PCB Tech - Covering film processing-double-sided FPC flexible circuit board manufacturing process

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 flexible circuit board cover film is divided into three parts:
1. FPC cover film
2. Screen printing of FPC overlay
3. FPC photocoating layer

1. FPC flexible circuit board cover film

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 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 cover film opening window uses a CNC drilling and milling machine or a punching machine, and the rotation speed of the CNC drilling and milling should not be too high. This kind of operating cost is high, and this method is generally not used for mass production. Lay 10-20 sheets of cover film with release paper together and fix them with upper and lower cover pads before processing. Semi-cured adhesive is easy to adhere to the drill bit, resulting in poor quality. Therefore, it should be inspected more frequently than when drilling copper foil plates, and the debris generated during drilling should be removed. A simple die can be used when processing the window of the cover film by the punching method, and the die is used for the processing of batch holes with a diameter of less than 3mm. When the hole of the window is large, use a punching die, and small and medium batches of small holes are processed by CNC drilling and punching together.

After removing the release film from the cover film with the opened window holes, paste it on the substrate of the etched circuit. Before lamination, the surface of the circuit should be cleaned to remove surface contamination and oxidation. Chemical methods for surface cleaning. After removing the release film, there are many holes of various shapes on the cover film, which completely becomes a film without a skeleton. It is particularly difficult to operate. It is not easy to use the positioning hole to overlap the position on the line. . At present, mass production plants still rely on manual alignment and stacking. The operator first accurately locates the cover film window hole and the connection plate and terminal of the circuit pattern, and temporarily fixes it after confirmation. In fact, if the size of either the flexible printed board or the cover film changes, it cannot be accurately positioned. If conditions permit, the cover film can be divided into several pieces before lamination positioning. If the covering film is forcibly stretched for alignment, the film will be more uneven and the size will change more. This is an important cause of wrinkles in the board.

Temporary fixing of the cover film can be done with an electric soldering iron or simple pressing. This is a process that completely relies on manual operation. In order to improve production efficiency, various factories have thought of many methods.

The fixed cover film must be heated and pressurized to completely cure the adhesive and integrate the circuit. The heating temperature of this process is 160-200 degree Celsius, and the time is 1.5-2h (one cycle time). In order to improve production efficiency, there are several different solutions, the most commonly used is to use a heat press. Put the printed board with the cover film temporarily fixed between the hot plates of the press, overlap in sections, and heat and press at the same time. Heating methods include steam, thermal media (oil), electric heating, etc. The cost of steam heating is low, but the temperature is basically 160°C. Electric heating can be heated to above 300°C, but the temperature distribution is uneven. The external heat source heats the silicone oil. The heating method using silicone oil as a medium can reach 200°C, and the temperature is evenly distributed. Recently, this heating method has gradually increased. Taking into account that the adhesive can be fully filled into the gaps of the circuit pattern, it is ideal to use a vacuum press. The equipment is expensive and the pressing cycle is slightly longer. However, it is cost-effective to consider the qualification rate and production efficiency. The introduction of vacuum presses is also increasing.

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 structure of the flexible printed board and the material used, the material and structure of the laminated board are also different.

2. 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.

3. FPCflexible circuit board 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 missed 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 a relatively new technology in the process of 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 can not be coated, and the coating can be continuously applied. For mass production.

The ink used for screen printing is epoxy resin and polyimide, both of which are two-components. Mix with curing agent before use. Add solvent to adjust viscosity as needed. Drying is required after printing. Double-sided lines can be lighted. After the coated side is temporarily dried, the other side is coated on the other side and temporarily dried, and then dried and cured after exposure and development.

The pattern exposure of the photocoating layer requires a positioning mechanism with a certain accuracy. If the size of the disk is about 100um, the position accuracy of the covering layer is at least 30-40 m. As discussed during the pattern exposure, if the mechanical capability of the device is guaranteed, this precision requirement can be achieved. However, after the flexible printed circuit board has been processed by multiple processes, it is difficult to meet higher requirements due to its own size expansion or partial deformation accuracy.

There are no major problems in the development process. Pay attention to the development conditions for precise patterns. The developer is the same sodium carbonate solution as the resist pattern developer. Avoid sharing the same developer with pattern development even in small batches. In order to completely cure the developed photocoating layer resin, post-curing must also be performed. The curing temperature will vary depending on the resin, and it must be cured in an oven for 20-30 minutes.