New PCB Ink - Liquid Photoresist Ink (Part I)

With the rapid development of the electronics industry, the requirements for the soldering process and soldering quality of printed circuit boards (PCBs) have become higher and higher. Both the conventional heat-cured and UV-cured resists were printed with a screen pattern. However, The full coverage of the lines, dimensional accuracy and other aspects, the method of using silk screen graphics has been incompatible, so the researchers developed a dry film resist. However, due to its inherent limitations, dry film resists can not be completely To meet the PCB's performance requirements, its resolution can reach nearly 25μm technically, but the scale of production can only be achieved 75 ~ 100μm. And the development of electronic technology, has required high-density PCB on the cutting-edge of the resolution of ≤ 75μm, And will be developed to 50μm, or even finer lines. Therefore, there is an urgent need for new photoresists, to improve the resolution to a higher level, especially multi-plate inner layer manufacturing. In addition, PCB price The competition has also become increasingly fierce, forcing PCB manufacturers to do everything possible to reduce the manufacturing cost of PCBs under the precondition of ensuring the quality and performance of PCBs. According to the structural characteristics of dry film resist itself, the cost is hard to reduce. In addition, high-density PCBs require minimum pad area, even with “no” pads, and dry film resists are difficult to adapt to this requirement. As a result, many PCB manufacturers have turned their attention to liquid photoresists [ 1 to 5].

The current representatives of the new generation of liquid photoresists are liquid photosensitive imaging inks (also known as wet film resists) and electrodeposited liquid photoresists (ED resists). Using these new types of resists, High resolution is easy to obtain. For example, with a non-collimated light source and a standard developing device, a resolution of 50 μm can be obtained after the development. If a collimated light source is used, as long as the corresponding clean environment and base map conditions are guaranteed, its resolving power can be achieved. It can reach 25μm. In the subsequent outer layer or inner layer etching process, the liquid photoresist can give excellent etching effect compared to the dry film. This high resolution makes thin line PCB makers can produce A product with a low defect density and a high yield.

The liquid photosensitive resist ink is an ink developed to solve the fine wire pattern production, commonly known as wet film. It overcomes some problems in the production process of thermosetting resist ink and dry film, and is suitable for fine wires and superfine wires. Production. Three wires (0.125 mm) or four wires (0.075 mm) can be formed between the two pads centered at 2.54 mm at the finest line width, and can also be used for high-precision craftwork, hollowing out. The use of stencils and pad printing for gravure printing can also be used for the production of fine wires in the inner layer of multilayer boards. It consists of photosensitive resin, sensitizers, fillers, auxiliaries, pigments and solvents. The resolution of inks is 50 to 100 μm, and The copper clad foil has good adhesion, and there are no defects such as wire burrs, nicks, short circuits, etc. caused by interfacial bubbles in the production of dry film. The ink is usually operated in a safe yellow light range, and the storage period is long. About one year. The brief process flow of liquid photosensitive electroplating inks is as follows:

Coating → Pre-bake → Cooling → Exposure → Development → Curing → Plating → Ink removal → Etching → Post-processing

The liquid photosensitive resist can be printed on the printed board by screen printing, spraying or curtain coating, without positioning, after pre-heating, after the surface is dried, the photoplate is used to locate the ultraviolet exposure and development. Accurate resist patterns are obtained. Compared with conventional thermosetting, photo-curable and anti-corrosive dry films, firstly, the pattern accuracy is high, and the solder joint pattern of 40 to 50 μm can be easily obtained; in addition to the substrate and the copper wire, Good combination, high heat resistance, good filling in wire gap, low defective rate; and, due to its use of epoxy resin as a thermosetting component, it is resistant to chemicals, gold plating, moisture, heat and electricity Insulation is very good. In view of the above advantages, both at home and abroad in the production of high-precision circuit boards and multilayer circuit boards have been used liquid photosensitive resist.

1 Liquid photosensitive resist composition

1. 1 photosensitive resin

1. 1. 1 Alkali-Soluble Photocurable Resins [6-8] In order to significantly increase the density of the wiring, the lands should be shrunk. This requires highly sensitive photosensitive resin with high resolution. Currently, it is more commonly used. There are several types of alkali-soluble light-curable resins:

(1) Phenolic condensation epoxy resin and acid anhydride reaction products. The main characteristics of this type of resin is the production of convenient, low cost, small thermal expansion coefficient, dimensional stability. At present the most commonly used.
(2) Mixed product of acrylic epoxy resin and anhydride, unsaturated isocyanate. Compared with 1, it can be seen that its unsaturated olefin functional group is more, so it has the characteristics of fast photocuring.

(3) Mixed products of acrylic epoxy resin and anhydride, alkyl diketene reaction. Such resins have a small number of carboxyl groups, low acid value and slow development speed. However, due to the presence of COCH2CO groups, The bonding strength of copper foil is quite high, suitable for occasions with special requirements for bonding strength.

