The appearance and development of Printed Circuit Boards (PCBs) have brought significant changes to the electronics industry. It has become an indispensable component in various electronic devices and instruments. With the continuous development of screen printing technology, new screen printing materials, screen printing technologies and testing equipment for the PCB industry have been improved, making the current screen printing process technology adaptable to high-density PCB production. The application of screen printing in PCB manufacturing mainly has the following three aspects: a. Application of PCB surface solder mask; b. The application of the anti-corrosion and anti-electroplating layer during the transfer of PCB pattern; c. The surface of the PCB is quite different from that of the application layer. As the first two applications are difficult to achieve in technology, this article will be targeted to introduce, while the screen printing materials for the PCB process for a brief introduction. —, screen printing materials 1. Wire mesh screen is the most important part of the same printing plate, because it is the key to control the ink flowability and printing thickness, and it determines the durability and quality of the screen. In recent years, the introduction of some high-tension, monofilament, and plain-woven polyester screens has made screen printing widely used in the manufacture of SMT templates, printed circuit manufacturing technologies, and membrane switch manufacturing technologies. In addition, the excellent combination of screen and screen printing materials is also an important factor in the production of high quality, high precision printing plates. In order to ensure a good combination of screen and photosensitive materials, the traditional approach is to roughen and degrease the new screen, which will ensure screen quality and extend screen life. 2. The plate sensitization material plate printing plate photosensitive material is commonly used diazo sensitizer, photosensitive film and so on. Diazo-type emulsions are commonly used in screen printing screen printing. The photosensitive film has the characteristics of uniform thickness controllable layer, high resolution image knife, high definition, wear resistance, and strong adhesion to the screen, and has been widely used in character printing of printed boards. 3. The material of the frame net frame and the shape of the cross section are very important. If the strength of the frame is not enough relative to a certain frame size, the consistency of the tension cannot be guaranteed. Nowadays, high tension aluminum frames are generally used. 4. The following printing inks are mainly used to introduce some screen printing inks used in the PCB industry. The specific uses and characteristics are shown in Table 1. Second, the application of PCB surface solder mask Printed circuit board solder mask is a permanent protective layer, it not only has the function of welding, protection, improve insulation resistance and other effects, but also the appearance of the quality of the circuit board Great influence. Early solder mask printing uses the solder mask to make the same pattern, and then prints the UV-curable solder mask ink. After each printing, excess solder mask remains on the pad due to screen deformation, misalignment, etc. It takes a long time to come and save, and it consumes a lot of manpower and time. Liquid photo-resist masking inks do not require the production of screen patterns, using the same printing, contact exposure. This process has high alignment accuracy, strong solder mask adhesion, good soldering resistance, and high production efficiency. It has gradually replaced photo-curable inks. 1. Process Flow Solder Mask Negatives Positioning Holes Punching Plate Positioning Holes - Cleaning Printed Plates - Preparing an Ink for Printing on Both Sides - Pre-bake Exposure A developing a thermoset 2. Key process analysis (1) Pre-baking Table 1 Usage and characteristics of some PCS inks Variety Uses and characteristics Development type liquid resisting plating resist inks Used for multilayer PCB inner layer fabrication and fabrication of high-precision, high-frequency PCB resist patterns , Double-sided and single-sided PCB board resist pattern production. The exposure time is short, the development speed is high, the release is easy, and the plating resistance and the etching resistance are excellent. UV curable liquid resist inks are used for the fabrication of single-sided and non-metallized hole PCB resist patterns. Curing speed, excellent plating resistance and etching resistance. Developing liquid photosensitive solder resist ink for high density PCB insulation protection. With high development accuracy, wide exposure range, rapid development, heat and acid and alkali resistance, resistance to plating, moisture resistance, excellent adhesion with copper foil. UV Curing Type Solder Resist Inks UV-curing two-component type character inks are used for the insulation protection of general PCBs, and are excellent in heat and acid resistance, alkali resistance, plating resistance, and moisture resistance. For multilayer and single-sided printed board surface mark symbol layer. Strong adhesion, screen printing characters full, clear and beautiful. The purpose of the pre-baking is to evaporate the solvent contained in the ink to make the solder mask non-tacky. For the different inks, the pre-baking temperature and time are different. Prebake too high a temperature, or drying time is too long, it will lead to poor development, reduce the resolution; pre-baking time is too short, or the temperature is too low, will stick to the film in the exposure, during development, the solder mask will be subject to sodium carbonate solution The erosion causes the surface to lose luster or the solder mask expands and falls off. (2) Exposure exposure is the key to the entire process. For the positive image, when overexposed, due to scattering of light, the solder resist film on the edge of the pattern or line reacts with light (mainly the photosensitive polymer contained in the solder resist strand reacts with light) to form a residual film, thereby reducing the resolution. As a result, the developed image becomes smaller and the lines become thinner. If the exposure is insufficient, the result is opposite to the above, and the developed image becomes larger and the lines become thicker. This situation can be reflected in the test: the exposure time is long, the measured line width is a negative tolerance; exposure time is short, the measured line width is a positive tolerance. In the actual process, "Optical Energy Integrator" can be used to determine the optimal exposure time. (3) Adjustment of ink viscosity The viscosity of the liquid photosensitive resist ink is mainly controlled by the ratio of the hardener to the main agent and the amount of diluent added. If the amount of hardener is not enough, there may be an imbalance in ink characteristics. After the hardening agent is mixed, the reaction proceeds at room temperature and the viscosity changes as follows. Within 30min: Ink main agent and hardener have not been fully integrated yet, fluidity is insufficient, and the screen will be blocked during printing. 