Liquid photosensitive ink application process flow chart: >
Substrate surface treatment -> coating (silkscreen) -> pre-baking -> exposure -> development -> drying -> inspection -> etching -> faded film -> inspection (note: within Laminate)
Substrate surface treatment-->coating (silk screen printing)-->pre-baking-->exposure-->development-->drying-->inspection-->electroplating-->degraded film-->etching-->inspection (Note: Outer Board)
one. Liquid Photoresist
Liquid photoresist (abbreviated as wet film) is made of photosensitive resin, with sensitizers, colorants, fillers, and solvents, etc., and after photoirradiation, photopolymerization reaction occurs to obtain a pattern, which is a negative photosensitive polymerization type. Compared with traditional resist ink and dry film, it has the following features:
a) No need to make silkscreen stencils. With the Contact Printig, it is possible to avoid defects such as penetration, smearing, shading, and image distortion caused by screen printing. Resolution has been greatly improved, with traditional ink resolution of 200um and wet film up to 40um.
b) Because it is a photo-curing reaction conjunctiva, its film adhesion, Etch resistance, and its resistance to plating are better than conventional inks.
c) The wet film coating method is flexible, diverse, and has strong process operability and is easy to master.
d) Compared with the dry film, the liquid wet film adheres well to the substrate and can be filled with slight pits, scratches, and other defects on the surface of the copper foil. In addition, the wet film can be as thin as 5 to 10 μm, only about one-third of the dry film, and there is no cover film on the upper layer of the wet film (the upper layer of the dry film is covered with a polyester cover film of about 25 μm thick), so the resolution of the pattern is High resolution. For example, when the exposure time is 4S/7K, the dry film resolution is 75um, while the wet film can reach 40um. To ensure product quality.
e) Frequently used dry film has problems such as lifting, electroplating, plating, and line irregularities. The wet film is a liquid film with no warping, permeation plating, and neat lines. It allows the shelf time up to 48 hrs from the coating process to the morphing process, solves the contradiction between production processes, and improves production efficiency.
f) For today's increasingly popular electroless nickel-plated gold process, generally the dry film is not resistant to gold plating, while the wet film is resistant to gold plating.
g) Because it is a liquid wet film, it has strong flexibility and is especially suitable for the production of Flexible Printed Board.
h) Because the thickness of the wet film itself is reduced and the cost of the material is reduced, and compared with the dry film, a Polyester Cover sheet and a Polyettylene Separator Sheet are not required, and There is no waste as large as dry film cutting, and no subsequent waste film needs to be processed. Therefore, using a wet film can save about 30 to 50% per square meter.
i) wet film is a single liquid ink easy storage and storage, general placement temperature of 20 ± 2 °C, relative humidity of 55 ± 5%, sealed in the shade, storage period (Storage Life): 4 to 6 months.
j) Wide range of applications, which can be used as MLB inner layer circuit patterning and hole plate plating pattern, and can also be combined with hole blocking process as masking hole etching pattern resist, and can also be used for the production of graphic templates.
However, the uniformity of the thickness of the wet film is not as good as that of the dry film, and the degree of drying after coating is also not easy to grasp, which increases the exposure difficulty. Therefore, be careful when operating. In addition, the volatilization of auxiliary agents, solvents, and initiators in the wet film causes environmental pollution, especially to the operator. Therefore, the workplace must be well ventilated.
At present, the liquid photoresist used has a viscous appearance and its color is mostly blue. Such as: GSP1550 produced by Taiwan Refinement Corporation, APR-700 produced by Taiwan Yinging Company, etc. These are all single-liquid inks and can be coated by a simple screen printing method, developed with dilute alkaline water, and used with an acidic or weakly alkaline etching solution. Etching.
Lifespan of Liquid Photoresist: Its service life is related to operating environment and time. General temperature ≤ 25 °C, relative humidity ≤ 60%, clean room under the yellow light operation, the service life of 3 days, the best use within 24hr.
two. Liquid photoresist pattern transfer
Liquid photoresist process:
The previous process → Pretreatment → Coating → Prebake → Positioning → Exposure → Development → Drying → Inspection → Etching or Plating → Stripping → Crossing
1. Pre-cleaning
The main purpose of the pretreatment is to remove Grease, Oxidized Layer, Dust and Particle residues, Moisture, and Chemicals, especially Alkaline, on the copper surface. ) To ensure the cleanliness and roughness of the copper surface, to produce a uniform and suitable copper surface, and to increase the bonding force between the photoresist and the copper foil. The requirements for the wet film and the dry film are different. It is more focused on the cleanliness.
The methods of pretreatment include mechanical grinding, chemical pretreatment, and a combination of both.
