1 Introduction
A solder mask is a permanent protective layer applied to the surface of the printed board. It selectively protects the surface of the printed board and prevents short circuits and bridging between the wires and the pads when the components are soldered. In addition, it is also moisture-proof, mold-proof, and salt-fog-proof. Therefore, the quality of the solder mask not only affects the appearance of the printed board, but also affects the service life of the printed board.
With the development of printed circuit boards, curtain coating (or silk screen printing) Liquid Photo-imageable Ink (abbreviated as LPI) has become the dominant method for solder mask processing of printed boards. Liquid photo-resistance solder mask ink, mainly composed of photo-curing and heat-curing resin, is composed of photosensitizer, heat-curing agent, filler, pigment and various additives. It is coated on the etched substrate by curtain coating or silk screen printing, followed by a series of processes such as pre-baking, exposure, development and post-cure, etc. Finally, a cross-linked and cured polymer coating is formed on the surface of the printed board. .
After the solder mask is processed, it must meet the requirements of strong adhesion, high hardness, solvent resistance, acid and alkali resistance, and heat-resistant oil. Due to the complexity of the process, it is important not only to select solder mask ink materials, but also to strictly control the process during production to ensure the quality of the solder mask. Following the actual production, talk about the Japanese DSR 2200NC solder mask ink process characteristics and process control requirements.
2 Process Characteristics of DSR-2200NC Photoresist Solder Ink
2.1 Solder Mask Processing Process
Liquid photo-sensitive solder mask processing process shown in Figure 1. In each process, process control must be strengthened to ensure the quality of solder mask processing. There are many other factors that must be taken into account.
2.2 Process Characteristics of DSR-2200NC Ink
The ink is a two-component liquid photosensitive heat-curing solder mask ink. Before use, the main agent and hardener should be prepared at a mass ratio of 3:1. Stir thoroughly and let stand for 10 minutes until the bubbles disappear. After the film is processed, the color of the solder resist film is green, and the brightness is between the high-light type and the matt type, and the physical properties are good.
2.2.1 Service life
The main agent and curing agent of DSR-2200NC liquid photosensitive ink should be stored in the dark at temperatures below 25°C. The storage period is 6 months. Use within the storage period, get good results. Judging from the actual application situation, if the process is strictly controlled, it will not affect the quality of the product. (However, for military printed boards with strict requirements on product quality, expired ink must not be used.) The mixed ink of the two groups has a service life of only 24 hours. Since there is a chemical reaction between the two components of the mixed ink, the ink cannot be used for more than 24 hours even if the storage conditions are strictly controlled. Therefore, in order to avoid waste, it is required to use the present 10 minutes before use according to the amount of usage.
2.2.2 Operating Conditions
The general environmental requirements for control of screen printing operations are as follows:
(1) Temperature: 18°C ​​to 25°C;
(2) Relative humidity: 50% to 60%;
(3) purification level: 10000 or more;
(4) Security light: yellow light.
The ambient temperature mainly affects the viscosity of the ink. Low temperature, large ink viscosity, difficult screen printing operation, increased solder mask thickness, high temperature, low ink viscosity, high fluidity, and silk screen solder mask too thin to meet requirements. The relationship between viscosity and temperature of DSR-2200NC solder mask ink is shown in Figure 2.
If the pre-drying of the substrate before silk screen printing is insufficient or is naturally cooled after drying, the relative humidity of the surrounding environment will be relatively large, which will cause the substrate to absorb moisture again, which will have an adverse effect on the binding force and the copper surface condition of the screen printing coating. Causes the oxidation of the copper foil under the solder mask, resulting in dark spots, affecting the appearance of the printed board; or, when subjected to thermal shock, the solder mask blisters and falls off. Therefore, the humidity of the operating room environment should not be too high, generally should be controlled at 50% to 60% relative humidity. If it cannot be guaranteed due to limited conditions, strict pre-drying before screen printing should be controlled. Especially when there are a large number of substrates, they should be batch-baked and silk screened in batches. In addition, when printing on the B side, it is also necessary to control the cooling time to prevent the substrate surface from absorbing moisture again.
Before the solder resist ink is cured without exposure, the dust, paper dust, cotton fiber, and hair present in the surrounding environment are easily adsorbed on the ink, and then applied to the surface of the substrate to become unnecessary on the solder resist film. Not only affects the appearance of the product, but also part of the conductive dust will also affect the electrical properties of the printed circuit board, resulting in substrate scrap. Therefore, the degree of cleanliness in the operation room should reach a certain level. In actual operation, if the conditions are not met, it is very important to do a good job of sanitation. It is required that at least ground and equipment surfaces should be kept clean and no dust should be brought into the operation room.
DSR-2200NC solder resist ink is a photosensitive material, and its spectral absorption wavelength is from 310nm to 440nm. Therefore, the operation must be performed within the safe spectral wavelength range, and it is generally operated in a yellow light region with a wavelength of ≥460 nm.
(to be continued)