PCB ink Selection Knowledge (1)

Introduction: In the PCB manufacturing process (Technology), whether it is a single, double-sided or multi-layer board (MLB), one of the most basic and critical processes is the transfer of graphics, that is, the transfer of the photographic plate (Art-work) graphics to copper plating foil substrates. Graphics transfer is a key control point in production and also a technical difficulty. There are many process methods, such as screen printing pattern transfer process, dry film pattern transfer process, liquid photoresist pattern transfer process, electrodeposition photoresist production process and laser direct imaging technology. Today, the first liquid photoresist pattern transfer process that can replace the dry film pattern transfer process is widely used due to the advantages of thin film, high resolution, low cost and low operating conditions. This paper analyzes the liquid photoresist and its production process in PCB pattern transfer.

Liquid photosensitive ink application process flow chart: >

Substrate surface treatment –> coating (screen printing) –> pre-baking –> exposure –> development –> drying –> inspection –> etching –> film removal –> inspection (Note: inside laminate)

Substrate surface treatment –> coating (screen printing) –> pre-baking –> exposure –> development –> drying –> inspection –> electroplating –> stripping –> etching –> inspection (Note: Outer layer board)

A. Liquid Photoresist

Liquid photoresist (referred to as wet film) is made of photosensitive resin, combined with photosensitive agent, colorant, filler and solvent, etc. After light irradiation, it produces a photopolymerization reaction to obtain a pattern, which is a negative photopolymerization type. Compared with traditional resist ink and dry film, it has the following characteristics:

a) No need to make silk screen stencils. The use of negative film contact exposure imaging (Contact Printig) can avoid defects such as penetration, stains, shadows, and image distortion caused by screen printing. The resolution is greatly improved, the resolution of traditional ink is 200um, and the wet film can reach 40um.

b) Since it is a photo-curing reaction conjunctiva, its film adhesion, bonding, etch resistance and electroplating resistance are better than those of traditional inks.

c) The wet film coating method is flexible and diverse, with strong process operability and easy to master.

d) 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. In addition, the wet film can be as thin as 5-10um, only about 1/3 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 with a thickness of about 25um), so the resolution of the graphics is, high definition. For example, when the exposure time is 4S/7K, the resolution of the dry film is 75um, while the wet film can reach 40um. Thus ensuring product quality.

e) Problems such as warping, electroplating, and irregular lines that often occur when using dry film in the past. The wet film is a liquid film, which can not warp, seep plating, and has neat lines. The allowed laying time from the coating process to the developing process can reach 48hrs, which solves the contradiction between the production processes and improves the production efficiency.

f) 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.

g) Because it is a liquid wet film, it is highly flexible, especially suitable for the production of flexible printed boards.

h) The material cost of wet film is reduced due to its reduced thickness, and compared with dry film, it does not need a carrier polyester cover sheet and a protective polyethylene separator sheet. There is no waste as much as the dry film cutting, and there is no need to deal with the subsequent waste film. The use of wet film can save about 30 to 50% of the cost per square meter.

i) Wet film is a single-liquid ink that is easy to store. Generally, the temperature is 20±2℃, the relative humidity is 55±5%, and it is sealed and stored in a cool place. Storage Life: 4 to 6 months.

j) It can be used in a wide range of applications. It can be used for the production of MLB inner layer circuit patterns and the production of electroplating resistant patterns for perforated boards.

However, the uniformity of wet film thickness is not as good as that of dry film, and the degree of drying after the coating is not easy to grasp. Therefore, be careful when operating. In addition, the volatilization of additives, solvents, initiators, etc. in the wet film causes pollution to the environment, especially to operators. Therefore, the workplace must be well ventilated.

At present, the liquid photoresist used has a viscous appearance and is mostly blue in color. Such as: GSP1550 produced by Taiwan Jinghua Company, APR-700 produced by Taiwan Tiying Company, etc. These are all single-liquid inks, which can be applied by simple screen printing, developed with dilute alkaline water, and used acid or weak alkaline etching solution etching.

Lifespan of liquid photoresist (Lifespan): Its service life is related to the operating environment and time. The general temperature is less than or equal to 25°C, the relative humidity is less than or equal to 60%, and it is operated under the yellow light of the clean room. The service life is 3 days.

B. Liquid Photoresist Pattern Transfer

Liquid photoresist process flow:

The previous process → pre-treatment → coating → pre-baking → positioning → exposure → development → drying → inspection and revision → etching or electroplating → film removal → handing over to the next process

1. Pre-Cleaning

The main purpose of pre-treatment is to remove grease, oxide layer, dust and particle residue, moisture and chemical substances, especially alkaline substances on the copper surface. ) Ensure the cleanliness and roughness of the copper surface, create a uniform and suitable copper surface, and improve the bonding force between the photosensitive adhesive and the copper foil. The wet film and dry film requirements are different, and it focuses more on cleanliness.

Pretreatment methods include: mechanical grinding, chemical pretreatment and a combination of the two.

1) Mechanical Grinding Method

Grinding Condition:
Pickling time: 6 to 8s.
H2SO4: 2.5%.
Washing: 5s～8s.
Nylon Brush: 500-800 mesh, most of which use 600 mesh.
Grinding speed: 1.2～1.5m/min, interval 3～5cm.
Water pressure: 2～3kg/cm2.

Strictly control the process parameters to ensure the drying effect of the board surface, so that the ground board surface is free of impurities, glue traces and oxidation. It is best to carry out anti-oxidation treatment after grinding the board.

2) Chemical Pretreatment Method

For the MLB inner layer board, because the substrate is thin, it is not suitable to use the mechanical grinding method, and the chemical pretreatment method is often used.

Typical chemical pretreatment process:

Degreasing → cleaning → micro-etching → cleaning → drying

Remove Oil:
Na3PO4 40～60g/l
Na2CO3 40～60g/l
NaOH 10～20g/l
Temperature: 40～60℃

Mi-Croetehing:
NaS2O8 170～200g/l
H2SO4(98%) 2%V/V
Temperature: 20～40℃

The chemically treated copper surface should be pink. Regardless of whether the mechanical grinding method or the chemical pretreatment method is used, it should be dried immediately after treatment.

Inspection method: The water film test is used, and the principle of the water film rupture test is based on the interfacial chemistry between the liquid phase and the liquid phase or the liquid phase and the solid phase. If the water film can be kept without rupture for 15-30s, it is clean.

Note: After cleaning, the board should be handled with clean gloves and immediately coated with photosensitive adhesive to prevent the copper surface from reoxidizing.