The hottest topic is the application and selection

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Talk about the application and selection of photo imaging anti-corrosion and anti Plating Ink (Part 2)

5. The selection of wet film for anti plating

first of all, it should meet the pattern electroplating of double-sided boards and multilayer boards, as well as the electroplating of sign panels and decorative metal paintings

5.1 good thixotropy

for double-sided boards and multilayer boards, please replace 1 stable normal power supply pattern. The selection of wet film for electroplating is relatively strict. First, the wet film should have good thixotropy and the specified viscosity. During printing, with the increase of the number of times of moistening (that is, to make the wet film moisten the plate, put a piece of clean white paper on the substrate before printing, scrape it on the paper several times; break off the paper during formal printing), the ink becomes thinner one by one, while it is printed on the substrate and the flow is small, maintaining the thickness of the ink, so as to prevent the wet film from flowing into the hole, showing good thixotropy

5.2 development resistance

its development resistance should show a large tolerance within the specified range. Under the specified and correct exposure conditions, the lines of the pattern cannot become thicker or thinner after the wet film on the edge of the wire hole or the pad is displayed. There must be no residual film

5.3 good electroplating resistance

first of all, it can resist the acidic or alkaline degreasing solution before electroplating, can resist copper plating, tin lead, nickel gold, pure tin (including black tin plating), and should not peel, the wet film should not fade (should not pollute the liquid), should not penetrate, and should not fall off (described in part above). It is easy to fade the film within the specified process conditions

5.4 requirements for solid content in wet film components

the solid content of the wet film used for electroplating resistance should be maintained at 70% - 75%, and no diluent should be added in use. If the viscosity is large, it can be solved by stirring or rough printing

5.4.1 relationship between solid content, thickness and coating of wet film

the rising layer of double-sided plate and multi-layer plate is carried out during pattern plating: secondary copper plating, pure tin plating (or lead tin or nickel gold). The coating on the hole wall and surface should be increased by 0.015 ~ 0.025mm as specified in the standard, and the wet film resistant to electroplating should also reach or approximate this thickness (after drying). If the film layer is very thin, the coating will appear protruding edge (also known as blank holder). After removing the film, a very thin wet film will be pressed by the coating on both sides of the line. After etching, the edge will be irregular, which cannot achieve the effect of steepness, and it will be scrapped in serious cases

if the solid content is low, no matter what method is used, the required thickness cannot be printed at one time, and the hole plugging phenomenon will also occur, which will bring trouble to the next process

5 the relevant person in charge of the Engineering Department of a construction team in Hebei said 4.2 meet the necessary conditions of thickness and calculation

in order to meet the wet film thickness required during electroplating, it is very necessary to select the purpose. Nylon should be selected from the material of silk, because it has low cost, small friction coefficient, good ink permeability and smoothness, which saves the operator effort in manual printing and obtains more ink permeability. There are many factors that affect the thickness of wet film in printing time. There are two constants: the thickness of silk and the solid content of wet film. The variables are: the hardness of the scraper, the angle between the scraper and the substrate during scraping, the pressure of the scraper, the times of scraping, and the speed of scraping. Here is a calculation formula: ft · s

in the formula, FT is the thickness of silk and S is the solid content. In fact, FT · s is only the theoretical value of film thickness, but the data provided by relevant materials are quite different from this formula: that is, the wet film left on the substrate after scraping is only 25 ~ 30% thick. The above five variable factors must be considered in accurate calculation. Many materials also list different thicknesses printed with different mesh numbers. The practice shows that the printing becomes thicker with the decrease of the number, but it is quite different from the marked data. Therefore, we can only choose the silk according to the film thickness required by our products, and carefully consider the five variables, so as to calculate the more accurate film thickness

6. Defects of wet film in use: polymerization inhibition of oxygen

when the wet film is exposed, the polymerization inhibition of oxygen can slow down the photochemical cross-linking speed until it is stopped, and in serious cases, it can also make the film surface sticky. The polymerization inhibition of oxygen has two aspects: one is the influence on the initiator, but the photosensitizer sudden drop in the excited state reduces the efficiency of the initiator. Second, it has polymerization inhibition in the polymerization process

