In the manufacturing process of PCB, one of the most basic and critical processes, whether it is single-panel, double-panel or multi-panel, is graphic transfer, that is, transferring the Art-work graphic to the copper-clad substrate. Graphic transfer is the key control point in production, and it is also the technical difficulty. There are many technological methods, such as screen printing pattern transfer technology, dry film pattern transfer technology, liquid photoresist pattern transfer technology, electro-deposition photoresist (ED film) manufacturing technology and laser direct imaging technology. Nowadays, the first liquid photoresist pattern transfer technology can replace dry film pattern transfer technology. This technology is widely used with its advantages of thin film, high Resolution, low cost and low operating conditions.
Flow chart of liquid photosensitive ink application process:
Surface treatment of substrate-> coating (screen printing)-> pre-baking-> exposure-> development-> drying-> inspection-> etching-> stripping-> inspection (remarks: inner plate)
Surface treatment of substrate-> coating (silk screen printing)-> pre-baking-> exposure-> development-> drying-> inspection-> electroplating-> stripping-> etching-> inspection (remarks: outer plate)
1. Liquid Photoresist (liquid photoresist)
Liquid photoresist (referred to as wet film for short) is made of photosensitive resin, together with photosensitive agent, pigment, filler and solvent, etc. After being irradiated by light, it produces photopolymerization reaction to obtain the pattern, which belongs to negative photosensitive photopolymerization type. Compared with traditional anti-corrosion ink and dry film, it has the following characteristics:
Compared with the dry film, the liquid wet film has good adhesion to the substrate, and can fill the slight pits, scratches and other defects on the surface of the copper foil. Then the rewetting film is as thin as 5 ~ 10μ m, only about 1/3 of that of the dry film, and there is no cover film on the upper layer of the wet film (there is a polyester cover film with a thickness of about 25μ m on the upper layer of the dry film), so the resolution and definition of the figure are high. For example, when the exposure time is 4S/7K, the resolution of dry film is 75um, while that of wet film can reach 40um. Thereby ensuring the product quality.
Because of the liquid wet film in PCB manufacturing, it has strong flexibility, so it is especially suitable for manufacturing Flexible Printed Board.
For the increasingly popular electroless nickel-gold plating process, generally dry film is not resistant to gold plating solution, while wet film is resistant to gold plating solution.
Compared with dry film, wet film does not need carrier Polyester Cover sheet and protective Polyettylene Separator Sheet, and there is no waste like dry film cutting, so there is no need to treat subsequent waste film? Play? Using wet film can save the cost by about 30 ~ 50% per square meter.
Problems such as warping, electroplating and infiltration, and irregular lines often occurred when dry film was used before. The wet film is a liquid film, which has no warping, infiltration plating and neat circuit, and the allowable shelving time from coating process to developing process can reach 48hr, which solves the correlation contradiction between production processes and improves the production efficiency.
The wet single-liquid ink is easy to store and keep. Generally, the storage temperature is 20 2℃, the relative humidity is 55 5%, and it is sealed in the shade. The storage life is 4 ~ 6 months.
It has a wide application range, can be used for making MLB inner-layer circuit patterns and electroplating-resistant patterns of perforated plates, can also be used as resist for mask etching patterns in combination with hole blocking process, and can also be used for making pattern templates, etc.
However, the Thickness uniformity of wet film is not as good as that of dry film, and the drying degree after coating is not easy to grasp? Are you brave? It’s difficult, so be careful when operating. In addition, the volatilization of additives, solvents, initiators, etc. in the wet film will cause pollution to the environment, especially to the operators. Therefore, the workplace must be well ventilated.
Lifespan of liquid photoresist: Its lifetime is related to the operating environment and time. General temperature ≤25℃, relative humidity ≤60%, operation in yellow light in clean room, service life of 3 days, preferably within 24hr hours.
II. Pattern transfer of liquid photoresist
Process flow of liquid photoresist:
The last working procedure → pretreatment → coating → pre-baking → positioning → exposure → development → drying → checking for revision → etching or electroplating → removing film → handing in the next working procedure.
1.PCB board Pre-cleaning
The main purpose of PCB pretreatment is to remove Grease, Oxidized Layer, Dust and Particle residue, Moisture and Chemicals, especially Alkaline substances, from the Copper surface to ensure the cleanliness and roughness of the copper surface, to manufacture a uniform and suitable copper surface and to improve the sensitivity.
