Flexible Printed Circuit Board (Flexible Printed Circuit abbreviated as FPC) is made of polyimide or polyester film as a substrate with a high degree of reliability, and excellent flexible printed circuit board. With high wiring density, lightweight, thin thickness, and good bending characteristics.
Flexible Printed Circuit (FPC) is a technology developed by the United States in the 1970s for the development of space rocket technology. It is made of polyester film or polyimide as a substrate, which is highly reliable and flexible. By embedding circuit design in a flexible thin plastic sheet, a large number of precision components can be stacked in a narrow and limited space to form a flexible circuit. Such circuits can be bent and folded at will, are lightweight, small in size, dissipate heat well, and are easy to install, breaking through the traditional interconnection technology. In the structure of flexible circuits, the materials are insulating films, conductors, and adhesives. Flexible printed circuits are the only solution to meet the miniaturization and mobility requirements of electronic products. Flexible printed circuits can greatly reduce the size and weight of electronic products and are suitable for the development of electronic products in the direction of high density, miniaturization, and high reliability.
1. FPC synthetic materials
(1) Insulating film
The insulating film forms the base layer of the circuit, and the adhesive bonds the copper foil to the insulating layer. In multilayer designs, it is bonded to the inner layer. They are also used as a protective cover to shield the circuit from dust and moisture and to reduce stress during bending. Copper foil forms the conductive layer. In some flexible circuits, the use of rigid elements made of aluminum or stainless steel provides dimensional stability, physical support for the placement of components and wires, and stress relief. Adhesives bond the rigid element to the flexible circuit. In addition, another material sometimes used in flexible circuits is an adhesive layer, which is formed by applying an adhesive to both sides of an insulating film. The adhesive layer provides environmental protection and electrical insulation and eliminates one film with the ability to bond multiple layers with a small number of layers.
There are many types of insulating film materials, but the most commonly used are polyimide and polyester materials. Nearly 80% of all U.S. manufacturers of flexible circuits use polyimide film materials and about 20% use polyester film materials. Polyimide materials are non-flammable, geometrically stable, have high tear strength, and resist soldering temperatures. Polyester, also known as polyethylene terephthalate (PET), has physical properties similar to those of polyimide, with a lower dielectric constant and less moisture absorption, but is not resistant to high temperatures. Polyesters have a melting point of 250°C and a glass transition temperature (Tg) of 80°C, which limits their use in applications requiring extensive end-soldering. In low-temperature applications, they exhibit rigidity. However, they are suitable for cell phones and other products that do not require exposure to harsh environments.
Polyimide insulating films are usually combined with polyimide or acrylic adhesives, and polyester insulations are usually combined with polyester adhesives.
(2) Conductors
Copper foil is suitable for use in flexible circuits. It can be electrodeposited (ED) or electroplated. Electrodeposited copper foil has a shiny surface on one side, while the processed surface on the other side is dull. It is a flexible material that can be made in many thicknesses and widths. The matte side of ED copper foil is often specially treated to improve its bonding ability. Forged copper foils are characterized by rigidity and smoothness in addition to flexibility. It is suitable for applications requiring dynamic deflection.
(3) Adhesives
In addition to bonding insulating films to conductive materials, adhesives can also be used as a cover layer, as a protective coating, and as a covering coating. The main difference between the two is the application method used. Said covering layer is bonded to said covering insulating film to form a circuit with a laminated structure. Screen printing techniques are used for the covering and application of the adhesive. Not all laminate structures contain adhesive and laminates without adhesive form thinner circuits and greater flexibility. Better thermal conductivity than adhesive-based laminate structures. Thermal conductivity is improved due to the thin structure of the adhesive-free flexible circuits and the elimination of the thermal resistance of the adhesive. It can be used in operating environments where flexible circuits based on adhesive laminate structures cannot be used.
2. Advantages and disadvantages of FPC
(1) Advantages of flexible printed circuits
Flexible printed circuit boards are printed circuits made from flexible insulated substrates, and have many advantages that rigid printed circuit boards do not have:
– Can be freely bent, wound, folded, can be arbitrarily arranged according to the requirements of the space layout, and can be moved in three-dimensional space, expansion, thus realizing the integration of component assembly and wire connection.
– The use of FPC can greatly reduce the size and weight of electronic products, suitable for electronic products to high-density, miniaturization, and high-reliability direction. Therefore, FPC has been widely used in aerospace, military, mobile communications, notebook computers, computer peripherals, PDAs, digital cameras, and other fields or products.
-FPC also has the advantages of good heat dissipation and solderability, easy to assemble, and low overall cost. The combination of soft and hard design also makes up to a certain extent the slight shortage of flexible substrate in the component carrying capacity.
(2) The disadvantages of flexible printed circuits
– High one-time initial cost: Because flexible PCB is designed and manufactured for special applications, the initial circuit design, wiring, and master photography require higher costs. Unless there is a special need to apply flexible PCB, it is usually best not to use it in a small number of applications.
– Difficulty of FPC Replacement and Repair: Once a Flexible PCB has been fabricated, it must be changed from the base map or the programmed light drawing program, so it is not easy to change. The surface is covered with a protective film, which must be removed before repair and restored after repair, a relatively difficult task.
– Size Limitations: Flexible printed circuit boards are usually manufactured by batch methods when they are not yet popular. As a result, they cannot be made very long or wide due to the size of the production equipment.
– Susceptible to damage: Improper handling by installers and connectors can easily cause damage to circuits, and their soldering and reworking require trained personnel.
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