One of the unique processes of the flexible printed board manufacturing process is the processing of the cover layer. The processing methods of the cover layer can be divided into three categories: cover film, screen printing, and photo coated layer. Recently, newer technologies have been introduced, expanding the range of choices.
1. Silkscreen Printing of FPC Coverlay
The stencil cover layer has poorer mechanical properties than the laminated cover film, but the material and processing costs are lower. The most used are civilian products and flexible printed boards on automobiles that do not require repeated bending. The process and equipment used are the same as printing solder masks on rigid printed boards, but the ink materials are entirely different. To select inks suitable for flexible printed boards, there are UV-curable and heat-curable inks on the market. The former has a short curing time and is convenient but generally has poor mechanical properties and chemical resistance. It is sometimes inappropriate if used in bending or harsh chemical conditions, especially for electroless gold plating. The plating solution will penetrate the cover layer from the end of the window, which will seriously cause the cover layer to peel off. Since curing thermosetting ink takes 20 to 30 minutes, the drying tunnel for continuous curing is also relatively long, and a batch oven is generally used.
2. FPC Cover Film
Cover film is the earliest technology for flexible printed board cover applications. It is a semi-cured adhesive film coated with the same adhesive as the copper-clad laminate on the same film as the base film of the copper-clad laminate and sold and supplied by the copper-clad laminate manufacturer. When supplied, a layer of release film (or paper) is attached to the adhesive film. The epoxy resin adhesive in the semi-cured state will gradually cure at room temperature, so it should be stored in low-temperature refrigeration. It should be kept in a refrigerated warehouse at about 5°C until use or sent by the manufacturer before use. The general material manufacturer guarantees 3 to 4 months of use, and if it is refrigerated, it can be used for 6 months. Acrylic adhesives hardly cure at room temperature and can be used for more than half a year even if they are not stored in refrigeration. Of course, the lamination temperature of such adhesives must be high.
One of the most critical issues for cover film processing is the flow 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 and storage conditions, the adhesive can guarantee a service life of 3 to 4 months. Still, within the validity period, the fluidity of the adhesive is not fixed but gradually decreases with time. . Generally, the cover film that has just been shipped from the factory has a large fluidity of the adhesive. The adhesive easily flows out during lamination and contaminates the terminal parts and connection pads. Adhesives at the end of their useful life have little or no fluidity. If the lamination temperature and pressure are not high, a cover film that fills the pattern voids with high bond strength cannot be obtained.
The cover film needs to be processed by opening, but it cannot be processed immediately after taking it out of the refrigerator. Especially when the ambient temperature is high, and the temperature difference is significant, water droplets will condense on the surface. When the base film is polyimide, it will absorb moisture quickly, which will affect the subsequent process. Therefore, the roll cover film is generally 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, the cover film can be taken out from the sealed bag for processing.
Use a CNC drilling and milling machine or a punching machine to open the window of the cover film. The CNC drilling and milling rotation speed should not be too high. This operating cost is high, and this method is generally not used in mass production. Stack 10 to 20 sheets of cover film with release paper and fix them with upper and lower cover plates before processing. Semi-cured adhesives tend to stick to the drill bit, resulting in poor quality. Therefore, it should be inspected more frequently than when drilling a copper foil board, and the debris generated when drilling should be removed. When processing the window of the cover film by the punching method, a simple punching die can be used, and the punching die is used to process batch holes with a diameter of 3 mm or less. When the window hole is large, a punch is used, and small and medium batches of small holes are processed by CNC drilling and punching.
After removing the release film from the cover film that has been opened, it is attached to the substrate with the etched circuit. Before lamination, the surface of the circuit should be cleaned to remove surface contamination and oxidation. After removing the release film, there are many holes of various shapes on the cover film, which completely becomes a film without a skeleton, which is particularly difficult to operate. It is not easy to use the positioning holes to overlap the circuits’ positions. At present, the mass production factories still rely on manual alignment and lamination. The operator first accurately locates the cover film window hole and the connection pad and terminal of the circuit pattern and then temporarily fixes it after confirmation. 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, and then the lamination positioning can be carried out. If the cover film is forced to be elongated for alignment, the film will be more uneven, and the size will change more, which is an important reason for the wrinkling of the board.
To temporarily fix the cover film, an electric soldering iron or simple pressing can be used. This is a process that relies entirely on manual operations. To improve production efficiency, various factories have tried many methods.
The positioned cover film is also heated and pressurized so that the adhesive is fully 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). There are several different solutions to improve production efficiency, and the most common is the use of hot presses. Put the printed board with the cover film temporarily fixed between the hot plates of the press, overlap in sections, and heat and pressurize at the same time. Heating methods include steam, heat medium (oil), electric heating, etc. Steam heating cost is low, but the temperature is 160 ℃. Electric heating can be heated to more than 300 ℃, but the temperature distribution is not uniform. The external heat source heats the silicone oil, and the heating can reach 200 ℃ by using the silicone oil as the medium, and the temperature distribution is uniform. Recently, this heating method has gradually increased. Considering that the adhesive can be filled into the gap of the circuit pattern, it is ideal to use a vacuum press, which is expensive and has a slightly longer pressing cycle. But it is still cost-effective in terms of pass rate and production efficiency. The introduction of vacuum presses is also increasing.
The lamination has a significant influence on the state of the adhesive filled between the lines and the bending resistance of the finished flexible printed board. There are commercially available general-purpose lamination materials. Considering the cost of mass production, each flexible board factory makes its lamination materials. Depending on the construction of the flexible printed board and the materials used, the material and structure of the laminate vary.
3. FPC Photo Coating
The basic process of the photocopying layer is the same as that of the photoresist film for rigid printed boards. The materials used are also dry film type and liquid ink type. In fact, solder mask dry film and liquid ink are still different. Although the coating process of dry film and liquid types is entirely different, the same device can be used for exposure and subsequent processes. Of course, the specific process conditions will vary. The dry film should be filmed first, and all the circuit diagrams should be covered with dry film. The standard dry film method is easy to have air bubbles remaining between the lines, so a vacuum film machine is used.
The ink type is to apply the ink on the circuit pattern by screen printing or spraying. Screen printing uses more coating methods, the same as the rigid printed board process. However, the thickness of the ink coated in one missed printing is relatively thin, basically 10-15um. Due to the directionality of the line, the thickness of the ink in the first printing is uneven, and even skip printing occurs. The second missed printing should be changed after the missing printing direction to improve the reliability. The spraying method is still a relatively new technology in printed boards. The nozzle can adjust the spraying thickness, and the adjustment range is also wide, the coating is uniform, there are almost no parts that cannot be coated, and the coating can be carried out continuously.
The inks used in screen printing are epoxy resin and polyimide types, both of which are two-component, mixed with curing agent before use, add solvent to adjust the viscosity as needed and need to be dried after printing. After temporary drying on one side of the coating, the other side is reversely coated and temporarily dried and then dried and cured after exposure and development.
The pattern exposure of the photo-coated layer requires a positioning mechanism with a certain precision. If the disk size is about 100um, the positional accuracy of the cover layer is at least 30-40um.
There is no major problem in the development process. For precision graphics, full attention should be paid to the development conditions. The developer solution is the same as the resist pattern developer solution. It is an aqueous solution of sodium carbonate. Even if it is produced in small batches, avoiding sharing the same developer solution with graphics development is necessary. Post-curing must also be performed to cure the developed photo coating resin fully. The curing temperature will vary depending on the resin, and it must be cured in an oven for 20 to 30 minutes.
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