Keywords circuit board, screen printing, printed material
Printed circuit board production is inseparable from screen printing and printing materials. Ink, also known as ink, is widely used in printed circuit manufacturing processes. Today, electronic products are moving toward high-precision, high-density, high-reliability, and micro-technologies, and require higher requirements for screen printing included in printed circuit manufacturing technology. With the rapid development and advancement of screen printing materials in recent years, many new types of printing materials have been developed and developed, and they have been continuously improved to meet the production process requirements for finished printed circuit boards.
At present, the printed board manufacturing industry has formed a series of special printing materials. Its types are roughly classified as follows:
(1) Anti-etching printing materials;
(2) Electroplating resist printing materials;
(3) Solder resist printing materials;
(4) characters, marking materials;
(5) plug hole printing materials;
(6) Conductive printing materials;
(7) Peelable printing materials;
(8) Solder paste printing material.
First, anti-etching printing materials
Anti-etching printing material is one of the earliest developed and used printing materials in PCB production and is used in subtractive process. Its role is to screen print on the copper foil, protecting the circuit pattern from being etched away by the etching solution. A large number of brands have been formed at present, but they are grouped into two major categories: (1) self-drying or thermosetting; (2) light-curing.
(1) Dry or thermosetting
Self-drying or thermosetting resists are one-component type inks composed of synthetic resins, fillers, pigments, auxiliaries and solvents.
Self-drying printing materials are dried at room temperature for one hour or less; thermosetting printing materials are required to be dried in a hot air circulation oven at 80 to 100° C. and 8 to 10 minutes (minutes). Hardness up to 2H. Some varieties are only resistant to acidic etching solutions, while others are resistant to both acidic and alkaline etching solutions. The removal of the film is usually carried out with a 1% to 3% NaOH solution. The print has a shelf life of one year and the pigment is mostly blue or black.
(2) Photosetting
Light-cured resist printing materials generally consist of photosensitive resins, photosensitive monomers, sensitizers, fillers, additives, and pigments. Its main feature is that it does not dry on the screen when it is screen printed, and it does not block the screen, but under ultraviolet (UV) light irradiation, photocrosslinking polymerization occurs within a few seconds. Rapid curing, the film layer hardness is also up to 2H, the same resistance to acidic and alkaline etching liquid; at the same time because of no solvent, it will not pollute the environment, suitable for single-sided printed circuit board automated production line. The storage period of printing materials is approximately 4 to 6 months.
The usual curing conditions for printing materials are as follows: high pressure mercury lamp 80 W/cm×3, irradiation distance 10 cm, light curing machine conveying speed 4 to 6 m/min, removing film 3% to 5% NaOH solution, temperature 30～ 50°C.
The resolution of the thermosetting and photosetting printing materials is about 200 μm, and patterns with line widths and pitches above 0.25 mm are produced.
Second, anti-electroplating printing materials
Anti-electroplating printing materials and anti-etching printing materials, is also a one-liquid dilute alkaline dissolved printing materials. It is mainly used for the production of double-sided or multi-layer printed circuit board outer circuit patterns. Its function is to screen negative pattern. It can prevent metal ions from forming a plating layer on the pattern plating. Anti-electroplating printing materials can also be used as Resistant to etching materials.
Electroless plating resists are mainly divided into two major categories: thermosetting and liquid-sensitive.
(1) Thermosetting
Thermosetting inks consist of synthetic resins, fillers, additives, pigments, and solvents. Film drying temperature is 80 ~ 100 °C, time 8 ~ 10 min, pencil hardness after drying is about 2H, resistant to electroplating of copper, tin-lead alloy, nickel, gold plating solution and acid or alkaline etching solution, It is suitable for the production of PCBs with a line width and pitch of 0.25 mm or more.
The printing of anti-electroplating printing materials requires that the film thickness after drying reach a certain requirement. If the film layer is too thin, the pattern of the circuit will extend to both sides during the pattern plating, resulting in plating overhang, which is the so-called “mushroom†phenomenon. It may also cause a short circuit, causing difficulties in removing the ink or removing ink from the ink when the printed material is removed. The residual film may cause the pattern wires to be serrated or etched uncleanly during etching.
The use of light-cured printing materials is not very popular. Although light-cured printing materials have the advantages of good screen printing performance and thicker coatings, the difficulty in controlling the degree of curing and the inconvenience of ink removal affect its popularization and use.
The average level of screen printing technology is about 0.2 mm in line width and spacing. It is very difficult to use the thermosetting printing material to operate the process to break through this level.
Deinking uses 3% to 5% NaOH solution, and the storage period of the printing material is one year.
(2) liquid photosensitive type
The liquid photosensitive resisting electroplating printing material is a kind of printing material developed by solving the production of fine wire patterns, commonly known as wet film. It overcomes some difficulties in the production process of thermosetting electroless plating resists and dry film and is suitable for the production of fine wires and superfine wires. The thin wire can be formed with three wires (0.125 mm) or four wires (0.075 mm) between the two pads centered at 2.54 mm. It can also be used for high-precision crafts, STM stencils, and pad printing. use.
The liquid sensitized electroplating printing material can also be used for the production of fine wires in the inner layer of a multilayer board, and is used as an anti-etching printing material.
It consists of photosensitive resins, sensitizers, fillers, additives, pigments, and solvents. The resolution of the printed matter is 50 to 100 μm, and the adhesion of the copper-clad laminates is good. There are no defects such as wire burrs, nicks, short circuits, etc. caused by edge leaching during the production of dry film.
The printing material is usually operated within a safe yellow light area and the storage period is approximately one year.
The brief process flow of the liquid sensitization plating resist is as follows:
Coating  pre-baking  cooling  exposure  display           a a a a a a a a a a a a a a a a a a a a a a a a a a a a a In the electroplating process of the pattern, a lower mesh screen is used. When the etching process for the inner layer of the multilayer board is performed, a mesh with a high mesh number is selected.
b. Pre-baking hot air circulation oven 80 ~ 100 °C, 8 ~ 10 min, A, B surface baking on both sides at the same time.
After baking, remove it and cool it to room temperature.
c. Exposure after exposure to cooling, the UV wavelength is 300-400 nm, the exposure energy is controlled at 100-300 mJ/cm2, and the Stuffer 21-class optical densitometer is 7-8.
d. A 1% Na2CO3 solution was developed, the developer temperature was 28-32°C, the spray pressure was 0.2-0.3 MP/cm2, and the time was 30-60 seconds (seconds).
e. Electroplating electroplating copper, tin-lead alloy, nickel, gold solvent.
f. To remove 3% to 6% NaOH solution, the film swells and swells at 50-60°C.
g. Etch the acid or alkaline etching solution, but the temperature of the etching solution should not be too high.
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