3 Process Control of Solder Mask Ink
3.1 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 from the substrate surface and the side of the printed wire. 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.
3.2 Solder Mask Processing
3.2.1 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.
3.2.2 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 to 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.
3.2.3 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.
3.2.4 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 printingâ€, “flying white†and uneven coating are found, reprint should be promptly remedied.
3.2.5 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.
(to be continued)