Pretreatment methods include mechanical grinding, chemical pretreatment and the combination of both.
1) mechanical grinding method in PCB manufacturing
Grinding conditions:
Soaking time: 6 ~ 8s.
H2SO4： 2.5%。
Washing: 5s ~ 8s.
Nylon brush: 500 ~ 800 meshes, most of which are 600 meshes.
Grinding speed: 1.2 ~ 1.5m/min, interval 3 ~ 5cm.
Water pressure: 2 ~ 3 kg/cm2.
Strictly control the process parameters to ensure the drying effect of the board surface, so that the ground board surface has no impurities, glue marks and oxidation phenomena. It is best to carry out anti-oxidation treatment after grinding the board.
2) Chemical pretreatment method in 2)PCB manufacturing
For MLB Inner Layer Board, because the base material is thin, mechanical grinding is not suitable, but chemical pretreatment is often used.
Chemical pretreatment process in typical PCB manufacturing;
Oil removal → cleaning → microetching → cleaning → drying.
Oil removal:
Na3PO4 40～60g/l
Na2CO3 40～60g/l
NaOH 10～20g/l
Temperature: 40 ~ 60℃
Micro-etching (Mi-croetehing):
NaS2O8 170～200g/l
H2SO4(98%) 2%V/V
Temperature: 20 ~ 40℃
The chemically treated copper surface should be pink. Whether mechanical grinding or chemical pretreatment is used, it should be dried immediately after treatment.
Methods: Water film test was used. The principle of water film rupture test was based on the chemical interaction between liquid phase and liquid phase or between liquid phase and solid phase. If the water film can be kept unbroken for 15 ~ 30s, it is clean.
Note: The cleaned board should be handled with clean gloves and coated with photosensitive glue immediately to prevent the copper surface from re-oxidation.
PCB Coating
Coating means that the copper surface is evenly covered with a layer of liquid photoresist. There are many methods, such as centrifugal coating, dip coating, screen printing, curtain coating and roll coating.
Screen printing is a commonly used coating method at present, with low equipment requirements, simple and easy operation and low cost. However, it is not easy to coat both sides at the same time, the production efficiency is low, and the uniformity of the film can not be completely guaranteed. Generally, 100 ~ 300 mesh screen is used for full screen printing? What is the frequency of the ditch? 50 mesh screen. This method is welcomed by most small and medium-sized manufacturers.
Curtain coating is also suitable for large-scale production, and it can control the thickness of coating layer evenly, but the equipment requirements are high, and only one side can be coated before the other side, which affects the production efficiency.
Working conditions: clean room with yellow light, room temperature of 23 ~ 25℃, relative humidity of 55 5%, clean workplace, avoiding direct sunlight and fluorescent lamps.
If the photo-induced coating film is too thick, it is easy to cause underexposure, underexposure, high pressure sensitivity and easy adhesion to the negative film; The film is too thin, which is prone to overexposure, poor insulation against electroplating and the phenomenon of electroplating on metal, and the film removal speed is slow.
Roller coating can realize simultaneous coating on both sides, with high automatic production efficiency and controllable coating thickness. It is suitable for large-scale production of plates of various specifications, but requires equipment investment.
PCB coating operation should pay attention to the following aspects
If there are pinholes in the coating layer, it may be that there is something unknown in the photoresist. Clean it with acetone and replace it with a new resist.
When printing, if the photo-coated film is too thick, it is because the mesh number of the screen is too small; The film is too thin, which may be caused by too large screen mesh. If the thickness of the coating layer is uneven, a thinner should be added to adjust the viscosity of the resist or the coating speed.
Try to prevent ink from entering the hole when PCB is coated.
No matter what method is adopted, the Photoimageable covercoating should be uniform in thickness, free of pinholes, bubbles, inclusions, etc., and the thickness of the film should reach 8 ~ 15μ m after drying.
Because the liquid photoresist contains solvent, the workplace must have good ventilation.
Wash your hands with soap after work.