1) Mechanical grinding
Plate condition:
Pickling time: 6 ~ 8s.
H2SO4: 2.5%.
Washing: 5s~8s.
Nylon Brush: 500 to 800 mesh, most of which uses 600 mesh.
Grinding plate speed: 1.2 ~ 1.5m/min, interval 3 ~ 5cm.
Water pressure: 2 to 3 kg/cm2.
Strictly control the process parameters to ensure that the board surface drying effect, so that the board surface without impurities, traces of glue and oxidation. It is best to carry out anti-oxidation treatment after grinding the board.
2) Chemical Pretreatment
For the MLB inner layer board (Inner Layer Board), due to the thin substrate, it is not appropriate to adopt the mechanical grinding method and often adopt the chemical pretreatment method.
Typical chemical pretreatment process:
Degreasing → Cleaning → Microetch → Cleaning → Drying
remove oil:
Na3PO4 40～60g/l
Na2CO3 40~60g/l
NaOH 10～20g/l
Temperature: 40～60°C
Mi-croetehing:
NaS2O8 170～200g/l
H2SO4 (98%) 2%V/V
Temperature: 20～40°C
The chemically treated copper surface should be pink. Whether using mechanical or chemical pretreatment methods, they should be dried immediately after treatment.
Inspection method: Water film test is used. The principle of water film rupture test is based on the interface chemistry between liquid and liquid phases or between liquid and solid phases. If you can maintain the water film 15 ~ 30s is not broken that is clean.
Note: The cleaned board should be worn with clean gloves and immediately coated with photo resist to prevent reoxidation of the copper surface.
2. Coating
Coating refers to uniformly covering the copper surface with a layer of liquid photoresist. There are many methods for this, such as centrifugal coating, dip coating, screen printing, curtain coating, roller coating, and the like.
Screen printing is a commonly used coating method, which requires low equipment, 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 cannot be completely guaranteed. In general screen printing, full-screen printing uses 100 to 300 mesh screens for electroplating using 150-mesh screens. This method is welcomed by most small and medium manufacturers.
Roll coating can realize double-sided coating at the same time. It has high automation production efficiency and can control the coating thickness. It is suitable for large-scale production of various specifications, but requires equipment investment.
Curtain coating is also suitable for large-scale production, but also can uniformly control the thickness of the coating layer, but the equipment requirements are high, and can only be painted on one side before coating the other side, affecting the production efficiency.
Photo-induced coating film is too thick, easy to produce underexposure, lack of development, high pressure sensitivity, easy to stick the film; film is too thin, prone to overexposure, poor resistance to plating insulation and easy to produce metal plating film phenomenon, Slow film removal.
Working conditions: Clean room under yellow light, room temperature is 23 ~ 25 °C, relative humidity is 55 ± 5%, the workplace is kept clean, avoid sunlight and direct sunlight.
Coating operation should pay attention to the following aspects
1) If the coating layer has pinholes, the photoresist may be unclear, wash with acetone and replace the new resist. It may also be that particles in the air fall on the surface of the board or the board surface is not clean due to other reasons. It should be carefully inspected and cleaned before coating.
2) If the photocoating film is too thick during screen printing, it is because the number of meshes is too small; if the film is too thin, it may be caused by too many meshes. If the coating thickness is not uniform, thinner should be added to adjust the viscosity of the resist or adjust the speed of the coating.
3) Try to prevent the ink from entering the hole while applying the film.
4) Regardless of the method used, Photoimageable cover coating should achieve uniform thickness, no pinholes, bubbles, inclusions, etc., and the film thickness should reach 8-15 μm after drying.
5) Because the liquid photoresist contains solvents, the workplace must be well ventilated.
6) Wash hands with soap after work.
3. Pre-curing
The pre-baking means that the surface of the liquid photoresist film is dried by heating and drying to facilitate contact exposure and development of the film to produce a pattern. Most of this process is performed in the same chamber as the coating process. Pre-baking methods are most commonly used in both drying tunnels and ovens.
Generally, oven drying is used. The first-side pre-baking temperature on both sides is 80±5° C. and 10 to 15 minutes. The second-side pre-bake temperature is 80±5° C. and 15 to 20 minutes. This pre-and post-bake pre-bake makes the degree of pre-curing of the wet film on both sides different, and the effect of the development is also difficult to ensure complete consistency. Ideally, both sides are coated simultaneously and pre-baked at a temperature of 80±5°C for about 20-30 minutes. This double-sided pre-curing at the same time and can ensure that the two-sided development of the same effect, and save man-hours.
