PCB Technical - FPC cover film processing and matters needing attention

1. Multi-layer FPC soft board cover film processing

Multi-layer FPC flexible board cover film processing-double-sided FPC manufacturing process, one of the unique processes of the flexible circuit board manufacturing process is the processing process 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, newer technologies have been developed to expand the range of options.

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

1. FPC cover film

2. Screen printing of FPC overlay

3. FPC photocoating layer

1. FPC cover film

Cover film is the earliest and most used technology for flexible circuit 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 cold storage at low temperature. 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. The material manufacturer adjusts the fluidity of the adhesive to a specific range before the cover film leaves the factory. Under proper temperature refrigeration storage conditions, 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 extremely 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.

Two, FPC circuit board cover film processing matters needing attention

The FPC circuit board cover film needs to be processed with 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. When the base film is polyimide, it is short. Moisture will also be absorbed within time, which will have an impact on 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 of 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 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 operation cost is high, and this method is generally not used in 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 for small and medium batches of small holes, use CNC drilling and punching together for processing, and processing of the cover film

After removing the release film from the cover film with the opened window holes, paste it on the substrate with the etched circuit. Before lamination, the circuit surface must 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, the large-volume FPC 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 cover film is forced to be 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 relies entirely on manual operation. In order to improve production efficiency, various factories have thought of many methods.