PCB Pre-curing
Pre-baking refers to drying the liquid photoresist film surface by heating and drying, so as to facilitate the contact exposure and development of the negative film to make patterns. This process is mostly operated in the same room as the coating process. The most commonly used ways of pre-drying are drying tunnel and oven.
Generally, oven drying is adopted, and the pre-drying temperature of the first side of both sides is 80 5℃ for 10 ~ 15 minutes; The pre-baking temperature of the second side is 80 5℃ for 15 ~ 20 minutes. This kind of pre-baking first and then makes the pre-curing degree of wet films on both sides different, and the development effect is difficult to guarantee the complete consistency. Ideally, both sides should be coated at the same time and pre-baked at 80 5℃ for about 20 ~ 30 minutes. In this way, both sides are pre-cured at the same time, and the same developing effect on both sides can be ensured, and the working hours can be saved.
It is important to control the Temperature and Time of pre-baking. Too high temperature or too long time, difficult to develop and difficult to remove the film; If the temperature is too low or the time is too short, the drying is incomplete, the film is pressure sensitive, and it is easy to stick to the negative film, resulting in poor exposure and damage to the negative film. Therefore, proper pre-baking, rapid development and film removal, and good graphic quality.
How to fix the position in PCB manufacturing
With the increasing application of high-density interconnection technology (HDI), resolution and positioning have become a major challenge for PCB manufacturers. The higher the circuit density, the more accurate the positioning is required. The positioning methods include visual positioning, movable pin positioning, fixed pin positioning and so on.
Visual positioning is to use Diazo film to overlap and align with printed board holes through graphics, and then stick adhesive tape for exposure. Diazo sheet is in brown or orange translucent state, which can ensure good coincidence and alignment accuracy. Silver Film can also be used in this method, but a transparent positioning plate must be made on the negative film to position it.
The movable pin positioning system includes a photographic film punch and a double round hole out-of-stock locator. Its method is: firstly, align the front and back two bottom film, punch two positioning holes outside the effective pattern with the film punch, and choose any one to program the drilling program, so that the drilling machine can be used to drill holes at one time. After metallized holes of printed boards and pre-copper plating, the double round hole out-of-stock locator can be used for positioning and exposure.
The fixing pin positioning is divided into two systems, one for fixing the photographic base plate and the other for fixing the PCB. By adjusting the positions of the two pins, the coincidence and alignment between the photographic base plate and the PCB can be realized.
5. exposure (exposure)
After being irradiated by UV light (300 ~ 400 nm), the liquid photoresist undergoes cross-linking polymerization, and the irradiated part of the photoresist forms a film and hardens without being affected by the developer. Usually, the light source of exposure lamp is high brightness, medium pressure mercury lamp or metal halide mercury lamp. The lamp tube is 6000W, the exposure is 100 ~ 300 MJ/cm2, and the density is measured by a 21-level optical density meter (Stouffer21) to determine the best exposure parameters, usually 6 ~ 8 levels. Liquid photoresist is not strictly required to use parallel light for exposure, but its photosensitive speed is not as fast as that of dry film, so a high-efficiency exposure machine (Drawer) should be used.
The photopolymerization reaction depends on the light intensity and exposure time of the lamp, and the light intensity of the lamp is related to the excitation voltage and the service time of the lamp tube. Therefore, in order to ensure enough light energy for photopolymerization, it must be controlled by a light energy integrator. Its function principle is to ensure that when the light intensity changes during exposure, the exposure time can be automatically adjusted to keep the total exposure energy constant, and the exposure time is 25 ~ 50 seconds.
Factors affecting exposure time:
(1) The closer the light is, the shorter the exposure time is;
(2) The thicker the liquid photoresist, the longer the exposure time;
(3) The higher the air humidity, the longer the exposure time;
(4) The higher the pre-baking temperature, the shorter the exposure time.
When overexposed, it is easy to form astigmatism refraction, reduce the line width and make it difficult to develop. When the exposure is insufficient, defects such as pinholes, hairiness, shedding and the like are easy to appear in the development, and the corrosion resistance and electroplating resistance are reduced. Therefore, choosing the best exposure parameters is an important condition to control the developing effect.
