FPC flexible circuit board prototype production cover film

FPC flexible circuit board prototype production cover film

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

1. FPC cover film

2. Screen printing of FPC overlay

3． FPC Photocoating

    1. FPC cover film

   Cover film is the earliest used technology for flexible printed board cover. It is the same film as the base film of the copper-clad laminate, which is coated with the same adhesive as the copper foil to make it a semi-cured adhesive film. It is sold and supplied by copper-clad laminate manufacturers. At the time of delivery, a layer of release film (or paper) is attached to the adhesive film. The semi-cured epoxy resin adhesive will gradually solidify at room temperature. Therefore, it should be stored in a cold storage at low temperature. Before use, it should be kept in a cold storage room at about 5 ° C or sent by the manufacturer before use. The general material manufacturer guarantees a use period of 3 to 4 months, and it can be used for 6 months if it is refrigerated. The acrylic adhesive hardly cures at room temperature. Even if it is not stored under refrigerated conditions, it can still be used for more than half a year. Of course, the laminating temperature of this adhesive must be high.

    One of the most important issues for the processing of cover films is the management of the fluidity of the adhesive. The material manufacturer adjusts the fluidity of the adhesive to a specific range before the cover film leaves the factory. Under the proper temperature and refrigerated storage conditions, it can guarantee a service life of 3 to 4 months. However, during the effective period, the adhesive The fluidity of the agent is not fixed, but gradually decreases with time. Generally, the cover film that has just left the factory is very fluid because of the adhesive. During lamination, the adhesive easily flows out and contaminates the terminal part and the connection pad. At the end of the service life, the adhesive has little or no fluidity. If the lamination temperature and pressure are not high, a cover film that fills the gaps in the pattern and has high bonding strength cannot be obtained.

   The cover film needs to be processed by opening the window, 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, water droplets will condense on the surface. When the base film is polyimide, it will also be processed in a short time. It absorbs moisture and affects subsequent processes. Therefore, the roll-shaped cover film is generally sealed in a polyethylene plastic bag. After taking out from the refrigerator, the sealed bag should not be opened immediately, but should be left in the bag for several hours. When the temperature reaches room temperature, it can be removed from the sealed bag. Remove the cover film for processing.

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

    After removing the release film from the cover film of the window hole, paste it on the substrate of the etched circuit. Before lamination, the circuit surface must be cleaned to remove surface contamination and oxidation. Chemical cleaning of surfaces. After removing the release film, there are many holes of various shapes on the cover film, which completely becomes a thin 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, large-scale production factories still rely on manual alignment for lamination. Operators first accurately position the cover film window holes and the connection disks and terminals of the circuit pattern, and then temporarily fix them after confirmation. In fact, if the size of either the flexible printed board or the cover film is changed, accurate positioning cannot be achieved. If conditions permit, the cover film can be divided into several pieces, and then laminated positioning is performed. If the cover film is stretched and aligned forcibly, it will cause the film to be more uneven and cause a larger change in size, which is an important cause of wrinkles in the board.

   The cover film can be temporarily fixed 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 ways.

   The cover film in place must be heated and pressurized so that the adhesive is completely cured and integrated into the circuit. The heating temperature in this step is 160 to 200 ° C, and the time is 1.5 to 2 hours (one cycle time). In order to improve production efficiency, there are several different schemes. The most commonly used is a hot press. The printed board with the cover film temporarily fixed is placed between the hot plates of the press, overlapping in sections, and heated and pressurized at the same time. Heating methods include steam, heat medium (oil), and electric heating. Steam heating is low cost, but the temperature is basically 160 ° C. Electric heating can be heated above 300 ° C, but the temperature distribution is uneven. The external heat source heats the silicone oil, and the silicone oil is used as a medium to heat up to 200 ° C, and the temperature distribution is uniform. Recently, this heating method has gradually been used. Considering that the adhesive can be fully filled into the gap 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 still cost-effective to consider in terms of qualification rate and production efficiency. The introduction of vacuum presses is also increasing.