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. This involves transferring the photographic plate (Artwork) graphics to copper plating foil substrates. Graphics transfer is a key control point in production and a technical challenge. There are various methods for this process, including screen printing pattern transfer, dry film pattern transfer, liquid photoresist pattern transfer, electrodeposition photoresist production, and laser direct imaging technology. Among these methods, the liquid photoresist pattern transfer process has gained popularity due to its advantages of thin film, high resolution, low cost, and low operating conditions. This paper will analyze 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)
Liquid photoresist, known as wet film, is composed of photosensitive resin, photosensitive agent, colorant, filler, solvent, and so on. When exposed to light, it undergoes a photopolymerization reaction to form a pattern, following a negative photopolymerization type. Compared to traditional resist ink and dry film, liquid photoresist offers several advantages:
a) Eliminates the need for silk screen stencils, utilizing negative film contact exposure imaging to improve resolution to 40um.
b) Exhibits superior film adhesion, bonding, etch resistance, and electroplating resistance compared to traditional inks.
c) Provides flexible and diverse coating methods, with easy process operability.
d) Offers good adhesion to the substrate, fills surface defects, and achieves high-definition resolution without a cover film layer, ensuring product quality.
e) Rectifies issues like warping, seepage plating, and irregular lines, ensuring uniformity and efficiency in production.
f) Suitable for electroless nickel-gold plating due to its resistance to gold plating solution.
g) Highly flexible for flexible printed circuit board production.
h) Reduces material cost, waste, and handling issues, saving up to 30 to 50% of the cost per square meter.
i) Easy to store as a single-liquid ink, with a storage life of 4 to 6 months under specified conditions.
j) Versatile for a wide range of applications, including MLB inner layer circuit patterns and electroplating resistant patterns.
However, liquid photoresist may have challenges with thickness uniformity and drying control, necessitating careful operational oversight. Additionally, volatile additives, solvents, and initiators in the wet film can pose environmental and health risks, requiring proper ventilation in the workplace.
The liquid photoresist currently in use typically has a viscous, blue appearance and is applied through simple screen printing, developed with dilute alkaline water, and etched using acid or weak alkaline solutions. The service life of liquid photoresist is influenced by operating environment conditions, with a recommended service life of 3 days under specified conditions.
Liquid photoresist process flow:
The previous process → pre-treatment → coating → pre-baking → positioning → exposure → development → drying → inspection and revision → etching or electroplating → film removal → moving to the next process
Pre-treatment aims to ensure cleanliness and roughness of the copper surface, enhancing the bonding force between the photosensitive adhesive and the copper foil. Mechanical grinding and chemical pretreatment methods are commonly used for this purpose, with a focus on cleanliness for wet film requirements.
In the mechanical grinding method, strict control of parameters like pickling time, brush mesh, grinding speed, and water pressure is crucial to achieve a clean board surface. Chemical pretreatment is preferred for MLB inner layer boards, involving steps like degreasing, cleaning, micro-etching, and drying to prepare the copper surface.
Regardless of the pre-treatment method used, immediate drying is essential post-treatment. Inspection through a water film test can determine the cleanliness of the board surface. After cleaning, handling with clean gloves and prompt application of photosensitive adhesive can prevent copper surface reoxidation.
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)
Liquid photoresist, known as wet film, is composed of photosensitive resin, photosensitive agent, colorant, filler, solvent, and so on. When exposed to light, it undergoes a photopolymerization reaction to form a pattern, following a negative photopolymerization type. Compared to traditional resist ink and dry film, liquid photoresist offers several advantages:
a) Eliminates the need for silk screen stencils, utilizing negative film contact exposure imaging to improve resolution to 40um.
b) Exhibits superior film adhesion, bonding, etch resistance, and electroplating resistance compared to traditional inks.
c) Provides flexible and diverse coating methods, with easy process operability.
d) Offers good adhesion to the substrate, fills surface defects, and achieves high-definition resolution without a cover film layer, ensuring product quality.
e) Rectifies issues like warping, seepage plating, and irregular lines, ensuring uniformity and efficiency in production.
f) Suitable for electroless nickel-gold plating due to its resistance to gold plating solution.
g) Highly flexible for flexible printed circuit board production.
h) Reduces material cost, waste, and handling issues, saving up to 30 to 50% of the cost per square meter.
i) Easy to store as a single-liquid ink, with a storage life of 4 to 6 months under specified conditions.
j) Versatile for a wide range of applications, including MLB inner layer circuit patterns and electroplating resistant patterns.
However, liquid photoresist may have challenges with thickness uniformity and drying control, necessitating careful operational oversight. Additionally, volatile additives, solvents, and initiators in the wet film can pose environmental and health risks, requiring proper ventilation in the workplace.
The liquid photoresist currently in use typically has a viscous, blue appearance and is applied through simple screen printing, developed with dilute alkaline water, and etched using acid or weak alkaline solutions. The service life of liquid photoresist is influenced by operating environment conditions, with a recommended service life of 3 days under specified conditions.
Liquid photoresist process flow:
The previous process → pre-treatment → coating → pre-baking → positioning → exposure → development → drying → inspection and revision → etching or electroplating → film removal → moving to the next process
Pre-treatment aims to ensure cleanliness and roughness of the copper surface, enhancing the bonding force between the photosensitive adhesive and the copper foil. Mechanical grinding and chemical pretreatment methods are commonly used for this purpose, with a focus on cleanliness for wet film requirements.
In the mechanical grinding method, strict control of parameters like pickling time, brush mesh, grinding speed, and water pressure is crucial to achieve a clean board surface. Chemical pretreatment is preferred for MLB inner layer boards, involving steps like degreasing, cleaning, micro-etching, and drying to prepare the copper surface.
Regardless of the pre-treatment method used, immediate drying is essential post-treatment. Inspection through a water film test can determine the cleanliness of the board surface. After cleaning, handling with clean gloves and prompt application of photosensitive adhesive can prevent copper surface reoxidation.
