Flexible circuit board related introduction

Product introduction

Flexible circuit board (FPC), also known as flexible circuit board, flexible circuit board, its light weight, thin thickness, free bending and folding and other excellent characteristics are favored. However, the domestic quality inspection of FPC mainly relies on manual visual inspection, which is high cost and low efficiency. With the rapid development of the electronics industry, circuit board design is becoming more and more high-precision and high-density, and the traditional manual detection method can no longer meet the production needs, and the automatic detection of FPC defects has become an inevitable trend of industrial development.

Flexible circuit (FPC) is a technology developed by the United States for the development of space rocket technology in the 1970s. It is a printed circuit with high reliability and excellent flexibility made of polyester film or polyimide as the substrate. By embedding the circuit design on a flexible thin plastic sheet, a large number of precision components are embedded in a narrow and limited space. Thus forming a flexible circuit that is flexible. This circuit can be bent and folded at will, light weight, small size, good heat dissipation, easy installation, breaking through the traditional interconnection technology. In the structure of a flexible circuit, the materials composed are an insulating film, a conductor and a bonding agent.

Component material 1, insulation film

The insulating film forms the base layer of the circuit, and the adhesive bonds the copper foil to the insulating layer. In a multi-layer design, it is then bonded to the inner layer. They are also used as a protective covering to insulate the circuit from dust and moisture, and to reduce stress during flexure, the copper foil forms a conductive layer.

In some flexible circuits, rigid components formed by aluminum or stainless steel are used, which can provide dimensional stability, provide physical support for the placement of components and wires, and release stress. The adhesive binds the rigid component to the flexible circuit. In addition, another material is sometimes used in flexible circuits, which is the adhesive layer, which is formed by coating the two sides of the insulating film with a adhesive. Adhesive laminates provide environmental protection and electronic insulation, and the ability to eliminate one thin film, as well as the ability to bond multiple layers with fewer layers.

There are many types of insulating film materials, but the most commonly used are polyimide and polyester materials. Nearly 80% of all flexible circuit manufacturers in the United States use polyimide film materials, and about 20% use polyester film materials. Polyimide materials have a flammability, stable geometrical dimension and has high tear strength, and have the ability to withstand the welding temperature, polyester, also known as polyethylene double phthalates (Polyethyleneterephthalate referred to as: PET), whose physical properties are similar to polyimides, has a lower dielectric constant, absorbs little moisture, but is not resistant to high temperatures. Polyester has a melting point of 250 ° C and a glass transition temperature (Tg) of 80 ° C, which limits their use in applications requiring extensive end welding. In low temperature applications, they show rigidity. Nevertheless, they are suitable for use in products such as telephones and others that do not require exposure to harsh environments. Polyimide insulating film is usually combined with polyimide or acrylic adhesive, polyester insulating material is generally combined with polyester adhesive. The advantage of combining with a material with the same characteristics can have dimensional stability after dry welding or after multiple laminating cycles. Other important properties in adhesives are low dielectric constant, high insulation resistance, high glass conversion temperature and low moisture absorption.

2. Conductor

Copper foil is suitable for use in flexible circuits, it can be Electrodeposited (ED), or plated. The copper foil with electric deposition has a shiny surface on one side, while the surface of the other side is dull and dull. It is a flexible material that can be made in many thicknesses and widths, and the dull side of ED copper foil is often specially treated to improve its bonding ability. In addition to its flexibility, forged copper foil also has the characteristics of hard and smooth, which is suitable for applications requiring dynamic bending.

3. Adhesive

In addition to being used to bond an insulating film to a conductive material, the adhesive can also be used as a covering layer, as a protective coating, and as a covering coating. The main difference between the two lies in the application used, where the cladding bonded to the covering insulation film is to form a laminated constructed circuit. Screen printing technology used for coating the adhesive. Not all laminates contain adhesives, and laminates without adhesives result in thinner circuits and greater flexibility. Compared with the laminated structure based on adhesive, it has better thermal conductivity. Because of the thin structure of the non-adhesive flexible circuit, and because of the elimination of the thermal resistance of the adhesive, thereby improving the thermal conductivity, it can be used in the working environment where the flexible circuit based on the adhesive laminated structure cannot be used.

Prenatal treatment

In the production process, in order to prevent too much open short circuit and cause too low yield or reduce drilling, calender, cutting and other rough process problems caused by FPC board scrap, replenishment problems, and evaluate how to select materials to achieve the best results of customer use of flexible circuit boards, pre-treatment is particularly important.

Pre-treatment, there are three aspects that need to be dealt with, and these three aspects are completed by engineers. The first is the FPC board engineering evaluation, mainly to evaluate whether the customer’s FPC board can be produced, whether the company’s production capacity can meet the customer’s board requirements and unit cost; If the project evaluation is passed, the next step is to prepare materials immediately to meet the supply of raw materials for each production link. Finally, the engineer should: The customer’s CAD structure drawing, gerber line data and other engineering documents are processed to suit the production environment and production specifications of the production equipment, and then the production drawings and MI (engineering process card) and other materials are sent to the production department, document control, procurement and other departments to enter the routine production process.

Production process

Two-panel system

Opening → drilling → PTH → electroplating → pretreatment → dry film coating → alignment → Exposure → Development → Graphic plating → defilm → Pretreatment → Dry film coating → alignment exposure → Development → etching → defilm → Surface treatment → covering film → pressing → curing → nickel plating → character printing → cutting → Electrical measurement → punching → Final inspection → Packaging → shipping

Single panel system

Opening → drilling → sticking dry film → alignment → Exposure → developing → etching → removing film → Surface treatment → coating film → pressing → curing → surface treatment → nickel plating → character printing → cutting → Electrical measurement → punching → Final inspection → Packaging → Shipping