PCB cover film processing technology

One of the unique processes in the manufacturing process of flexible printed boards is the processing of the overlay. During the PCB proofing process, there are three types of overlay processing methods: cover film, overlay screen printing, and photo coating. Recently, new technologies have been added to expand the selection.


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

    1, FPC cover layer silk screen printing

    2, FPC cover film

    3, FPC coating


    First, FPC cover layer silk screen printing

    Printed overlays are inferior to laminating overlays in terms of mechanical properties, but they require lower materials and processing costs. The most used products are civil products that do not require repeated bending and flexible printed boards on automobiles. The process and equipment used are essentially the same as the rigid printed board solder mask, but the ink materials used are completely different. PCB manufacturing should use inks suitable for flexible printed boards. Commercially available inks are UV-curable and heat-curable. The former has short curing time and convenience, but generally has poor mechanical properties and poor resistance to chemical properties. If it is used for bending or harsh chemical conditions, it may sometimes be inappropriate, and electroless gold plating must be avoided in particular, because the plating solution will seep from the end of the window to the underside of the cover layer, which will cause serious peeling of the cover layer. Since the thermosetting ink requires 20 to 30 minutes for curing, the continuous curing drying path is also relatively long, and a batch type oven is generally used.


    Second, FPC cover film

    The cover film is the earliest used technology for the application of flexible printed board overlays. The same adhesive as the copper foil board is coated on the same film as the base film of the copper-clad laminate to make it a semi-cured adhesive film, which is sold and supplied by the copper-clad laminate factory. When supplied, the adhesive film is affixed with a release film (or paper), and the semi-cured epoxy adhesive will gradually solidify at room temperature, so it should be stored under low temperature, printed circuit manufacturing plant It should be kept in a freezer warehouse at around 5°C before use or sent by the manufacturer before use. The general material manufacturer guarantees a period of 3 to 4 months of use, if it can be used for 6 months under refrigerated conditions. Acrylic adhesives hardly cure at room temperature and can be used for more than half a year even if they are not stored under refrigeration. The laminating temperature of this adhesive must be high.

    One of the most important issues in the processing of cover film is the fluidity management of the adhesive. The material manufacturer adjusts the flowability of the adhesive to a specific range before the cover film is shipped from the factory. Under the proper temperature and refrigerated storage conditions, it can guarantee a service life of 3 to 4 months, but within the effective period, the adhesive The fluidity of the agent is not fixed but gradually becomes smaller with time. Since the adhesive film has a large flowability during the manufacture of a normally-covered film made of PCB, the adhesive easily flows out and contaminates the terminal portion and the land when laminating. At the end of the service life, the adhesive has little or no fluidity. If the laminating temperature and pressure are not high, it will not be possible to obtain a cover film that is filled with pattern voids and high adhesive strength.

    Cover film to open the window processing, but can not be processed immediately after removal from the refrigerator, especially when the ambient temperature is high and the temperature difference is large, the surface will condense water droplets, when the base film is polyimide, in a short time Will absorb moisture, will have an impact on the subsequent process. Therefore, the general roll cover film is sealed in a polyethylene plastic bag, and the sealed bag should not be opened immediately after being taken out of the refrigerator, but should be put in the bag for several hours. When the temperature reaches room temperature, the sealed bag can be removed from the bag. Remove the cover film for processing.

    The use of numerically controlled milling or milling machines or punch presses to open the cover film, and the rotational speed of CNC drilling and milling cannot be too large. This kind of operation cost is high, and mass production generally does not use this method. Overlays 10 to 20 sheets with release paper are superimposed and fixed with upper and lower caps. Semi-cured adhesives tend to adhere to the drill, resulting in poor quality. Therefore, it should be tested more frequently than when drilling copper sheets, and debris generated during drilling should be removed. A simple punch can be used to machine the window of the cover film by the punching method, and punching of the batch hole having a diameter of 3 mm or less can be used for punching. When the hole of the window is large, the punch is used, and small and medium-sized batches of holes are processed by using CNC drilling and punching.

