2-3. Major compositions and functions of solder mask
Currently, most of the solder mask is liquid photo imageable solder mask mainly made of UV-hardened and heat-hardened resin in combination with photoinitiator, thermal hardener, filler, pigment, and other aids. The physical property of the hardened material differs depending on the raw material, blending proportion and operating conditions. The hardened products have an IPN (Interpenetrating Polymer network) structure.
Major components are described below:
2-3-1. Synthetic resin
This is photosensitive base epoxy resin or urethane resin. Since diluted alkaline water solution is needed for development, a carboxyl group (COOH) or hydroxyl group (OH) must be introduced. This resin is a prepolymer and framed with bisphenol A epoxy resin, phenolic epoxy resin, O-Methyl phenolic epoxy resin, and urethane resin.
2-3-2. Photoinitiator, photosensitizer, thermal hardener
The photoinitiator, photosensitizer and UV hardener share the same solder mask. However, since the UV light must pass through a layer of glass and a PET film for exposure, an applying additional UV sensitizer must be considered. Thermal hardener contains aromatic amine and acid anhydride. The photoinitiator contains benzophenone, benzoin ether, acetopheonone, and Lewis acid and base. The photosensitizer contains amine.
The more filler, the drier, and the stress is reduced after UV and heat hardening with better heat resistance and thermal cycling. However, excessive filler may reduce the resolution. Hence, the proportion and type of the filler is very important in the formula. Filler is usually comprised of SiO2+BaSO4+TiSO4…
The selection of the thermal hardener, pigment and filler is similar to traditional solder mask. As for binding agent and photoinitiator, the following must be taken into account for the selection and design of the photosensitive monomer:
a.High photosensitivity (surface hardening)
b.Solvable in weak base solution (1% Na2CO3) prior to exposure
c.Resistant to the erosion of the weak base solution after exposure (development resistance)
d.Resistant to strong base solution (10% NaOH) after thermal hardening
The thixotropy of the solder mask is another important factor. The thixotropic agent, also named anti-sagging agent, can flow more easily in the movement or swinging of the solder mask, persists in its shear resistance when put aside and does not move easily. This makes solder mask roll easily during printing and keeps it fixed and immovable after printing to avoid the sagging of solder mask from the board or accumulation of ink when it is put aside. In practice, it is not really good with higher or lower thixotropy. It should be appropriate for the application. During screening printing, the solder mask rolls and squeezes under the push power of the scraper. The viscosity of the solder mask is reduced in favor of penetration. When the solder mask is printed to the PCB substrate, the slow restoration of the viscosity facilitates the slow flow of the solder mask. When the solder mask reaches a state of balance , the edge of the printed pattern attains satisfactory straightness.
The color of the ink is not directly related to its functionality. However, in consideration of the appearance and the preference of the operator, control of color becomes an important indicator. The purpose of the color is for easy visual or optical inspection. Blue and green are the most commonly used colors. Both are good complementary colors in contrast to the dark red or pink of the copper surface. There are organic and inorganic pigments. The former is non-mineral material with bright colors covering the full spectrum. However, the masking capability of organic pigment is not good. Inorganic pigment is mineral material with good masking capability and strong light and aging resistance. This is the most commonly used pigment in PCB solder mask. The pigment is adjusted upon request of the customer. The major components of the pigment are the same except for the proportion. Hence, the solder mask of different brands has slight differences in hues.
2-3-5 Leveling agent, etc., and other aids
The aids added to the solder mask are used to improve the physical properties of the solder mask, make it more suitable for printing, and improve the printing effect. Aids: Defoaming agent, dispersant, thinner, homogenizing agent, retention agent, color separation inhibiting agent, settlement inhibiting agent, plasticizer, coupling agent, UV absorber, activating agent, thickener, etc.
The major function of the organic solvent is to dissolve resin, pigment and aid, adjust the viscosity of the solder mask, and control its drying speed to improve the dissolution, penetration and adhesion to the printed material. Using thinner randomly may lead to gelation (agglomeration), stripping and uneven color of the solder mask and reduction of its chemical resistance, thermal shock resistance, and adhesion. Using “anti-white water” as a versatile solvent and pouring it in solder mask randomly is incorrect.
2-4. Hole plugging ink and coating ink
2-4-1. Coating ink
Main agent: hardener mix ratio 85: 15, 86:14, 75:25, 80:20
Solder mask viscosity: 190 ±30 PS.
Solid content: 70–75 %.
2-4-2 Hole plugging ink
Main agent: hardener mix ratio 70:30, 75:25.
Solder mask viscosity: 220 ± 30 PS —400 PS
Solid content: 80–90 %.
The above table shows that the viscosity, solid content and hardener mix proportion of the hole plugging ink are higher than the coating ink. To ensure the operability and coating capability of the ink, the manufacturer may adjust the hardener mix ratio, viscosity, rheological property, and so on of the ink according to its application.