Three main features of flexible circuit boards

14 January 2020
New

1. Flexibility and reliability of FPC flexible circuit boards

At present, FPC has four types: single-sided, double-sided, multi-layer flexible board and rigid-flexible board.

a. The single-sided flexible board is the lowest cost when the board is not required for electrical performance. For single-sided wiring, a single-sided flexible board should be used. It has a 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 ester, and polyvinyl chloride.
b. The double-sided flexible board is a conductive pattern made by etching each 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 design and use functions of the flexibility. The cover film protects the single and double-sided wires and indicates where the components are placed.
c. The multi-layer flexible board is a single-sided or double-sided flexible circuit of 3 or more layers laminated together, and a metalized hole is formed through a drill collar L and electroplating to form a conductive path between different layers. This eliminates the need for complex welding processes. Multilayer circuits have tremendous 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.
d. Conventional rigid-flexible sheets are composed of rigid and flexible substrates that are selectively laminated together. The structure is tight, and the metalized 孑L forms an electrically conductive connection. If a printed board has components on both the front and back sides, a rigid-flexible board is a good choice. However, if all the components are on one side, it is more economical to use a double-sided flexible board and laminate a layer of FR4 reinforcement on the back.
e.The flexible circuit of the hybrid structure is a multilayer board, and the conductive layer is composed of different metals. An 8-layer board uses FR-4 as the inner layer medium, using polyimide as the outer layer medium, and the leads are extended from three different directions of the mainboard, each of which is made of a different metal. Constantan, copper, and gold are used as separate leads. This kind of hybrid structure is mostly used in the relationship between electrical signal conversion and heat conversion and the low-temperature condition with relatively strict electrical performance, which is the only feasible solution.

It can be evaluated by the convenience and total cost of the interconnect design to achieve the best performance-price ratio.

2. Economics of FPC flexible circuit boards

If the circuit design is relatively simple, the total volume is small, and the space is suitable, the traditional internal connection method is mostly much cheaper. Flexible circuits are a good design choice if the wiring is complex, handles many signals, or has special electrical or mechanical performance requirements. Flexible assembly is the most economical when the size and performance of the application exceed the capabilities of rigid circuits. A 12 mil pad with 5 mils through holes and a 3 mil line and pitch flexible circuit can be fabricated on a single film. Therefore, it is more reliable to mount the chip directly on the film. Because it does not contain a flame retardant that may be a source of ion drilling. These films may be protective and cure at higher temperatures, resulting in higher glass transition temperatures. The reason for the cost savings of flexible materials over rigid materials is that connectors are eliminated.

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 raw materials for polyester flexible circuits with the lowest cost is 1.5 times that of raw materials used for rigid circuits; the performance of high-performance polyimide flexible circuits is 4 times or higher. At the same time, the flexibility of the material makes it difficult to automate the processing during the manufacturing process, resulting in a drop in yield; defects are likely to occur during the final assembly process, including peeling off flexible attachments and broken lines. This type of situation is more likely to occur when the design is not suitable for the application. In the high stress caused by bending or forming, it is often necessary to select a reinforcing material or a reinforcing material. Despite its high raw material cost and manufacturing complexity, the foldable, bendable, and multi-layer panel features reduce overall component size and material use, resulting in lower overall assembly costs.

The flexible circuit industry is in the midst of a small but rapid development. The polymer thick film process is an efficient, low-cost production process. The process selectively screens conductive polymer inks on inexpensive flexible substrates. A representative flexible substrate is PET. Polymer thick film conductors include silk screen metal fillers or carbon powder fillers. The polymer thick film process itself is very clean, using lead-free SMT adhesives, without etching. Due to the use of the addition process and low substrate cost, the polymer thick film method is 1/10 of the price of the copper polyimide film circuit; it is 1/2 to 1/3 of the price of the rigid board. The polymer thick film method is especially 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 printed circuit boards into polymer thick film circuits. It saves costs and reduces energy consumption.

In general, flexible circuits are indeed more expensive and costly than rigid circuits. In the manufacture of flexible sheets, in many cases, they have to face the fact that many parameters are out of tolerance. The difficulty in manufacturing flexible circuits lies in the flexibility of the material.

3. FPC flexible circuit board cost

Despite the above-mentioned cost factors, the price of flexible assemblies is declining and becomes close to conventional 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 is a very little material mismatch. Some of the newer materials have thinner copper layers that allow for more precise lines, making the components lighter and more suitable for small spaces. In the past, the copper foil was adhered to the adhesive-coated medium by a roll-pressing process, and now it is possible to directly form a copper foil on the medium without using an adhesive. These techniques can obtain a copper layer several microns thick, resulting in 3m. 1 even narrow lines with narrower widths. The flexible circuit after removing some of the adhesives has flame retardant properties. This will speed up the uL certification process and further reduce costs. Flexible circuit board solder masks and other surface coatings further reduce flexible PCB assembly costs.