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2022-12-06 17:30:13 Flexible Circuit Board 140 Viewed

SummaryFlexible circuit boards are a unique and important form of printed circuitry. IPC Standard IPC-T-50 "Terms and Definitions for Printed Boards" defines it as: "The pattern of printed circuits is arranged on a flexible substrate, which may or may not have a flexible cover". This is...

Flexible circuit boards are a unique and important form of printed circuitry. IPC Standard IPC-T-50 "Terms and Definitions for Printed Boards" defines it as: "The pattern of printed circuits is arranged on a flexible substrate, which may or may not have a flexible cover". This is an accurate but somewhat restrictive definition that does not adequately take into account the application of this technique. For example, flexible circuit boards can be used in static situations, such as in the "flex-to-fit" form, but also in dynamic situations, or intermittent like hinges, or Approximately contiguous as used on drive disks.

Flexible Circuit Board Technologies

Flexible circuit boards are the oldest and most versatile type of substrate. The pioneers of circuit boards first thought of using flexible circuit boards instead of flexible wiring. Flexible circuit boards are not only thinner than rigid circuit boards, but flexibility must be guaranteed throughout their service life, and must be able to withstand millions of repeated bendings, such as disk drive flexible circuit boards. The first difference, compared to rigid boards, is that there are no stiffeners in flex, at least for most products. The glass fabric often used in rigid circuit boards will affect the flexibility, and the glass fibers will eventually be broken.

Since there is no reinforcement with a low thermal expansion rate, the polymer of the flexible circuit board itself needs to have a low CTE. The most common flexible circuit board material is polyimide (PI), which is the first commercial PI film. PI is easy to match with copper's thermal expansion coefficient of 18ppm/°C. Polyimide contains nitrogen atoms, so it is inherently flame retardant and will not conflict with any upcoming bill against the use of halogen flame retardants.

Flexible Circuit Board Manufacturing

From many aspects, thin (25~125μm) flexible insulating materials are good printed circuit board materials. Polyimide is a very unique material among all laminates. It is very tough, heat-resistant, and can be etched and dissolved with a special alkaline etching solution without affecting the copper conductor. Two factors that limit the widespread use of polyimides are high moisture absorption (requires pre-drying during assembly) and high cost, the latter may be more important.

There are other polymers that can be used to make flexible circuit boards. The second type of widely used material is polyester material, which is the earliest polyester film. Polyester should only be used in applications where some degree of flammability can be tolerated (such as computer keyboards) and where soldering is not required. Polyester will start to shrink above 150°C, so it can only be welded by special local heating. Polyester is widely used due to its good electrical properties and low cost.

Classification of Flexible Circuit Boards

Flexible circuits are the most basic three-dimensional interconnection technology. Special design practices are required to produce reliable flexible circuit boards. Users should be familiar with the basic information of flexible circuit board design before designing flexible circuit boards to avoid possible problems. Several different types of flexible circuit boards are described below.

Single-sided flexible circuit board

Single-sided flexible circuit boards are the most common of these products, the lowest cost, and the most suitable for dynamic flex applications. Circuit patterns can be created by etching metal or printing conductive ink. If required, metal lines can be connected from one or both sides. In the latter case, the polymer-based film can be removed by appropriate methods (eg, with a laser). A special case variation is to include selective copper foil etching in which the copper thickness varies along the length of the circuit pattern, with thinner copper at flex locations and thicker copper where interconnects are required , and usually a cover layer is used to protect the flexible circuit board to improve the application life of dynamic deflection.

Double-sided flexible circuit board

Double-sided flexible circuit boards are the second common form of flexible circuit boards and are used where higher density interconnections are required. Obviously, this flexible circuit board has two metal layers and is normally interconnected by plated through holes, although this is not always necessary. For example, a flexible circuit board called "Type 5" has no plated through holes, and both metal layers are drawn from the same side.

Multilayer flexible circuit board

Multilayer flexible circuit boards are used where higher density interconnections are required. They are similar to the rigid structural parts of multilayer boards, but are often more complex and very dense in process. It seems amazing that making a highly practical multilayer flexible circuit board solves complex electrical and electronic interconnection problems. circuit board.

Rigid-flexible circuit board

Rigid-flex circuit boards are the last variant of flexible circuit boards, and they are a hybrid structure that combines both rigid and flexible circuit structural elements. The rigid part of most rigid-flex circuit boards is used to support connectors or components, and the flexible part is used for interconnection between rigid components. In the 1970s, the technology gained support from military product designers because it was a more reliable and lighter interconnect structure. Similar to multilayer flexible circuit boards, such circuits are often of high level fabrication, requiring detailed knowledge of their fabrication process in order to be rigorously designed.

Flexible Circuit Board Materials

The flexible circuit board is a kind of PCB with high reliability and high flexibility made of polyimide or polyester film and other materials, sometimes called flexible PCB or FPC, currently, widely used flexible circuit boards include polyester film, polyimide film and fluorinated ethylene propylene film.

1. Polyester film. The use temperature of this kind of flexible circuit board material can reach 80 ℃ ~ 130 ℃. It is characterized in that it can form a shrinkable coil when heated, which has good flexibility and good electrical performance. At the same time, it is affected by the surrounding environment. Humidity has little effect. It should be noted that the polyester film is prone to softening and deformation when the temperature is high, so the flexible circuit board should be extra careful during the soldering process.

2. Polyimide film. This flexible circuit board material can continue to work at temperatures up to 150°C, and the special fusion-bonded polyimide film using fluorinated ethylene propylene as the intermediate adhesive film can continue to work at temperatures up to 250°C. The main advantage of polyimide film is that it has good flexibility and good electrical properties. The disadvantage is that the humidity of the environment will have a certain impact on its performance.

3. Fluorinated ethylene propylene film: This fluorinated ethylene propylene film is usually combined with polyimide or glass cloth to make a laminate, and its welding temperature can reach 250°C. The main advantage of fluorinated ethylene propylene film is that it has good flexibility and good stability, and has good acid resistance, alkali resistance, moisture resistance and organic solvent resistance; The lower conductive pattern is prone to movement.

As a special basic material for electronic interconnection, flexible circuit board has remarkable characteristics. It can not only be bent statically, but also can be bent, curled and folded dynamically, and can also move and stretch freely in three-dimensional space. Wait. The use of flexible circuit boards can reduce the size and realize the characteristics of thin, light, and small products. Therefore, flexible circuit boards are rapidly shifting from military products to civilian use, and have been widely used.


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Flexible Circuit Board Description

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Flexible circuit board materials

Flexible circuit boards require adhesive and binder-free materials to attach their layers. Flexible adhesive material is the main body of flexible circuit board material. Binder-based materials are commonly used in single-sided and double-sided flexible circuit board designs. Glueless flexible materials do not have any adhesives. They are preferred for flexible circuit boards with large inner layers.

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