive technology and low substrate cost, polymer thick film circuit is 1/10 of the price of copper polyimide film circuit; it is 1/2 to 1/3 of the price of rigid circuit board. The polymer thick film method is particularly suitable for the control panel of the device. In mobile phones and other portable products, the polymer thick film method is suitable for converting components, switches and lighting devices on the printed circuit board into a polymer thick film method circuit. It not only saves costs, but also reduces energy consumption.
Generally speaking, flexible circuits are indeed more expensive and costly than rigid circuits. When manufacturing flexible boards, in many cases have to face the fact that many parameters are outside the tolerance range. The difficulty in manufacturing flexible circuits lies in the flexibility of the materials.
3. The cost of flexible circuits
Despite the aforementioned cost factors, the price of flexible assembly is declining, becoming close to traditional rigid circuits. The main reason is the introduction of newer materials, improved production processes and changes in structure. The current structure makes the thermal stability of the product higher, and there are few material mismatches. Some newer materials can produce more precise lines due to the thinner copper layer, making the components lighter and more suitable for small spaces. In the past, the copper foil was adhered to the adhesive-coated medium by using a rolling process. Nowadays, copper foil can be directly formed on the medium without using an adhesive. These techniques can get a few microns thick copper layer, get 3m. 1 Precision lines with even narrower widths. After removing certain adhesives, the flexible circuit has flame-retardant properties. This can speed up the uL certification process and further reduce costs. The flexible circuit board solder mask and other surface coatings further reduce the cost of flexible assembly.
In the next few years, smaller, more complex, and more expensive flex circuits will require more novel methods of assembly, and hybrid flex circuits will need to be added. The challenge for the flexible circuit industry is to use its technological advantages to keep pace with computers, remote communications, consumer demand, and active markets. In addition, flexible circuits will play an important role in the lead-free operation.