It is important to control the temperature and time of the pre-baking. If the temperature is too high or the time is too long, the development is difficult, and it is difficult to remove the film; if the temperature is too low or the time is too short, the drying is not complete, the film has pressure-sensitive properties, and the negative film may be easily exposed due to poor exposure, and the film may be easily damaged. Therefore, pre-baking is appropriate, development and removal of film is faster, and the graphics quality is good.
The operation should pay attention
(1) After pre-baking, the board should be air-cooled or naturally cooled before the film is exposed.
(2) Do not use natural drying, and drying must be complete, otherwise it is easy to stick the film and cause poor exposure. After pre-baking, the film thickness of the photosensitive film should be HB ~ 1H.
(3) If an oven is used, it must be equipped with blast air and constant temperature control so that the pre-bake temperature is even. And the oven should be clean, no impurities, so as not to fall on the board, damage the film surface.
(4) After the pre-baking, the coating film is allowed to develop for a maximum of 48 hrs, and the exposure time is as long as 12 hr.
(5) The requirements for different types of liquid photoresist are different. Read the instructions carefully and adjust the process parameters such as thickness, temperature, and time according to the production practice. 4. Positioned (Fixed Postion)
With the continuous expansion of high-density interconnect technology (HDI) applications, resolution and positioning have become major challenges for PCB manufacturers. The higher the circuit density, the more precise the positioning is required. The positioning methods include visual positioning, moving pin positioning, fixing pin positioning and other methods.
The visual positioning is to use a Diazo film to pass through the pattern to coincide with the hole in the printed board, and then expose it with adhesive tape. The diazo film is brown or orange and translucent, which can ensure good repositioning accuracy. Silver film can also be used in this method, but it must be made of transparent positioning plate in the film to locate.
The movable pin positioning system includes a photographic film punch and a double-round hole out-positioning device. The method is: firstly aligning the front and back two bottom film films, and using the film punch device to arbitrarily punch two positions outside the effective pattern. Holes, any one to take a drilling program, you can use a drill hole drilling, printed boards metallized holes and pre-plated copper, you can use the double-round hole out positioning locator positioning exposure.
Fixed pin positioning is divided into two sets of systems, a set of fixed photo plates and another set of fixed PCBs. By adjusting the positions of the two pins, a coincident alignment between the photo plate and the PCB can be achieved.
5. Exposuring
After the liquid photoresist is irradiated with UV light (300-400 nm), a cross-linking polymerization reaction occurs, and the light-irradiated portion is formed into a film and hardened without being affected by the developer. The source of the exposure lamp normally used is a high-brightness, medium-pressure mercury lamp or a metal halide mercury lamp. Lamp 6000W, exposure 100 ~ 300mj/cm2, density measurement using 21 optical density table (Stouffer21), to determine the optimal exposure parameters, usually 6 to 8 levels. The use of liquid photoresist for exposure to parallel light is not critical, but its speed is not as fast as that of dry film, so a high-efficiency exposing (Drawer) should be used.
The photopolymerization reaction depends on the light intensity of the lamp and the exposure time. The light intensity of the lamp is related to the excitation voltage and is related to the lamp usage time. Therefore, in order to ensure enough light energy for photopolymerization reaction, it must be controlled by the optical energy integrator. Its principle is to ensure that the exposure time is automatically adjusted to maintain the total exposure energy when the light intensity changes during exposure. Exposure time It is 25 to 50 seconds.
Factors affecting exposure time:
(1) The closer the light is, the shorter the exposure time;
(2) The thicker the thickness of the liquid photoresist, the longer the exposure time;
(3) The greater the humidity of the air, the longer the exposure time;
(4) The higher the pre-bake temperature, the shorter the exposure time.
When the exposure is excessive, the astigmatism is easily refracted, the line width is reduced, and the development is difficult. When the exposure is insufficient, the development is likely to cause defects such as pinholes, hairiness, and detachment, and the corrosion resistance and plating resistance are reduced. Therefore, selecting the optimal exposure parameter is an important condition for controlling the development effect.
The quality of the film has a direct impact on the quality of exposure. Therefore, the pattern of the film is clear and there must be no halo or imaginary phenomena. Pinholes and trachoma are required. The stability is good. The film requires a large black-and-white contrast: the silver salt film has a Density of DMAX ≥ 3.5, and DMIN ≤ 0.15; the diazo film has an optical density of DMAX ≥ 1.2 and DMIN ≤ 0.1.
In general, after the film has been produced, it is transferred from one process (factory) to another (factory), or stored for a period of time before it enters the yellow light chamber. This way of experiencing different environments, the dimensional stability of the film is difficult to guarantee. I think that the finished film should be directly into the yellow room, and each film should be discarded when it is used to make more than 80 boards. In this way, the micro-deformation of the graphics can be avoided, especially the micro-hole technology should pay more attention to this point.
