Application analysis of screen printing in the pro

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Application analysis of silk printing in PCB manufacturing (II)

2. Hard training "internal skill"

as an old PCB worker and old printer, I deeply realize that whether silk printing is accepted in PCB manufacturing technology completely depends on whether silk printing has excellent printing machine materials and good process suitability. With the great progress of silk printing technology in the past decade, PCB workers have to re recognize and gradually love and cannot live without silk printing. The development in recent years has made silk printing have good process adaptability and become a controllable and easy to control production process. Next, we briefly introduce the progress of printing machine materials in recent ten years

(1) silk printing

silk is the most important part of printing. This is because it controls the fluidity and ink consumption of the ink. At the same time, it also determines the durability and quality of the version, thus determining the quality of printing. In the past decade, people can see high modulus printing silk with good performance in the market, especially monofilament, plain woven polyester (PET) silk with high binding force, high tension (the diameter of the silk can be as small as 27 μ m. The introduction of flat woven silk with up to 150 threads per cm) has changed people's understanding of some important influencing factors in printing, which has been confirmed in SMT technology, integrated circuit manufacturing technology and film switch manufacturing technology of precision silk printing. In addition, the excellent crosslinking of the photosensitive adhesive or photosensitive film between the silk and the plate is also an important factor in making high-quality printing plates. In order to ensure the good combination of the silk and the photosensitive material of the plate, the traditional practice is to emphasize that the plate making operators should do a good job in the pretreatment of the stretched silk, which includes the mechanical coarsening and appropriate degreasing of the new silk. At present, there are already some in the market that do not need to be coarsened The high tension/high adhesion silk treated with fat removal is sold. In addition to the advantages of saving time and simplifying the process, this product is more durable, and its service life is 2 ~ 3 times longer than that of ordinary silk. At the same time, in the development and research of new silk, attention has also been paid to its wear resistance, high resolution, wide exposure time limit and excellent matching characteristics with photosensitive materials. The above-mentioned new silk has been commercialized and is very easy to buy in the domestic market

(2) photosensitive material

today, we can buy many brands of high-resolution silk photosensitive adhesive or photosensitive film in the market, especially water-based photosensitive film (direct sun photosensitive film). Its use process is simple, and the film thickness is uniform and controllable, especially the film thickness is 80 ~ 140 μ M water-based photosensitive film can be used in some special applications. At the same time, it also provides a reliable material foundation for silk printing to expand new fields

according to the latest printing information, the American yilinglong company recently launched 400 μ M-thick photosensitive film can be used for various electronic technology applications and high-density fabric printing

(3) printing materials (inks)

the progress of printing materials, especially the development of functional inks, not only benefits ordinary silk printing (for example, UV inks are never blocked), but also directly leads to the high-tech of silk printing technology and products. For example, conductive silver paste printing in film switches; Tin paste (lead tin alloy) printing that interconnects SMD and SMC (chip element device) in SMT technology with PCB; The semiconductor layers CDs (cadmium sulfide), CdTe (cadmium telluride) and electrode layers carbon (c), silver (Ag), silver + experimental force display and the clearing operation of the display results of the whole part in the solar cell. The silk printing of indium (Ag + in), magnetic ink, thick film IC ink, etc. As far as PCB printing materials are concerned, the quality and printing process of PCB printing materials today are quite different from those 20 years ago

it is particularly gratifying that the PCB Printing Materials researched, developed and manufactured by our Chinese people can be completely comparable to the high-quality PCB printing materials produced abroad! In recent years, the high-precision, high-density, multi-layer PCBs produced by applying domestic PCB printing materials have been successfully equipped on China's new missile destroyers that have sailed to more than 10 countries, such as the United States, Mexico, Peru, Australia, the Philippines, Malaysia, Pakistan, etc; At the same time, more PCBs made of silk are also equipped with graphene materials, which can also be used in photovoltaic, energy storage and other fields, nuclear submarines, underwater missile launches, underwater robots and other most modern ships in China. Building warships is a complex system engineering. Modern warships are the epitome of a country's modern industry, almost including a country's high-tech and cutting-edge technologies in many aspects: ships, machinery, electronics, weapons and more than a dozen other systems. Under each large system, there are several subsystems, such as warning and detection system, combat intelligence system, sea missile system, air missile system, defense main gun system, underwater acoustic system, anti submarine system, helicopter system, electronic warfare system, etc. The PCB produced with domestic PCB printing materials has been widely used in the core technology of the above-mentioned modern warships. In this regard, it should be said that this is enough to make every silk printing worker and the whole silk printing industry proud

in order to make more silk printers and PCB workers have a deeper understanding of the performance, characteristics and process technology of domestic PCB printing materials, the author will focus on some domestic PCB printing materials

