Speaking of flexible circuit boards, many friends are very unfamiliar. I do n’t know what this is. The flexible circuit board is also called FPC circuit board. It is a technology that has only been developed in recent years, mainly used in some sophisticated In products, mobile phones, computers, digital cameras, flexible circuit boards are generally used. Compared with ordinary circuit boards, flexible circuit boards have great advantages, light weight, and can be bent. Today, I will introduce you to flexible circuits The composition of the board.

Material of flexible circuit board 1. Insulating base material

The insulating substrate is a flexible insulating film. As the insulating carrier of the circuit board, choosing a flexible dielectric film requires a comprehensive investigation of the material’s heat resistance, coating performance, thickness, mechanical properties and electrical properties. Now commonly used in engineering are polyimide (PI: Polyimide, trade name KaptON) film, polyester (PET: Polyester, trade name Mylar) film and polytetrafluoroethylene (PTFE: Ployterafluoroethylene) film. Generally, the thickness of the film is selected in the range of O.0127 ~ O.127mm (O.5 ~ 5mil).

Material of flexible circuit board

The role of the bonding sheet is to bond the film and the metal foil, or to bond the film and the film (covering film). Different types of adhesive sheets can be used for different film substrates. For example, the adhesive sheet for polyester is different from the adhesive sheet for polyimide. The adhesive sheet of polyimide substrate is divided into epoxy and acrylic. Choosing the bonding sheet mainly investigates the fluidity and thermal expansion coefficient of the material. There are also polyimide copper-clad plates without adhesive sheets, which have better chemical resistance and electrical properties.

Due to the low glass transition temperature of the acrylic adhesive sheet, the large amount of contamination generated during the drilling process is not easy to remove, which affects the quality of the metallized holes and other undesirable materials such as other adhesive materials. Polyimide materials are commonly used for tablets. Because they are matched with polyimide substrates, the CTE (coefficient of thermal expansion) they ask is consistent, which overcomes the problem of dimensional instability in multilayer flexible circuits, and other properties are satisfactory .

Material of flexible circuit board and road board III. Copper foil

The copper foil covers the conductor layer adhered to the insulating substrate, and then forms a conductive circuit after selective etching. The vast majority of this copper foil is rolled copper foil (Rol led Copper Foil) or electrolytic copper foil (Electrodeposited Copper Foil). The ductility and bending resistance of rolled copper foil are better than electrolytic copper foil. The elongation of rolled copper foil is 20% to 45%, and the elongation of electrolytic copper foil is 4% to 40%. The thickness of the copper foil is most commonly 35um (1oz), but also has a thin 18um (O.5oz) or a thick 70um (2oz), or even 105um (30z). Electrolytic copper foil is formed by electroplating, and its copper particles are in the form of vertical needles. It is easy to form vertical line edges during etching, which is conducive to the production of precision circuits; The structure is prone to breakage; therefore, the flexible circuit substrate is mostly made of rolled copper foil, and its copper particles have a horizontal axis-like structure, which can adapt to multiple windings.

Material of flexible circuit board

The covering layer is an insulating protective layer covering the surface of the flexible circuit board, which protects the surface wires and increases the strength of the substrate. There are generally two types of protective materials for outer graphics.

The first type is the dry film type (covering film), which uses polyimide material, and directly presses the circuit board that needs to be protected after etching without laminating. This cover film needs to be pre-formed before pressing to expose the part to be welded, so it cannot meet the requirements of finer assembly.

The second type is photosensitive development type. The first type of photosensitive development type is to cover the dry film with a laminating machine and then expose the welded part through the photosensitive development method, which solves the problem of high-density assembly; the second type is the liquid screen printing type covering material, commonly used Thermosetting polyimide material, and solder resist ink for photosensitive developing flexible circuit board, etc.

Such materials can better meet the requirements of fine-pitch, high-density flexible boards.

Material of flexible circuit board

The reinforcement board is bonded to the local position of the flexible board, and the super-supporting and reinforcing effect on the flexible film substrate is convenient for the connection, fixing or other functions of the printed circuit board. Reinforced board materials are selected according to different applications. Commonly used polyester, polyimide sheet, epoxy fiberglass cloth board, phenolic paper board or steel plate, aluminum plate, etc.

The manufacturing requirements of flexible circuit boards are very high. In China, there are few manufacturers of such flexible circuit boards. In the past few years, before flexible circuit boards were invented, rigid circuit boards were often used. , That is, what we often say is the coil line, it will often break when it is used for a long time. Now that the mobile phone uses a flexible circuit board, the mobile phone will not have the previous problems. This is also the biggest advantage of the flexible circuit board, and it is also a technology in the future. An important sign of industry renewal.