3min to 10h: The ink main agent and hardener have been fully blended with proper fluidity. After 10h: The reaction between the materials of the ink itself has been actively carried out, resulting in greater fluidity, poor printing, the longer the time after the hardening agent is mixed, the more fully the reaction of the resin and the hardener, and the ink gloss is also changed accordingly. it is good. In order to make the gloss of the ink even and the printability is good, it is better to start printing after the curing agent is mixed for 30 minutes. If the diluent is added too much, it will affect the heat resistance and hardenability of the ink. In a word, the viscosity adjustment of the liquid photosensitive resist ink is very important: the viscosity is over-regulated, the printing is difficult, the screen is easy to stick the network, the viscosity is too thin, and the amount of volatile solvent in the ink is more, which brings difficulties to the pre-curing. The viscosity of the ink is measured using a rotary viscometer. In production, it is also necessary to adjust the optimum value of the viscosity according to different inks and solvents. Third, the application of anti-corrosion and anti-electroplating layer in the PCB pattern transfer process In the manufacturing process of the printed circuit board, the pattern transfer is the key process, and the commonly used dry film process is used to transfer the printed circuit pattern. At present, the wet film is mainly used for the manufacture of inner-layer circuit patterns of multilayer printed boards and the production of outer-layer circuit patterns of double-sided and multi-layered poles. 1. Process before processing ~ screen printing ~ baking an exposure ~ development ~ anti-plating or anti-corrosion ~ to film ~ the next process 2. Analysis of key processes (1) Selection of coating methods The wet film coating methods include the same type of printing, roller coating, curtain coating, and dip coating. In these methods, the surface layer of the wet film made by the roll-coating method is not uniform, and it is not suitable for producing a high-precision printed board. The wet film surface layer made by the method of the Z curtain coating is uniform and the thickness can be precisely controlled, but The curtain coater coating equipment is expensive and suitable for mass production; the wet film surface layer made by the dip coating method has a thin thickness and poor plating resistance. According to the current PCB production requirements, the same printing method is generally used for coating. (2) Pre-treatment The coupling of the wet film and the printed board is accomplished by chemical bonding. Usually, the wet film is an acrylate-based polymer that is a free-moving polymerized acrylate group. Steel combination. This process adopts the method of photochemical cleaning and mechanical cleaning to ensure the above-mentioned bonding, so that the surface is free of oxidation, oil-free, and water-free traces. (3) Control of twist and thickness The relationship between ink viscosity and diluent is shown in Figure 1. As can be seen from the figure, at a point of 5%, the wet film has a twist of 150 PS, which is lower than the thickness of the print, which does not meet the requirements. Wet film printing does not add thinner in principle. If it is to be added, it should be controlled within 5%. The thickness of the wet film is calculated by the following formula: hw-[hs-(S×hs)]×P% where hw is the wet film thickness h−is the screen thickness; hs is the fill area ZP is the ink solids content . Taking a screen of 100 mesh as an example: screen thickness: 60 μm; open area: 30%; ink solids: 50%. Figure 1 Relationship between ink viscosity and diluent Thickness of wet film = [60-(60 x 70%)] x 50% = 9 μm When a wet film is used for corrosion resistance, its film thickness is generally required to be 15-20 μm; when used In the anti-plating, the film thickness is generally required for 2O-30μm. Therefore, when the wet film is used for anti-corrosion, it should be printed twice. At this time, the thickness is about 18 μm, which meets the requirement of anti-corrosion; when it is used for anti-electroplating, it should be printed three times, at this time, the thickness is about 27 μm, which is in accordance with the anti-plating film thickness. Claim. When the wet film is too thick, it is easy to produce shortcomings such as underexposure, short-term development, poor etching resistance, etc., and it will be eroded by the syrup when it is electroplated, resulting in dislocation phenomenon, high visual pressure, and easy generation of sticky film when the film is attached. When the film is too thin, there are disadvantages such as overexposure, poor plating insulation, release film, and occurrence of metal plating on the film layer. In addition, when the film is exposed too much, the film removal speed is also slow. IV. Conclusion With the development of screen printing technology, screen printing technology has become increasingly important in the printed circuit manufacturing industry. Its application not only improves the production efficiency of printed circuits, but also reduces the production of printed circuits. The cost also improves the quality of printed circuit board processing. On the other hand, due to the research, development and application of printed materials, especially the development of functional inks, the application fields of screen printing have been diversified and have been continuously expanded, such as silver paste printing in membrane switch manufacturing technology. ZSMT manufacturing technology, solder paste printing and template manufacturing ZPCB peelable ink applications. (Author: East China Institute of Electronics Engineering, 230033; Received: 2001-O3-26)
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