Exposure process operation precautions
(1) The exposure machine is absolutely necessary for vacuum drying and the degree of vacuum is ≥ 90%. Only by evacuating the film and the workpiece, it can be ensured that the image is not distorted so as to improve the accuracy.
(2) When the exposure operation, if the production of viscous negative film, may be caused by insufficient pre-baking or too strong vacuum drying and other reasons, it is necessary to adjust the pre-baking temperature and time or check the vacuum drying conditions.
(3) After the exposure is stopped, the board should be taken out immediately. Otherwise, the excess light in the lamp will cause excess glue after development.
(4) The working conditions must reach: a dust-free yellow light operation room with a cleanliness level of 10,000 to 100,000 and air-conditioning facilities. The exposure machine should have a cooling exhaust system.
(5) When the film is exposed, the film side of the film must face downwards to make it close to the surface of the photosensitive film to increase the resolution.
6. Developing
The development removes (dissolves) the non-photosensitive, non-patterned wet film, leaving the photohardened graphic portion. The method is generally manual development and machine spray development.
The working conditions of this process are the same as the coating process.
Machine Development Formulas and Process Specifications
Na2CO3 0.8 to 1.2%
Antifoaming agent 0.1%
Temperature 30±2°C
Development time 40±10 seconds
Spray pressure 1.5～3kg/cm2
The Breok Point Control is controlled at 1/3 to 1/2. In order to ensure the development quality, it is necessary to control the developer concentration, temperature, and development time within an appropriate operating range. Temperatures that are too high (above 35°C) or development times that are too long (more than 90 seconds) can result in reduced film quality, hardness, and chemical resistance.
After development, there are more glues, most of which are related to the process parameters. There are mainly the following possibilities:
1 the development temperature is not enough;
2Na2CO3 concentration is low;
3 spray pressure is small;
4 The transmission speed is faster and the development is not complete;
5 overexposure;
6 stacked plates.
The operation precautions
(1) If a wet film enters the hole during production, it is necessary to increase the ejection pressure and lengthen the development time. After development, carefully check whether the hole is clean. If there is residue, rework should be repeated.
(2) After the developer has been used for a period of time, the capacity has dropped and new fluid should be replaced. Experiments have shown that when the pH of the developer drops to 10.2, the developer has lost its activity. To ensure the image quality, the plate-making capacity at PH=10.5 is defined as the cylinder change time.
(3) After the development, wash it thoroughly so that the alkaline solution is not brought into the etching solution.
(4) If open circuit, short circuit, exposed copper and other phenomena occur, the reason is generally that there is damage or debris on the film.
7. dry
In order to make the film have excellent anti-corrosion and anti-plating ability, it should be dried after developing. The conditions are temperature 100°C and time 1 to 2 minutes. The cured film hardness should reach 2H ~ 3H.
8. Check revision
The revised version is actually a self-inspection. Its purpose is mainly to repair the defects on the graphics circuit and remove the parts that are not related to the graphics requirements. That is, the removal of extraneous burrs, glue spots, etc., and make up for missing holes such as pinholes and gaps. , broken lines and so on. The general principle is to scrape after make up, so that it is easy to guarantee the quality of revision.
The commonly used cleaning solutions include shellac, asphalt, and acid-resistant inks. The simplest one is the shellac solution. Its formula is as follows:
Shellac 100~150g/l
Methyl violet 1～2g/l
Amount of anhydrous ethanol
Revision requirements: the pattern is correct, the alignment is accurate, and the accuracy is in accordance with the process requirements; the conductive pattern has neat and smooth edges, no residue, oil, fingerprints, pinholes, gaps, and other impurities, and there is no residual film and foreign matter on the hole wall; 90% of repairs The workload of the plate is caused by the uncleanness of the exposure tool. Therefore, the film should be frequently inspected during the operation, and the sunscreen and film should be cleaned with alcohol to reduce the amount of revision. Care should be taken in the revision of the gloves to prevent sweat contamination of the layout. If the first two processes are done very well, there is almost no revision, and the revision process can be omitted.
9. Strip
After Etching or Plating, the protective film must be removed. Usually, a 4-8% NaOH solution is used to remove the film, and the film is heated and expanded to achieve differentiation. Methods include manual film removal and machine spray coating.
Spraying machine is used to spray the film. Its injection pressure is 2 ~ 3kg/cm2. The quality of the film is good, the removal is clean, and the production efficiency is high. Increasing the temperature can increase the speed of film removal, but the temperature is too high and black holes are easy to occur. Therefore, the temperature should generally be 50-60°C.
After the film is removed, it must be cleaned. If there is residual glue on the surface after removing the film, the main reason is that the process parameters of the baking process are not correct, and the overbaking is generally excessive.
