Cover film is the earliest and most used technology for FPC cover layer applications. It is to coat the same adhesive as the copper clad laminate on the same film as the copper-clad laminate base film 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. 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 cure 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 proper temperature refrigeration storage conditions, the service life of 3 to 4 months can be guaranteed, but within the validity period, the adhesive 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.
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, it 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 operation cost is high, and this method is generally not adopted 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 the copper foil plate drilling, and the debris generated during the drilling should be removed. A simple punching die can be used when processing the window of the cover film by the punching method, and the punching 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 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 mass production factories 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.
To temporarily fix the cover film, you can use an electric soldering iron or do 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 so that the adhesive is completely cured and integrated with the circuit. The heating temperature of this process is 160-200℃, 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 more than 300 ℃, but the temperature distribution is not uniform. 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. Laminate materials are commercially available general products. Taking into account the cost of mass production, each flexible board factory has self-made laminate 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.
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