① it is showing double-digit growth. Liquid photosensitive, corrosion-resistant and electroplating ink

single component, dilute alkaline aqueous solution development, contact exposure liquid photosensitive, corrosion-resistant and electroplating ink is mainly used to manufacture the inner layer of high-precision, high-density PCB and multilayer board. Using silk full-scale printing, when exposed, the photographic film is in direct contact with the photosensitive film. Its resolution is higher than that of the general anti-corrosion dry film, and there will be no residual glue problem of the anti-corrosion dry film. The binding force between the photosensitive film and the substrate is stronger than that of the anti-corrosion dry film, and the electroplating resistance is better. The cost is saved by about 50% compared with that of the anti-corrosion dry film

a technical indicators (see Table 2)

Table 2 technical indicators of liquid photosensitive corrosion-resistant electroplating ink

project indicators

color green

solid content (%) 75

viscosity (cps/20 ℃) 6000 ± 500

composition single component

adhesion (cross cutting method) 100/100

hardness 1H (before exposure), 2~3h (after exposure)

corrosion resistance acidic CuCl2, FeCl3 alkaline CuCl2, etc.

electroplating resistance Cu, sn/pb, Ni Ni/Au

Special diluent for dilution

b process flow

substrate pretreatment → silk full screen printing (silk mesh number 100 ~ 200) → pre drying (70 ~ 80 ℃, 10 ~ 20min) → exposure (4kw, 35 ~ 70s) → development (1% anhydrous sodium carbonate solution, 20 ~ 30 ℃, 45 ~ 60s) → drying (90 ~ 100 ℃, 5min) → etching or electroplating → deinking (3 ~ 5% sodium hydroxide solution, 40 ~ 60 ℃, 1 ~ 3min)

C precautions

a silk full page printing: the thickness of the photosensitive film after drying is generally 15 ~ 25 μ m. When used for electroplating, the film should be thicker, the number of wire mesh used should be smaller (100 ~ 150 mesh), and when used for etching, the number of wire mesh can be higher (140 ~ 200 mesh)

b ﹐ ultraviolet light should be avoided during printing

C pre drying: the drying condition of single panel is 70 ~ 75 ℃, 15min; Double sided plate 70 ~ 75 ℃, about 10min, too long pre drying time or too high temperature will lead to thermal polymerization/crosslinking of photosensitive film, resulting in residual glue after development; On the contrary, if the drying is not sufficient, some solvent is still left in the photosensitive film, and the photosensitive film will stick to the negative film and have poor development resistance during exposure. The pre dried boards should be handled with care, and must not be stacked together to avoid mutual adhesion. The film blowing machine should develop high-end technology exposure within 48 hours after natural cooling

d exposure: the thicker the photosensitive film is, the longer the exposure time is required; The thinner the film, the shorter the exposure time required. The exposure time of 4 ~ 7KW exposure machine is 35 ~ 70s. If the exposure time is too short, the development resistance and corrosion resistance of the photosensitive film are poor, and the electrical insulation is reduced; The exposure time is too long, the resolution decreases, and the development time becomes longer. Pay attention to the ambient temperature and humidity of the workshop during exposure. It is best to work in a clean environment. The exposed board can be developed after being placed for 15min

e ﹐ washing and drying: the developed board should be fully washed to neutral, and dried within the range of 90 ~ 110 ℃ for 3 ~ 5min, which can improve the adhesion and electrical insulation between the cured film and the substrate, and have better electroplating resistance

f ﹐ etching plate: the solidified film can resist the corrosion of FeCl3 (ferric chloride), acidic or alkaline copper chloride (CuCl2) and other etching solutions

g ﹐ electroplating: when used for electroplating, the thickness of the solidified film should be greater than the thickness of the coating to avoid coating overflow and short circuit during electroplating

h ﹐ deinking: the higher the temperature, the greater the mass fraction (concentration) of NaOH solution, the shorter the deinking time; The lower the temperature is, the smaller the mass fraction of NaOH is, and the longer the deinking time is. The influence of temperature on the deinking time is more obvious than the mass fraction of NaOH. Generally, the mass fraction of NaOH is 3% ~ 5%, the temperature is 40 ~ 60 ℃, and the cured film expands and falls off in sheets for 1 ~ 3min

(to be continued)

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