Substrate surface treatment -> coating (silkscreen) -> pre-baking -> exposure -> development -> drying -> inspection -> etching -> faded film -> inspection (note: within Laminate)
Substrate surface treatment-->coating (silk screen printing)-->pre-baking-->exposure-->development-->drying-->inspection-->electroplating-->degraded film-->etching-->inspection (Note: Outer Board)
one. Liquid Photoresist
Liquid photoresist (abbreviated as wet film) is made of photosensitive resin, with sensitizers, colorants, fillers, and solvents, etc., and after photoirradiation, photopolymerization reaction occurs to obtain a pattern, which is a negative photosensitive polymerization type. Compared with traditional resist ink and dry film, it has the following features:
a) No need to make silkscreen stencils. With the Contact Printig, it is possible to avoid defects such as penetration, smearing, shading, and image distortion caused by screen printing. Resolution has been greatly improved, with traditional ink resolution of 200um and wet film up to 40um.
b) Because it is a photo-curing reaction conjunctiva, its film adhesion, Etch resistance, and its resistance to plating are better than conventional inks.
c) The wet film coating method is flexible, diverse, and has strong process operability and is easy to master.
d) Compared with the dry film, the liquid wet film adheres well to the substrate and can be filled with slight pits, scratches, and other defects on the surface of the copper foil. In addition, the wet film can be as thin as 5 to 10 μm, only about one-third of the dry film, and there is no cover film on the upper layer of the wet film (the upper layer of the dry film is covered with a polyester cover film of about 25 μm thick), so the resolution of the pattern is High resolution. For example, when the exposure time is 4S/7K, the dry film resolution is 75um, while the wet film can reach 40um. To ensure product quality.
e) Frequently used dry film has problems such as lifting, electroplating, plating, and line irregularities. The wet film is a liquid film with no warping, permeation plating, and neat lines. It allows the shelf time up to 48 hrs from the coating process to the morphing process, solves the contradiction between production processes, and improves production efficiency.
f) For today's increasingly popular electroless nickel-plated gold process, generally the dry film is not resistant to gold plating, while the wet film is resistant to gold plating.
g) Because it is a liquid wet film, it has strong flexibility and is especially suitable for the production of Flexible Printed Board.
h) Because the thickness of the wet film itself is reduced and the cost of the material is reduced, and compared with the dry film, a Polyester Cover sheet and a Polyettylene Separator Sheet are not required, and There is no waste as large as dry film cutting, and no subsequent waste film needs to be processed. Therefore, using a wet film can save about 30 to 50% per square meter.
i) wet film is a single liquid ink easy storage and storage, general placement temperature of 20 ± 2 °C, relative humidity of 55 ± 5%, sealed in the shade, storage period (Storage Life): 4 to 6 months.
j) Wide range of applications, which can be used as MLB inner layer circuit patterning and hole plate plating pattern, and can also be combined with hole blocking process as masking hole etching pattern resist, and can also be used for the production of graphic templates.
However, the uniformity of the thickness of the wet film is not as good as that of the dry film, and the degree of drying after coating is also not easy to grasp, which increases the exposure difficulty. Therefore, be careful when operating. In addition, the volatilization of auxiliary agents, solvents, and initiators in the wet film causes environmental pollution, especially to the operator. Therefore, the workplace must be well ventilated.
At present, the liquid photoresist used has a viscous appearance and its color is mostly blue. Such as: GSP1550 produced by Taiwan Refinement Corporation, APR-700 produced by Taiwan Yinging Company, etc. These are all single-liquid inks and can be coated by a simple screen printing method, developed with dilute alkaline water, and used with an acidic or weakly alkaline etching solution. Etching.
Lifespan of Liquid Photoresist: Its service life is related to operating environment and time. General temperature ≤ 25 °C, relative humidity ≤ 60%, clean room under the yellow light operation, the service life of 3 days, the best use within 24hr.
two. Liquid photoresist pattern transfer
Liquid photoresist process:
The previous process → Pretreatment → Coating → Prebake → Positioning → Exposure → Development → Drying → Inspection → Etching or Plating → Stripping → Crossing
1. Pre-cleaning
The main purpose of the pretreatment is to remove Grease, Oxidized Layer, Dust and Particle residues, Moisture, and Chemicals, especially Alkaline, on the copper surface. ) To ensure the cleanliness and roughness of the copper surface, to produce a uniform and suitable copper surface, and to increase the bonding force between the photoresist and the copper foil. The requirements for the wet film and the dry film are different. It is more focused on the cleanliness.
The methods of pretreatment include mechanical grinding, chemical pretreatment, and a combination of both.
1) Mechanical grinding
Plate condition:
Pickling time: 6 ~ 8s.
