Flexible Double-Panel FPC Manufacturing - Cover Film Processing Technology
One of the processes unique to the flexible printed board manufacturing process is the processing of the cover layer. The processing methods of the cover layer include the cover film, the screen leakage of the cover layer, and the photo-coating layer. Recently, there have been updated technologies to expand the selection range.
The processing of the FPC cover film is divided into three parts:
- 1. Screen printing of FPC overlay
- 2. FPC cover film
- 3. FPC photocoating layer
- 1. Screen printing of FPC overlay
The printed overcoat layer is inferior to the laminated cover film, but the material cost and processing cost are low. The most used are civilian products that do not require repeated bending and flexible printed boards on automobiles. The process and equipment used are essentially the same as those of the rigid printed board solder mask, but the ink materials used are completely different. To select inks suitable for flexible printed boards, commercially available inks have UV curing type and heat curing type. The former has a short curing time and is convenient, but generally has poor mechanical properties and chemical resistance. If it is used for bending or harsh chemical conditions, it may be inappropriate to use it for chemical gold plating, because the plating solution will penetrate from the end of the window to the underside of the window, which will seriously cause the cover layer to peel off. The thermosetting ink requires 20 to 30 minutes for curing, so the continuous curing tunnel is also relatively long, and a batch oven is generally used.
- 2. FPC cover film
The cover film is the earliest used technology for flexible printed board overlay applications. The same adhesive as the copper foil plate is coated on the same film as the copper foil laminate base film to make it a semi-cured adhesive film, which is sold by a copper clad laminate manufacturer. When supplied, 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, so it should be stored at low temperature and stored in a circuit manufacturing plant. It should be kept in a freezer at around 5 °C before use or sent by the manufacturer before use. The general material manufacturer guarantees a service life of 3 to 4 months, and can be used for 6 months under refrigerated conditions. Acrylic adhesives hardly cure at room temperature, even if they are not stored under refrigeration, they can be used for more than half a year. Of course, the adhesive must be laminated at a high temperature.
One of the most important issues with 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 leaving the film. When it is stored under the proper temperature, it can guarantee the service life of 3 to 4 months, but during the validity period, the adhesive is glued. The fluidity of the agent is not fixed, but gradually decreases over time. Generally, the mask film that has just been manufactured is very fluid because of the adhesiveness, and when laminating, the adhesive easily flows out and contaminates the terminal portion and the land. At the end of the service life, the adhesive has little or no fluidity. If the laminating temperature and pressure are not high, a coating film filled with a pattern void and a high bonding strength cannot be obtained.
The cover film should be opened for window processing, but it cannot be processed immediately after it is taken out 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, it is also short-time. It will absorb moisture and will have an impact on the after-process. Therefore, the general roll-shaped cover film is sealed in a polyethylene plastic bag. After being taken out of the refrigerator, the sealed bag should not be opened immediately. Instead, it should be placed in the bag for several hours. When the temperature reaches room temperature, it can be removed from the sealed bag. The cover film is taken out for processing.
The cover film opening window uses a numerical control drilling and milling machine or a punching machine. The rotational speed of the numerical control drilling and milling can not be too large, and such running cost is high, and this method is generally not used for mass production. 10 to 20 sheets of cover film with release paper are overlapped and fixed by upper and lower cover pads before processing. Semi-cured adhesives tend to adhere to the drill bit and result in poor quality. Therefore, it should be inspected more frequently than when drilling a copper foil, and the debris generated during drilling should be removed. When the window of the cover film is processed by the punching method, a simple die can be used, and the processing of the batch hole having a diameter of 3 mm or less is punched using a die. When the hole of the window is large, the die is used, and the small holes of the small and medium batch are processed by the numerical control drilling and the die, and the processing of the cover film is as shown in Fig. 10-8.