The quality of the negative directly affects the exposure quality. Therefore, the graphic circuit of the negative is clear, and there must be no fainting, false edges and other phenomena. It is required to have no pinholes and trachoma and good stability. It is required that the black-and-white contrast of negative film be large: the optical density of silver salt film DMAX ≥ 3.5 and DMIN ≤0.15; Optical density dmax of diazosheet is ≥ 1.2 and dmin is ≤ 0.1.
Generally speaking, after the film is made, it will be transported from one process (factory) to another, or stored for a period of time before entering the yellow room. As a result, it is difficult to guarantee the dimensional stability of the film in different environments. I think the finished negatives should go directly into the yellow room, and each negative should be discarded when more than 80 plates are made. In this way, the micro-deformation of graphics can be avoided, especially the micro-hole technology should pay more attention to this point.
Precautions for exposure operation?
(1) It is necessary for the exposure machine to vacuumize the box, and the vacuum degree is ≥90%. Only by vacuumizing the negative and the workpiece tightly can the image be guaranteed without distortion, so as to improve the accuracy.
(2) During the exposure operation, if the negative film is stuck, it may be caused by insufficient pre-baking or too strong vacuum in the drying box, etc., and the pre-baking temperature and time should be adjusted in time or the vacuum in the drying box should be checked.
(3) After the exposure stops, take out the plate immediately, otherwise, the corner of the lamp will cause excess glue after development.
(4) The working conditions must meet: a dust-free yellow light operating room with cleanliness of 10,000 ~ 100,000 and air conditioning facilities. Exposure machine should have cooling and exhaust system.
(5) When exposure, the film surface of the negative must face down to make it close to the photosensitive film surface, so as to improve the resolution.
6. Developing
That is, developing removes (dissolves) the wet film of the non-photographic part, leaving the photographic hardened part. The methods generally include manual development and machine spray development.
The working conditions of this process are the same as those of the coating process.
Machine development formula and process specification?
Na2CO3 0.8～1.2%
0.1% defoamer
The temperature is 30 2℃
The developing time is 40 10 seconds.
Spray pressure is 1.5 ~ 3 kg/cm2.
When operating, the Breok Point Control is controlled at 1/3 ~ 1/2. In order to ensure the development quality, it is necessary to control the concentration, temperature and development time of the developer in the appropriate operating range. If the temperature is too high (above 35℃) or the developing time is too long (over 90 seconds), the quality, hardness and chemical resistance of the film will be reduced.
After development, excess glue is produced, which is mostly related to process parameters. There are mainly the following possibilities:
① insufficient development temperature;
②Na2CO3 concentration is low;
③ low spray pressure;
④ The conveying speed is fast and the development is incomplete;
⑤ overexposure;
⑥ Laminating.
Matters needing attention in this process operation?
(1) If wet film is found to enter the hole during production, it is necessary to increase the injection pressure and extend the development time. After development, carefully check whether the hole is clean, and rework and reappear if there is residual glue.
(2) After the developer has been used for a period of time, its ability decreases, so it should be replaced with a new one. The experiment proves that when the PH value of the developer drops to 10.2, the developer loses its activity. In order to ensure the image quality, the plate-making quantity at PH=10.5 is set as the cylinder changing time.
(3) After development, it should be thoroughly washed to prevent alkali liquor from being brought into the etching solution.
(4) In case of open circuit, short circuit, copper exposure and other phenomena, the reason is generally that there is damage or sundries on the negative.
dry
In order to make the film have excellent resistance to corrosion and electroplating, it should be dried again after development, under the conditions of temperature 100℃ and time 1 ~ 2 minutes. The hardness of the cured film should reach 2h ~ 3h.
PCB Strip
After PCB Etching or Plating, the resist film must be removed. There are two methods: manual film removal and machine spray film removal.
The spray film remover is used, and its spray pressure is 2 ~ 3 kg/cm2, which has good film removing quality, clean and thorough removal and high production efficiency. Increasing the temperature can increase the speed of film removal, but if the temperature is too high, it is easy to produce black holes, so the temperature should generally be 50 ~ 60℃.
The PCB must be cleaned after removing the film. If there is excess glue on the surface after removing the film, the main reason is that the process parameters of the baking process are incorrect, usually excessive baking.
Only by adopting effective operation skills and techniques and strengthening total quality management (TQM) according to the technological equipment and technological level of each company, can the qualified rate of products be greatly improved.