    After removing the release film from the cover film with the opened window holes, affix it to the substrate with the etched circuit. Before laminating, the surface of the circuit should be cleaned to remove surface contamination and oxidation. Surface cleaning using chemical methods. There are many holes in various shapes on the cover film after the release film is removed, and it is completely a film without a frame, which is particularly difficult to handle. It is not easy to use the positioning hole to overlap with the position on the line. . At present, large-scale production of each plant still depends on manual alignment stacking. The operator first accurately positions the cover film window hole and the wiring board and terminal of the circuit pattern, and temporarily fixes after confirmation. In fact, if any of the dimensions of the flexible printed board or cover film changes, it cannot be accurately positioned. If the conditions permit, the cover film can be divided into several pieces and then the laminations can be positioned. If the cover film is forcibly stretched to align, it will cause the film to be more uneven and cause a larger change in size, which is an important reason for making the board crease.

    Temporary fixing of the cover film can use electric irons or simple press-fitting. This is a process that completely depends on manual operations. In order to increase production efficiency, various manufacturers have thought of many ways.

    A well-positioned cover film is also heated and pressurized so that the adhesive is completely cured and the circuit is integrated. The heating temperature in this process is 160-200°C and the time is 1.5-2 hours (one cycle time). In order to increase production efficiency, there are several different solutions. 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 and overlapped in sections while heating and pressurizing. Heating methods are steam, heat media (oil), electric heating and so on. Steam heating costs are low, but the temperature is essentially 160°C. Electric heating can be heated to above 300°C, but the temperature distribution is not uniform. An external heat source heats the silicone oil, heating it with silicone oil as a medium can reach 200° C., and the temperature distribution is uniform. Recently, this heating method has been used gradually. Taking into account the PCB proofing so that the adhesive can be fully filled into the circuit pattern gap using a vacuum press is ideal, its high equipment prices, slightly longer compression cycle. However, it is still cost-effective to consider the rate of conformity and production efficiency. The introduction of vacuum presses is also increasing.

    The laminating method has a great influence on the state of the adhesive filling between the lines and the bending resistance of the finished flexible printed board. Laminated materials are available on the market, taking into account the cost of mass production, flexible boards are made of laminated materials. Depending on the construction of the flexible printed board and the materials used, the material and structure of the stack are also different.


    Third, FPC coating

    The basic process of photo coating is the same as that for rigid printed boards. The materials used are also dry film and liquid ink types. Actually the solder mask dry film is different from the liquid ink. Although the dry film type and the liquid type coating process 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 filmed first, and all the circuit diagrams should be covered with dry film. The ordinary paste film method is easy to have bubble residue between the lines, so it is necessary to use a vacuum filming machine.

    Ink type is the use of screen printing or spraying method to apply ink on the circuit pattern. Screen printing is a relatively versatile coating method, which is the same as a rigid printed board process. However, the thickness of a missed coating ink is relatively thin, basically 10 ~ 15um, due to the line side. For directionality, the thickness of one-time printing ink is uneven, and even skipping occurs. In order to improve the reliability, the second miss-printing should be performed after changing the miss-printing direction. The spraying method is still a relatively new technique in the process of printing a printed board. The spray nozzle can be used to adjust the spray thickness, and the adjustment range is also wider, the coating is uniform, there is almost no uncoated part, and continuous coating can be applied. Mass production.

    Screen printing inks are epoxy and polyimide type, both are two-component, mixed with a curing agent before use, add solvent as needed to adjust the viscosity, drying after printing, double-sided lines can be light One side of the coating was temporarily dried, and the other side was reversely coated and temporarily dried. After exposure and development, the coating was dried and cured.

    The pattern exposure of the photo-curing layer requires a certain precision of the positioning mechanism. If the size of the disk is about 100 μm, the position accuracy of the cover layer is at least 30-40 μm. As discussed in graphical exposures, the accuracy of the device can be achieved if the mechanical capabilities of the device are guaranteed. However, after a flexible printed circuit board is processed through multiple processes, it may be difficult to meet higher requirements due to its own size, resulting in expansion or contraction or partial deformation accuracy.

    The development process is not a big problem. Precision graphics should pay attention to the development conditions. The developer and the resist pattern developer are all sodium carbonate aqueous solutions. Even small batch production must avoid sharing the same developer with the pattern development. In order to completely cure the developed photo coating resin, post-curing must also be performed. The curing temperature will vary depending on the resin and must be cured in the oven for 20 to 30 minutes.


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