INTRODUCTION: In the PCB manufacturing technology, one of the most basic and critical processes, whether single, double-faced or multi-layered (MLB), is the transfer of graphics, ie the transfer of artwork from artwork to copper. On the foil substrate. Graphics transfer is a key control point in production, and it is also a technical difficulty. There are many process methods such as screen printing pattern transfer process, dry film pattern transfer process, liquid photo resist pattern transfer process, electrodeposition photoresist ( ED film) manufacturing process and Laser Direct Image Technology (Laser Drect Image). The liquid crystal photoresist pattern transfer process that can replace the dry film pattern transfer process today is widely used because of its thin film, high resolution, low cost, and low operating condition requirements. This article analyzes the liquid photoresist and its manufacturing process in PCB pattern transfer.
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 characteristics:
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 much as dry film cutting, and no subsequent disposal of the waste film is required. Therefore, the use of 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 wet film thickness 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 pre-processing is to remove
Grease, Oxidized Layer, Dust, and Particle Residues, Moisture, and Chemicals, especially Alkaline, on Copper Surfaces Guarantee Copper Surfaces Cleanliness and roughness, the production of a uniform and suitable copper surface, improve the binding force of the photosensitive adhesive and copper foil, the wet film and the dry film requirements are different, it is more focused on 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.