6.1 adjust the wet film formula to reduce the polymerization inhibition of low oxygen

in order to avoid the interference of oxygen and make the wet film polymerize smoothly in the sun, ink manufacturers and experts have carried out long-term exploration and practice, and found many methods: for example, select an ideal photosensitive initiation system in the wet film formula to isolate oxygen and curing system; Increase the concentration of photosensitive initiator; Select appropriate resin, etc. Although these methods have achieved certain results, they only reduce the interference of oxygen to the greatest extent. This interference can not be considered when manufacturing general patterns. However, it must be carefully overcome when manufacturing thin lines or fine patterns, otherwise the expected effect will not be achieved (referring to the thin lines and points manufacturing 2mil)

6.2 calculation and example of light energy:

formula: e = I · T

where I = light intensity, Use mv/cmto express

t = time, in seconds,

e = light energy, and mj/cm to express

example: we calculate the light intensity of an exposure machine as 10mv/cm, and select the exposure time as 10 seconds:

common light energy E = I · t = 10 · 10 = 100mj/cm

similarly, the light intensity of another exposure machine is 5mv/cm, and the light energy of 100 mj/cm can also be obtained by selecting the exposure time of 20 seconds, which makes the vehicle out of balance. But after exposure and development, use 100 × Observation with a magnifying glass showed that the lines of the latter became thicker (male lines) and thinner (Yin lines), and it was difficult to develop in severe cases. Because the upper layer of the wet film will be cross-linked soon after being irradiated by UV light, but the latter will instantly enter the bottom due to low intensity of UV light. At this time, oxygen enters the anoxic exposure area from the non exposure area. Due to the extension of time and the diffusion of light, some cross-linking and polymerization cannot be completed in time, and oxygen stays in and around the exposure area. This phenomenon often occurs in products with thick wet film, such as molds, or product grids with high resolution. The pattern manufacturing of double-sided board and multilayer board lifting thin line is also in this category

6.3 polymerization inhibition to reduce low oxygen in the exposure process

another solution is to consider in the exposure process. As mentioned above, the solution is to increase the exposure intensity and relatively shorten the exposure time. In a short time, the speed of dissolved oxygen is faster than the speed of oxygen diffusion into the exposure area, that is, the whole exposure process has been completed before the oxygen in the non exposure area enters the exposure area, UV light can quickly enter the bottom of the wet film to complete polymerization and crosslinking

7. The popularity and sales war of the wet film market

is precisely the excellent corrosion resistance and electroplating resistance of the wet film, which is used in other industries besides circuit boards (as described above). Therefore, ink merchants at home and abroad look at the market and rush to land one after another. First, Japanese wet film entered the Chinese market, followed by Britain, Germany, the United States, Italy, South Korea and Singapore. In China, there are many brands in Taiwan, followed by Beijing, Guangdong, Jiangsu and Zhejiang. According to my incomplete statistics, there are nearly 30 different brands of wet films at home and abroad competing for sales in the market. There are so many manufacturers and brands that no country can compare with us. It is really the world's largest! Therefore, it is a buyer's market. We must choose the wet film with good environmental protection, good quality and reasonable price

8. Conclusion

although the wet film was initially used as a mask for pattern etching and pattern plating of circuit boards, it is widely used in the label, panel, building materials and decoration industries because of its excellent corrosion resistance, electroplating resistance and high resolution. Especially in the label panel industry, it replaces dichromate photosensitive adhesive, and eliminates the emission of hexavalent chromium, which is environmentally friendly 3 Reliability verification: according to the given reliability target value, it has made a great contribution to improve the working environment for the operator. When used in electroplating, the cost is greatly reduced, while the cost of using wet film in the circuit board industry is only 50% - 65% of the cost of dry film, which is its life. At present, its scope of use will continue to expand. Someone asked whether wet film will replace dry film in the future? I can't answer. I only know that some large PCB factories are carrying out technical transformation for the application of wet film recently...

source: Membrane Switch Technology Forum

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