H2SO4: 2.5%.
Washing: 5s~8s.
Nylon Brush: 500 to 800 mesh, most of which uses 600 mesh.
Grinding plate speed: 1.2 ~ 1.5m/min, interval 3 ~ 5cm.
Water pressure: 2 to 3 kg/cm2.
Strictly control the process parameters to ensure that the board surface drying effect, so that the board surface without impurities, traces of glue and oxidation. It is best to carry out anti-oxidation treatment after grinding the board.
2) Chemical Pretreatment
For the MLB inner layer board (Inner Layer Board), due to the thin substrate, it is not appropriate to adopt the mechanical grinding method and often adopt the chemical pretreatment method.
Typical chemical pretreatment process:
Degreasing → Cleaning → Microetch → Cleaning → Drying
remove oil:
Na3PO4 40～60g/l
Na2CO3 40~60g/l
NaOH 10～20g/l
Temperature: 40～60°C
Mi-croetehing:
NaS2O8 170～200g/l
H2SO4 (98%) 2%V/V
Temperature: 20～40°C
The chemically treated copper surface should be pink. Whether using mechanical or chemical pretreatment methods, they should be dried immediately after treatment.
Inspection method: Water film test is used. The principle of water film rupture test is based on the interface chemistry between liquid and liquid phases or between liquid and solid phases. If you can maintain the water film 15 ~ 30s is not broken that is clean.
Note: The cleaned board should be worn with clean gloves and immediately coated with photo resist to prevent reoxidation of the copper surface.
2. Coating
Coating refers to uniformly covering the copper surface with a layer of liquid photoresist. There are many methods for this, such as centrifugal coating, dip coating, screen printing, curtain coating, roller coating, and the like.
Screen printing is a commonly used coating method, which requires low equipment, 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 cannot be completely guaranteed. In general screen printing, full-screen printing uses 100 to 300 mesh screens for electroplating using 150-mesh screens. This method is welcomed by most small and medium manufacturers.
Roll coating can realize double-sided coating at the same time. It has high automation production efficiency and can control the coating thickness. It is suitable for large-scale production of various specifications, but requires equipment investment.
Curtain coating is also suitable for large-scale production, but also can uniformly control the thickness of the coating layer, but the equipment requirements are high, and can only be painted on one side before coating the other side, affecting the production efficiency.
Photo-induced coating film is too thick, easy to produce underexposure, lack of development, high pressure sensitivity, easy to stick the film; film is too thin, prone to overexposure, poor resistance to plating insulation and easy to produce metal plating film phenomenon, Slow film removal.
Working conditions: Clean room under yellow light, room temperature is 23 ~ 25 °C, relative humidity is 55 ± 5%, the workplace is kept clean, avoid sunlight and direct sunlight.
Coating operation should pay attention to the following aspects
1) If the coating layer has pinholes, the photoresist may be unclear, wash with acetone and replace the new resist. It may also be that particles in the air fall on the surface of the board or the board surface is not clean due to other reasons. It should be carefully inspected and cleaned before coating.
2) If the photocoating film is too thick during screen printing, it is because the number of meshes is too small; if the film is too thin, it may be caused by too many meshes. If the coating thickness is not uniform, thinner should be added to adjust the viscosity of the resist or adjust the speed of the coating.
3) Try to prevent the ink from entering the hole while applying the film.
4) Regardless of the method used, Photoimageable cover coating should achieve uniform thickness, no pinholes, bubbles, inclusions, etc., and the film thickness should reach 8-15 μm after drying.
5) Because the liquid photoresist contains solvents, the workplace must be well ventilated.
6) Wash hands with soap after work.
3. Pre-curing
The pre-baking means that the surface of the liquid photoresist film is dried by heating and drying to facilitate contact exposure and development of the film to produce a pattern. Most of this process is performed in the same chamber as the coating process. Pre-baking methods are most commonly used in both drying tunnels and ovens.
Generally, oven drying is used. The first-side pre-baking temperature on both sides is 80±5° C. and 10 to 15 minutes. The second-side pre-bake temperature is 80±5° C. and 15 to 20 minutes. This pre-and post-bake pre-bake makes the degree of pre-curing of the wet film on both sides different, and the effect of the development is also difficult to ensure complete consistency. Ideally, both sides are coated simultaneously and pre-baked at a temperature of 80±5°C for about 20-30 minutes. This double-sided pre-curing at the same time and can ensure that the two-sided development of the same effect, and save man-hours.