(4) Mixed reaction products of acrylic epoxy resin and anhydride, alcohol, TDI (diisocyanate). Such resin has good wetting ability on the filler powder surface in the resist, which facilitates the manufacture of the resist.

(5) Trisphenol is a reaction product of epoxy acrylic acid and acid anhydride. This type of resin has good resistance to plating and can also be used as a developing resist for plating resists, in addition to resist.
In addition to the above five resins, it is also possible to introduce synthetic rubber or long-chain alkyl ether structures in the alkali-soluble macromolecules to increase the flexibility or softness of the resins; it is also possible to use alkyl phenols or diacids or diamides. Partial substitution of acrylic acid and epoxy resin reaction to increase the resolution of the resin, while increasing the molecular weight of the resin, reducing the surface of the film layer stickiness.

1. 1. 2 Synthesis of alkali-soluble photo-setting resin [1~5]
Alkali-solubility photocurable resin is the main component of the preparation of liquid photosensitive resists. It plays an important role in the various properties of the printing materials, especially the process properties such as pre-baking, developing, and exposure, adhesion, hardness, and photo-curing speed. The role of determining which type of alkali-soluble light-curable resin to use is the key to successful printing. The synthesis method can be described simply as follows: some of the macromolecules are "mounted" on the basis of condensation or grafting. Aqueous groups (such as carboxyl, amide, etc.) and reactive groups (such as vinyl, propylene, etc.).

According to different needs, different alkali-soluble groups and reactive groups can be introduced to achieve the purpose.

1. 2 Thermosetting Epoxy The thermosetting epoxy resin is another major component of the liquid photosensitive resist. There are many kinds of thermosetting epoxy resins that are used in common with the photosensitive resin. For example: glycidyl epoxy Compounds, glycidyl ethers of polyvalent phenols, glycidyl ethers of polyhydric alcohols, etc., contain at least 2 or more epoxy groups per molecule. Among them, novolac epoxy resins, hydroquinone shrinkage are preferred. Glycerin ether, etc. These compounds are used as thermosetting components in liquid photosensitive resists, have good developability, and do not produce a residue film on the pad, and do not affect the photosetting speed of the liquid photosensitive resist. The thermal curing agent in the resist should be a potential thermal curing agent, ie it should be a material with good storage stability after addition. Among them, organic acid hydrazide and diaminomaleamide are preferred. At room temperature, the thermal curing resin does not Reaction, heating to about 150 °C to react with thermosetting resin.

1. 3 Reactive diluents The various properties of the liquid photosensitive resist are not only closely related to the structure and properties of the alkali-solubility photosensitive resin, but also to a large extent affected by reactive diluents and heat-curable components, especially reactive diluents. Through the control of the added amount and the selected type, the hardness, photospeed, developing difficulty and other physical and chemical properties of the resist film can be adjusted.

Reactive diluent is a functional monomer, which is also one of the basic components of the photo-curing solder resist. Its role is to adjust the viscosity of the ink, control the cross-linking density, and improve the physical properties of the cured film. Active agent is divided into monofunctional active diluents. Agents (ie, one double bond per molecule) and polyfunctional diluents (ie, several double bonds per molecule). From the dilution effect, monofunctional> bifunctional> polyfunctional; photocuring speed is multi-functional> bifunctional > Monofunctional [5, 8].

In actual operation, it is necessary to increase the speed of light curing, increase the degree of cross-linking, and increase the hardness. Use polyfunctional diluents such as trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate (PE2TA), etc., and need to improve the cured film. Flexibility uses bifunctional monomers such as tripropylene glycol diacrylate (TPGDA) and hexanediol diacrylate (HD2DA). In order to achieve a better overall effect, two or more active dilutions are generally used. The agent is used in combination, the best ratio of which is determined by the experimental results.

In addition to the above components, fillers have to be added in the liquid photosensitive resist to improve the screen printing properties. Defoamers should be added to eliminate air bubbles and pigments should be added to suit each user's requirements for color.

1.4 Photoinitiators In addition to reactive diluents, photoinitiators that determine the photosensitivity and hardening response of the resist also occupy important positions. Depending on the mode of photochemical activity, they can be roughly classified into the following two categories [8–11] :

(1) Homogeneous initiators: The main products are benzoin (benzophenone) and its derivatives. A large number of studies on the mechanism of its cleavage have shown that the process involves the carbon-carbon bonds bonded to the carbonyl groups. Homolytic cracking. However, benzoin itself is not a very effective photoinitiator, the current industrial products are benzoin ether, benzyl ketals and α, α′-dichloro acetophenone and so on.

(2) Hydrogen-initiating initiators: The mechanism can be briefly summarized as follows: Hydrogen is extracted from adjacent donor molecules to generate free radicals, and the process involves transitions between states. Benzophenone and its derivatives, thioxanthones Such as these are all such. Among them, Michler's ketone (4,4'-dimethylamine benzophenone) has been widely used in light curing formulations.