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, when the printed circuit board does not require high electrical performance. When wiring on one side, a single-sided flexible board should be used. It has a layer of chemically etched conductive pattern, and the conductive pattern layer on the surface of the flexible insulating substrate is rolled copper foil. The insulating substrate may be polyimide, polyethylene terephthalate, aramid fiber ester, and polyvinyl chloride.
② The double-sided flexible board is a conductive pattern made by etching a layer on both sides of the insulating base film. The metalized holes connect the patterns on both sides of the insulating material to form a conductive path to meet the flexible design and use functions. The cover film can protect single and double-sided wires and indicate the location of components.
③ Multi-layer flexible board is to laminate three or more layers of single-sided or double-sided flexible circuits together, and to form metallized holes by drilling and plating, and to form conductive paths between different layers. In this way, no complicated welding process is required. Multilayer circuits have huge functional differences in terms of higher reliability, better thermal conductivity, and more convenient assembly performance. When designing the layout, the interaction between assembly size, number of layers and flexibility should be considered.
④Traditional rigid flexible board is composed of rigid and flexible substrates selectively laminated together. The structure is tight, and the metallized metal L forms a conductive connection. If a printed board has components on both the front and back, rigid-flex board is a good choice. But if all the components are on one side, it is more economical to choose a double-sided flexible board and laminate a layer of FR4 reinforcing material on the back side.
⑤The flexible circuit with mixed structure is a multi-layer 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 the main board in three different directions, 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 relationship between electrical signal conversion and heat conversion and the low temperature conditions where the electrical performance is more demanding, and is the only feasible solution.
It can be evaluated by the convenience of the interconnection design and the total cost to achieve the best performance-price ratio.
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 much cheaper. If the circuit is complex, handles many signals, or has special electrical or mechanical performance requirements, flexible circuits are a good design choice. When the size and performance of the application exceeds the capacity of the rigid circuit, the flexible assembly method is the most economical. A 12 mil pad with 5 mil through holes and a 3 mil line and pitch flexible circuit can be made on a film. Therefore, it is more reliable to mount chips directly on the film. Because it does not contain a flame retardant that may be a source of ion drilling contamination. These films may be protective and cure at higher temperatures, resulting in higher glass transition temperatures. The reason that 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, and the cost of the lowest cost polyester flexible circuit is 1.5 times that of the rigid circuit; the 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 carry out automated processing during the manufacturing process, which leads to a decline in production; defects are prone to occur during the final assembly process, these defects include peeling off flexible accessories and broken lines. This type of situation is more likely to occur when the design is not suitable for the application. Under the high stress caused by bending or forming, it is often necessary to choose reinforcing materials or reinforcing materials. Despite its high raw material cost and cumbersome manufacturing, the foldable, bendable, and multi-layer puzzle functions will reduce the overall component size, reduce the materials used, and reduce the overall assembly cost.
The flexible circuit industry is in a small but rapid development. The polymer thick film method is an efficient and low-cost production process. This process selectively screen-prints conductive polymer ink on an inexpensive flexible substrate. Its representative flexible substrate is PET. Polymer thick film conductors include screen-printed metal fillers or carbon powder fillers. The polymer thick film method itself is very clean, using lead-free SMT adhesive, without etching. Because of its addition process and low base material cost, the polymer thick-film method 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 especially suitable for the control panel of the equipment. 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 method circuits. Not only save costs, but also reduce energy consumption.
Generally speaking, flexible circuits are indeed more expensive and costly than rigid circuits. In the manufacture of flexible boards, in many cases it is necessary 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 cost factors mentioned above, the price of flexible assembly is declining and becoming close to traditional rigid circuits. The main reason is the introduction of newer materials, improved production processes and structural changes. The current structure makes the product more thermally stable, 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 media coated with adhesives using a roller pressing process. Today, copper foil can be directly generated on the media without using adhesives. These techniques can get a few microns thick copper layer, get 3m. 1 Precision lines with even narrower width. After removing some adhesives, the flexible circuit has flame retardant properties. This can speed up the uL certification process and further reduce costs. Flexible circuit board solder masks and other surface coatings further reduce the cost of flexible assembly.
In the next few years, smaller, more complex and more expensive flexible circuits will require more novel methods of assembly and the need to add hybrid flexible circuits. The challenge for the flexible circuit industry is to use its technological advantages to keep pace with computers, telecommunications, consumer demand, and active markets. In addition, flexible circuits will play an important role in lead-free operations.