It is important to control the temperature and time of the pre-baking. If the temperature is too high or the time is too long, the development is difficult, and it is difficult to remove the film; if the temperature is too low or the time is too short, the drying is not complete, the film has pressure-sensitive properties, and the negative film may be easily exposed due to poor exposure, and the film may be easily damaged. Therefore, pre-baking is appropriate, development and removal of film is faster, and the graphics quality is good.
The operation should pay attention
(1) After pre-baking, the board should be air-cooled or naturally cooled before the film is exposed.
(2) Do not use natural drying, and drying must be complete, otherwise it is easy to stick the film and cause poor exposure. After pre-baking, the film thickness of the photosensitive film should be HB ~ 1H.
(3) If an oven is used, it must be equipped with blast air and constant temperature control so that the pre-bake temperature is even. And the oven should be clean, no impurities, so as not to fall on the board, damage the film surface.
(4) After the pre-baking, the coating film is allowed to develop for a maximum of 48 hrs, and the exposure time is as long as 12 hr.
(5) The requirements for different types of liquid photoresist are different. Read the instructions carefully and adjust the process parameters such as thickness, temperature, and time according to the production practice. 4. Positioned (Fixed Postion)
With the continuous expansion of high-density interconnect technology (HDI) applications, resolution and positioning have become major challenges for PCB manufacturers. The higher the circuit density, the more precise the positioning is required. The positioning methods include visual positioning, moving pin positioning, fixing pin positioning and other methods.
The visual positioning is to use a Diazo film to pass through the pattern to coincide with the hole in the printed board, and then expose it with adhesive tape. The diazo film is brown or orange and translucent, which can ensure good repositioning accuracy. Silver film can also be used in this method, but it must be made of transparent positioning plate in the film to locate.
The movable pin positioning system includes a photographic film punch and a double-round hole out-positioning device. The method is: firstly aligning the front and back two bottom film films, and using the film punch device to arbitrarily punch two positions outside the effective pattern. Holes, any one to take a drilling program, you can use a drill hole drilling, printed boards metallized holes and pre-plated copper, you can use the double-round hole out positioning locator positioning exposure.
Fixed pin positioning is divided into two sets of systems, a set of fixed photo plates and another set of fixed PCBs. By adjusting the positions of the two pins, a coincident alignment between the photo plate and the PCB can be achieved.
5. Exposuring
After the liquid photoresist is irradiated with UV light (300-400 nm), a cross-linking polymerization reaction occurs, and the light-irradiated portion is formed into a film and hardened without being affected by the developer. The source of the exposure lamp normally used is a high-brightness, medium-pressure mercury lamp or a metal halide mercury lamp. Lamp 6000W, exposure 100 ~ 300mj/cm2, density measurement using 21 optical density table (Stouffer21), to determine the optimal exposure parameters, usually 6 to 8 levels. The use of liquid photoresist for exposure to parallel light is not critical, but its speed is not as fast as that of dry film, so a high-efficiency exposing (Drawer) should be used.
The photopolymerization reaction depends on the light intensity of the lamp and the exposure time. The light intensity of the lamp is related to the excitation voltage and is related to the lamp usage time. Therefore, in order to ensure enough light energy for photopolymerization reaction, it must be controlled by the optical energy integrator. Its principle is to ensure that the exposure time is automatically adjusted to maintain the total exposure energy when the light intensity changes during exposure. Exposure time It is 25 to 50 seconds.
Factors affecting exposure time:
(1) The closer the light is, the shorter the exposure time;
(2) The thicker the thickness of the liquid photoresist, the longer the exposure time;
(3) The greater the humidity of the air, the longer the exposure time;
(4) The higher the pre-bake temperature, the shorter the exposure time.
When the exposure is excessive, the astigmatism is easily refracted, the line width is reduced, and the development is difficult. When the exposure is insufficient, the development is likely to cause defects such as pinholes, hairiness, and detachment, and the corrosion resistance and plating resistance are reduced. Therefore, selecting the optimal exposure parameter is an important condition for controlling the development effect.
The quality of the film has a direct impact on the quality of exposure. Therefore, the pattern of the film is clear and there must be no halo or imaginary phenomena. Pinholes and trachoma are required. The stability is good. The film requires a large black-and-white contrast: the silver salt film has a Density of DMAX ≥ 3.5, and DMIN ≤ 0.15; the diazo film has an optical density of DMAX ≥ 1.2 and DMIN ≤ 0.1.
In general, after the film has been produced, it is transferred from one process (factory) to another (factory), or stored for a period of time before it enters the yellow light chamber. This way of experiencing different environments, the dimensional stability of the film is difficult to guarantee. I think that the finished film should be directly into the yellow room, and each film should be discarded when it is used to make more than 80 boards. In this way, the micro-deformation of the graphics can be avoided, especially the micro-hole technology should pay more attention to this point.
