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PCB Tech
​Double-sided FPC manufacturing: cover film processing technology
PCB Tech
​Double-sided FPC manufacturing: cover film processing technology

​Double-sided FPC manufacturing: cover film processing technology

2021-11-06
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Author:Downs

One of the unique processes of the flexible printed circuit board manufacturing process is the processing procedure of the cover layer. There are three types of processing methods for the covering layer, covering film, screen printing of the covering layer, and photocoating layer. Recently, there has been an updated technology that has expanded the range of options.

The processing of FPC cover film is divided into three parts:

1. Screen printing of FPC overlay

2. FPC cover film

3. FPC photocoating layer

1. Screen printing of FPC overlay

The missing cover layer has worse mechanical properties than the laminated cover film, but the material cost and processing cost are lower. The most used are civilian products and flexible printed boards on automobiles that do not require repeated bending. The process and equipment used are basically the same as those for solder mask printing on rigid printed boards, but the ink materials used are completely different. It is necessary to choose inks suitable for flexible printed boards. There are UV curable and heat curable inks in the market. The former has a short curing time and is convenient, but the general mechanical properties and chemical resistance properties are poor. If it is used for bending or under harsh chemical conditions, it is sometimes inappropriate. In particular, it is necessary to avoid electroless gold plating, because the plating solution will penetrate from the end of the window to the cover layer, which will cause the cover layer to peel off. Since the curing of thermosetting inks requires 20-30 minutes, the drying tunnel for continuous curing is relatively long, and intermittent ovens are generally used.

2. FPC cover film

pcb board

Cover film is the earliest and most used technology for flexible printed board cover layer applications. The same film as the base film of the copper clad laminate is coated with the same adhesive as the copper clad laminate to make it into a semi-cured adhesive film, which is sold and supplied by the copper clad laminate manufacturer. At the time of delivery, a release film (or paper) is attached to the adhesive film. The semi-cured epoxy resin adhesive will gradually solidify at room temperature, so it should be stored in low temperature refrigeration. Printed circuit manufacturer Before use, it should be kept in a refrigerated warehouse at around 5°C or sent by the manufacturer before use. The general material manufacturer guarantees a use period of 3 to 4 months, if it can be used for 6 months under refrigerated conditions. Acrylic adhesives hardly solidify at room temperature. Even if they are not stored under refrigerated conditions, they can still be used after storage for more than half a year. Of course, the lamination temperature of this adhesive must be very high.

One of the most important issues for the processing of the cover film is the fluidity management of the adhesive. Before the cover film leaves the factory, the material manufacturer adjusts the fluidity of the adhesive to a specific range. Under the conditions of refrigerated storage at an appropriate temperature, the service life of 3 to 4 months can be guaranteed, but within the validity period, the adhesive The fluidity of the agent is not fixed, but gradually decreases over time. Generally, since the adhesive is very fluid when the cover film is just shipped from the factory, the adhesive is easy to flow out during lamination and contaminate the terminal part and the connection plate. At the end of the service life of the adhesive, its fluidity is very small or even no fluidity. If the lamination temperature and pressure are not high, a covering film that fills the pattern gaps and has high bonding strength cannot be obtained.

The cover film needs to be processed by opening windows, but it cannot be processed immediately after being taken out of the refrigerator. Especially when the ambient temperature is high and the temperature difference is large, the surface will condense water drops. Will absorb moisture and affect the subsequent processes. Therefore, the general roll cover film is sealed in a polyethylene plastic bag. The sealed bag should not be opened immediately after being taken out from the refrigerator, but should be placed in the bag for several hours. When the temperature reaches room temperature, it can be removed from the sealed bag. Take out the cover film for processing

The laminating method has a great influence on the state of the adhesive filled between the lines and the bending resistance of the finished flexible printed board. Laminated materials are commercially available general products. Taking into account the cost of mass production, each flexible board factory has self-made laminated materials. According to the FPC structure and the materials used, the material and structure of the laminated board are also different.

3. FPC photocoating layer

The basic process of the photocoating layer is the same as that of the photoresist film used for rigid printed boards. The materials used are also dry film type and liquid ink type. In fact, the solder mask dry film is still different from the liquid ink. Although the coating process of the dry film type and the liquid type are completely different, the same device can be used for the exposure and subsequent processes. Of course, the specific process conditions will be different. . The dry film must be pasted first, and all the circuit diagrams are covered with dry film. The ordinary dry film method is likely to have air bubbles between the lines, so a vacuum filming machine is used.

Ink type is to use screen printing or spraying method to coat the ink on the circuit pattern. Screen printing is the use of more coating methods, the same as the rigid printed board process. However, the thickness of the ink coated by a missed printing is relatively thin, basically 10 ~ 15um, due to the squareness of the circuit. Orientation, the thickness of the ink in one printing is not uniform, and even skip printing occurs. In order to improve the reliability, the missing printing direction should be changed and then the second missing printing should be performed. The spraying method is still a relatively new technology in the process of PCB printed boards. The spraying thickness can be adjusted by the nozzle, and the adjustment range is also wide, the coating is uniform, there are almost no parts that cannot be coated, and the coating can be continuously performed. Suitable for mass production.