Process Control of Liquid Photothermographic Solder Mask Ink

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 screen printing) of liquid photo-imageable inks (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.

First, the technical characteristics of DSR-2200NC photosensitive resist ink

1. Process of solder mask processing

Liquid photo-sensitive solder mask must be strengthened in each production process 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.

Second, the 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.

Third, the operating conditions

The general environmental requirements for control of screen printing operations are as follows:
(1) Temperature: 18°C~25°C;
(2) Relative humidity: 50%~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.

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 with a wavelength of 310 nm to 440 nm in the spectral absorption region. 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.

IV. Process Control of Solder Resist Ink

1, substrate processing

The purpose of the substrate processing is to remove the oxide layer of the printed circuit copper foil surface, roughen the copper surface, and clean the surface of the substrate to enhance the bonding force between the solder mask and the substrate.

Substrate processing generally includes steps such as acid treatment, water wash, brush plate, high pressure water rinse, and drying. Acid treatment generally uses 2% to 3% of dilute sulfuric acid solution to soak, if the copper surface oxidation is too serious, then extend the soaking time, but the acid concentration is generally not too high. After pickling, the substrate must be washed thoroughly to ensure that there is no residual acid on the substrate. The washed substrate should be immediately brushed and dried to prevent the copper surface from reoxidizing. Brush plate, high-pressure water flushing and drying process, it is best to complete in the grinding cleaning line (or pumice stone brush line). After grinding and cleaning, the substrate must also be placed in a hot air circulating oven at 80°C for 10 minutes to 15 minutes to completely remove water vapor on the sides of the substrate and printed lines. This process is especially important for substrates with thick plating lines. After the dried substrate is cooled to room temperature, it can be transferred to screen printing (or curtain coating) solder resist.

2, solder mask processing

3, ink preparation

10min before use, should be DSR  2200NC ink main agent and hardener according to the ratio of mass ratio of 3:1, according to the amount of preparation. Before mixing the two components, they should be fully stirred so that the components are evenly distributed, then mixed according to the ratio, fully stirred, allowed to stand for 10 minutes, and stirred again before use. All tools that touch the ink should be clean and free of dirt, dander, etc.

4, screen production

Screen silk screen used by the screen, generally choose a smooth surface, printing material permeability, elasticity, tensile strength, acid, alkali resistant, organic solvent-resistant nylon mesh.

First, according to the required thickness of the solder mask, select the appropriate number of screen meshes. The thickness of the solder mask is generally 15μm~35μm, so the 100 mesh screen is usually used. After blanking, washing nets, sealing nets, baking nets and other processes, make a blank screen. You can also use the printing method to create a screen dot screen.

5, scraper selection

The squeegee material used for screen printing and soldering is polyurethane rubber, and its hardness and length parameters should generally be considered. Scraper hardness is generally selected in the Shore 60 ° ~ 75 °, most choose Shore hardness 70 °. The length of the scraper should be determined according to the required size of the empty screen. It must be ensured that the length of the scraped sheet is 20mm larger than the empty screen, so that the phenomenon that the edge is thicker than the middle when the screen is printed does not occur.

6, screen printing solder resist

Screen printing solder resist can be done by manual, automatic, semi-automatic screen printing machine. Web version can be empty screen version, but also can use dot screen version. The use of empty screen printing method, the positioning requirements are not very accurate, as long as the integrity of the printed circuit board can be ensured, to solve the problem of ink into the hole, can be controlled by adjusting the angle of the scraper, in addition to the method of reversing screen printing . However, the reversing screen printing may sometimes leave traces of holes on the surface of the previous substrate when the screen is printed, thereby affecting the appearance of the printed circuit board coating. Therefore, it can only be used when the appearance requirement is not very strict.

The way to completely solve the problem of the ink entering the hole is to use a solder resist film to make a screen of the ink repellent dot, and then strictly position the printing during the screen printing. One type of ink repellent dot screen can only be used for screen printing of one kind of printed circuit board, which not only increases the cost of screen printing, but also makes the screen printing plate extend the processing cycle. Therefore, military products printed boards that are not strictly required for small batches, varieties, and appearances are usually not screened with silkscreening solder resists, but are often screen printed with blank screens.

Not only does the thickness of the solder resist are basically the same when printing on the wire screen, but the side coating of the printed wire should also be taken into account. Absolutely no "fly-printing" or "flying white" phenomenon is allowed, and the coating on both shoulders of the printed wire is required to have an appropriate thickness. Screen printing is usually performed by 1 person, 1 panel, and the screen printing quality is checked at any time. If “skip printing”, “flying white” and uneven coating are found, reprint should be promptly remedied.

7, pre-baking

After screen printing, the substrate should be allowed to stand still for 10 minutes (in the case of conditions not available, it should be placed on an upright stand). After the bubbles have completely disappeared, they are placed in a circulating hot air oven for pre-drying. The pre-baking process parameters of the DSR-2200NS solder resist ink are: pre-baking on the A surface for 15 minutes at 80°C, and prebaking on the B surface for 20 minutes. In actual operation, suitable prebaking parameters can be set according to the thickness of solder mask printed. Especially in the case of manual screen printing, the thickness of the solder mask is usually more than the standard value. If the pre-baking is not enough, the negative and the solder mask will stick together during exposure, resulting in an unsightly “cloudy” appearance on the surface of the solder mask. Printed board appearance quality. Pre-baking on the A side is to prevent the solder mask from sticking to the surface when printed on the B side. As long as this is achieved, the shorter the pre-baking time, the better. This way, the processed solder mask will not have much difference. Pre-baking at 80°C for 15 minutes usually meets the requirements. After the B side is printed, it can be pre-baked. According to the experiment, the quality of the next step of the development can be observed, and then the appropriate pre-bake time can be set. Under normal circumstances, pre-baking at 80 °C, B surface up to 35min, A surface pre-baking the total time of 50min, has not exceeded the DSR  2200NC solder mask ink pre-baking the maximum limit of 60min, will not reduce the ink Resolution.

8, exposure

The pre-baked substrate can be exposed by standing to cool to room temperature. The exposure should be completed within 24 hours after pre-baking, otherwise it will cause difficulty in development. The exposure is classified into contact exposure type and non-contact exposure type. At present, most of the exposure exposure type on the market represents the development direction of solder mask processing technology.

The exposure was performed with a silver salt backsheet or a diazo sheet as a mask on a vacuum exposure machine with a cooling system. Exposure machine power can not be too small, generally 5kW ~ 8kW. Exposure light source can be used to emit light with a wavelength of 310nm ~ 420nm halogen lamp to facilitate the absorption of photosensitive ink. DSR-2200NC solder resist ink exposure energy requirements in the 400mj/cm2 ~ 600mj/cm2, if the film is thick, the exposure time should be appropriately extended to make full exposure.

Exposure is a critical step in solder mask processing and largely determines the quality of solder mask processing. The most direct reflection of the lack of exposure energy is that during development, the solder mask is eroded and the surface loses its luster. In severe cases, the surface of the solder mask even appears as a "crack crack," resulting in scrap. The indirect reflection is that the physical properties of the solder mask, such as solvent resistance, acid and alkali resistance, and thermal shock resistance, are degraded, and the service life cannot be guaranteed. In addition, the exposure clip during exposure