Exposure process operation precautions
(1) The exposure machine is absolutely necessary for vacuum drying and the degree of vacuum is ≥ 90%. Only by evacuating the film and the workpiece, it can be ensured that the image is not distorted so as to improve the accuracy.
(2) When the exposure operation, if the production of viscous negative film, may be caused by insufficient pre-baking or too strong vacuum drying and other reasons, it is necessary to adjust the pre-baking temperature and time or check the vacuum drying conditions.
(3) After the exposure is stopped, the board should be taken out immediately. Otherwise, the excess light in the lamp will cause excess glue after development.
(4) The working conditions must reach: a dust-free yellow light operation room with a cleanliness level of 10,000 to 100,000 and air-conditioning facilities. The exposure machine should have a cooling exhaust system.
(5) When the film is exposed, the film side of the film must face downwards to make it close to the surface of the photosensitive film to increase the resolution.
6. Developing
The development removes (dissolves) the non-photosensitive, non-patterned wet film, leaving the photohardened graphic portion. The method is generally manual development and machine spray development.
The working conditions of this process are the same as the coating process.
Machine Development Formulas and Process Specifications
Na2CO3 0.8 to 1.2%
Antifoaming agent 0.1%
Temperature 30±2°C
Development time 40±10 seconds
Spray pressure 1.5～3kg/cm2
The Breok Point Control is controlled at 1/3 to 1/2. In order to ensure the development quality, it is necessary to control the developer concentration, temperature, and development time within an appropriate operating range. Temperatures that are too high (above 35°C) or development times that are too long (more than 90 seconds) can result in reduced film quality, hardness, and chemical resistance.
After development, there are more glues, most of which are related to the process parameters. There are mainly the following possibilities:
1 the development temperature is not enough;
2Na2CO3 concentration is low;
3 spray pressure is small;
4 The transmission speed is faster and the development is not complete;
5 overexposure;
6 stacked plates.
The operation precautions
(1) If a wet film enters the hole during production, it is necessary to increase the ejection pressure and lengthen the development time. After development, carefully check whether the hole is clean. If there is residue, rework should be repeated.
(2) After the developer has been used for a period of time, the capacity has dropped and new fluid should be replaced. Experiments have shown that when the pH of the developer drops to 10.2, the developer has lost its activity. To ensure the image quality, the plate-making capacity at PH=10.5 is defined as the cylinder change time.
(3) After the development, wash it thoroughly so that the alkaline solution is not brought into the etching solution.
(4) If open circuit, short circuit, exposed copper and other phenomena occur, the reason is generally that there is damage or debris on the film.
7. dry
In order to make the film have excellent anti-corrosion and anti-plating ability, it should be dried after developing. The conditions are temperature 100°C and time 1 to 2 minutes. The cured film hardness should reach 2H ~ 3H.
8. Check revision
The revised version is actually a self-inspection. Its purpose is mainly to repair the defects on the graphics circuit and remove the parts that are not related to the graphics requirements. That is, the removal of extraneous burrs, glue spots, etc., and make up for missing holes such as pinholes and gaps. , broken lines and so on. The general principle is to scrape after make up, so that it is easy to guarantee the quality of revision.
The commonly used cleaning solutions include shellac, asphalt, and acid-resistant inks. The simplest one is the shellac solution. Its formula is as follows:
Shellac 100~150g/l
Methyl violet 1～2g/l
Amount of anhydrous ethanol
Revision requirements: the pattern is correct, the alignment is accurate, and the accuracy is in accordance with the process requirements; the conductive pattern has neat and smooth edges, no residue, oil, fingerprints, pinholes, gaps, and other impurities, and there is no residual film and foreign matter on the hole wall; 90% of repairs The workload of the plate is caused by the uncleanness of the exposure tool. Therefore, the film should be frequently inspected during the operation, and the sunscreen and film should be cleaned with alcohol to reduce the amount of revision. Care should be taken in the revision of the gloves to prevent sweat contamination of the layout. If the first two processes are done very well, there is almost no revision, and the revision process can be omitted.
9. Strip
After Etching or Plating, the protective film must be removed. Usually, a 4-8% NaOH solution is used to remove the film, and the film is heated and expanded to achieve differentiation. Methods include manual film removal and machine spray coating.
Spraying machine is used to spray the film. Its injection pressure is 2 ~ 3kg/cm2. The quality of the film is good, the removal is clean, and the production efficiency is high. Increasing the temperature can increase the speed of film removal, but the temperature is too high and black holes are easy to occur. Therefore, the temperature should generally be 50-60°C.
After the film is removed, it must be cleaned. If there is residual glue on the surface after removing the film, the main reason is that the process parameters of the baking process are not correct, and the overbaking is generally excessive.
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