After removing the release film of the window hole, the film is pasted on the substrate on which the circuit has been etched, and the surface of the circuit is cleaned to remove surface contamination and oxidation before lamination. Surface cleaning uses chemical methods. There are many kinds of holes on the cover film after removing the release film, which is completely a film without a skeleton, which is particularly difficult to operate. It is not easy to use the positioning holes to overlap with the position on the line. . At present, large-scale production of each factory still relies on manual aligning. The operator firstly positions the splicing plate and the terminal of the cover film window hole and the line pattern accurately, and then temporarily fixes it after confirmation. In fact, if the size of either side of the flexible printed board or the cover film changes, it cannot be accurately positioned. If the condition allows, the cover film can be divided into several pieces, and then the lamination is positioned. If the film is forced to be stretched for alignment, the film will be more uneven and the size will be changed more, which is an important reason for the wrinkles of the plate.
Temporary fixing of the cover film can be done by using an electric soldering iron or simply pressing. This is a process that relies entirely on manual operation. In order to improve production efficiency, various factories have thought of a lot of methods.
The fixed cover film is also heated and pressurized to completely cure the adhesive and integrate with the line. The heating temperature in this step is 160 to 200 ° C, and the time is 1.5 to 2 h (one cycle time). There are several different options for increasing production efficiency, the most common being the use of hot presses. The printed board temporarily fixed with the cover film is placed between the hot plates of the press, overlapped in sections, and heated and pressurized. Heating methods include steam, heat medium (oil), electric heating, and the like. 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. The external heat source heats the silicone oil, and the silicone oil is heated by the medium to reach 200 ° C, and the temperature distribution is uniform. Recently, the heating method is gradually increased. It is desirable to use a vacuum press in order to allow the adhesive to be sufficiently filled into the gap of the wiring pattern, and the equipment is expensive and the pressing cycle is slightly longer. However, it is cost-effective to consider the qualification rate and production efficiency. Examples of the introduction of vacuum presses are also increasing.
The laminate lamination method has a great influence on the state in which the adhesive is filled between the lines and the bending resistance of the finished flexible printed board. The laminated material has a commercially available general product, and in consideration of the cost of mass production, each flexible board factory has a laminated material. The materials and construction of the laminate vary depending on the construction of the flexible printed board and the materials used.
- 3. FPC photocoating layer
The basic process of the photo-coating layer is the same as that of the photo-resistance film for a rigid printed board. The materials used are also dry film type and liquid ink type. In fact, 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 basically used for the exposure and subsequent processes. Of course, the specific process conditions will vary. . The dry film should first be filmed, and all the wiring patterns should be covered with a dry film. The ordinary dry film method is easy to have bubbles remaining between the lines, so a vacuum filming machine is used.
The ink type is applied to the line pattern by screen printing or spraying. Screen printing is a much more used coating method, the same as the rigid printed board process. However, the thickness of the ink that is applied by a smear is relatively thin, basically 10 to 15 um, due to the side of the line. The directionality, the thickness of the printing ink is uneven, and even the jump printing occurs. In order to improve the reliability, the printing direction should be changed and the second printing is performed. The spraying method is still a relatively new technology in the process of printed boards. The nozzle can be used to adjust the thickness of the coating, and the adjustment range is also wide, the coating is uniform, there is almost no coating, and the coating can be continuously applied. For mass production.
The ink used for screen printing is epoxy resin type and polyimide type. It is a two-component type. It is mixed with the curing agent before use. The solvent is added according to the need to adjust the viscosity. After printing, it needs to be dried. The double-sided line can be light. After the coating is temporarily dried, the other side is coated and temporarily dried, and exposed and developed, followed by drying and curing.
The pattern exposure of the photo-coating layer requires a positioning mechanism with a certain precision. If the size of the disk is about 100 um, the positional accuracy of the cover layer is at least 30 to 40 pm. As discussed in the graphical exposure, the mechanical capabilities of the device are guaranteed, and such accuracy requirements are achievable. However, after a multi-process processing of a flexible printed board, it is difficult to achieve high requirements due to the expansion or partial deformation accuracy of its own size.
There is no major problem in the development process. The precision pattern should pay full attention to the development conditions. The developer is the same as the resist pattern developer solution. Even in small batch production, it is necessary to avoid sharing the same developer with the pattern development. In order to completely cure the developed photo-coating resin, post-cure must also be performed. The curing temperature will vary depending on the resin and must be cured in an oven for 20 to 30 minutes.