The solder mask process in the printed board is to cover the printed board with a solder mask after screen printing with a photographic plate to cover the pads on the printed board, so that it will not be exposed to ultraviolet rays during exposure, and the solder mask will not be exposed. The protective layer is more firmly attached to the surface of the printed board after UV irradiation, and the pads are not exposed to UV light, which exposes the copper pads for lead and tin during the hot air leveling process.
The solar resist welding process can be roughly divided into three operating procedures:
The first procedure is exposure. First, check that the Mylar and glass frame of the exposure frame are clean before starting exposure. If it is not clean, wipe it with an anti-static cloth in time. Then, turn on the power switch of the exposure machine, then turn on the vacuum button to select the exposure program and shake the exposure shutter. Before starting the formal exposure, the exposure machine should be "empty exposure" five times. The function of "empty exposure" is to make the machine enter a saturated working state, and the most important thing is to make the energy of the UV exposure lamp enter the normal range. If it is not "empty exposure", the energy of the exposure lamp may not be in the best working state. Exposure can cause problems with printed boards. After five "empty exposures", the exposure machine has entered the best working state. Before using the photographic plate for alignment, check whether the quality of the plate is qualified. Check whether the film surface on the substrate has pinholes and exposed parts, and whether it is consistent with the pattern of the printed board, because checking the photographic plate in this way can avoid the rework or scrap of the printed board for some unnecessary reasons.
The second process is development. The developing operation is generally carried out in the developing machine, and the developing parameters such as the temperature of the developing solution, the conveying speed, and the spray pressure are well controlled to obtain a better developing effect. Development is to remove the solder resist layer on the pad with the developer in the light-shielding part. The developer is 1% anhydrous sodium carbonate, and the liquid temperature is usually between 30-35 degrees Celsius. Before formal development, the temperature of the developing machine should be raised to make the solution reach the predetermined temperature to achieve the best development effect.
The developing machine is divided into three parts:
The first stage is the spraying section, which is mainly to dissolve the unexposed solder resist by high-pressure spraying with anhydrous sodium carbonate;
The second stage is the washing stage. First, use a high-pressure pump to rinse the remaining solution with water, then enter the circulating water to rinse, and rinse thoroughly;
The third stage is the drying section. There are air knives before and after the drying section, which mainly use hot air to dry the plates. If the temperature in the drying section is high, the board can be dried. The correct development time is determined by the development point, which must be maintained at a constant percentage of the total length of the development segment. If the development point is too close to the outlet of the development section, the unexposed solder mask will not be fully developed. It causes residues of unexposed solder mask to remain on the board. If the display point is too close to the entrance of the developed part, the exposed solder mask may be corroded by prolonged contact with the developer. It becomes hairy and loses its luster.
Usually the display point is controlled within 40%-60% of the total length of the developing section. Another thing to note is that it is easy to scratch the board during development. The usual solution is to put the board on the operator during development. Gloves, the board should be handled with care, the size of the printed board is different, try to put the same size together. When placing the board, keep a certain distance between the board and the board to prevent the board from being crowded during transmission, causing "paper jams" and other phenomena. After the film is projected, place the printed board on the wooden stand.
The third is to repair the veneer. Repairing the circuit board includes two aspects, one is to repair the defects of the image, and the other is to remove the defects that are not related to the desired image. Wear gauze gloves during maintenance to prevent hand sweat from contaminating the board. Common circuit board defects are:
1. Tone skipping is also known as flying white. The main reason is that the electroplating current is too large and the plating layer is too thick, resulting in too high graphic lines. In the screen printing of the printed board, the lines on both sides of the line are too high due to the angle between the squeegee and the screen printing frame. No ink: Skip printing. Another reason is that there is a gap in the scraper, and the ink does not fill the gap, resulting in missing printing. The main solution is to control the plating current and check the scraper for gaps.
2. Oxidation. There is blackening of the gns on the copper foil line under the solder mask of the western printed board. The reason is that the water is not dried after wiping the board, and the surface of the printed board is splashed with liquid or hand-shaped before printing the solder mask. The method is to visually check whether the copper foil on both sides of the printed board is oxidized during screen printing.
3. The surface is uneven, the paper is not printed in time during the screen printing process, and the residual ink on the screen is removed, resulting in uneven surface. The solution is to print the paper in time to remove the residual ink on the screen.
4. In-hole solder resist. The reason is that the paper is not printed in time during the screen printing process, resulting in too much ink accumulation on the screen. Residual ink is printed into the holes under the pressure of the squeegee. The solution is to print the paper in time, and the mesh number of the screen is too small. , will also cause solder resist in the hole. It is necessary to select a high-mesh screen to make a board. The viscosity of the printing material is too low. Change to a higher viscosity printing material. The blade edge becomes round and the scraper edge becomes sharp.
5. The graphics have pinholes. The reason is that there is dirt on the photographic plate, so that the part of the printed board that should be exposed during the exposure process cannot be seen, resulting in pinholes in the pattern. The solution is to frequently check the cleanliness of the photographic plate during exposure.
6. There is dirt on the surface. Since the screen printing room of the printed board is a clean room, there should be electrostatic lines at the screen printing air outlet to absorb debris such as flies in the air. Therefore, in order to reduce the surface dirt, it is necessary to fully ensure the cleanliness of the clean room, and appropriately implement some specific measures: such as entering the clean room, fully ensure the cleanliness of the operators, avoid unrelated personnel from passing through the clean room, and clean the clean room regularly.
7. The colors on both sides are inconsistent. The reason may be that the number of knives printed on both sides is very different, and there are also mixed use of new and old inks. It is possible to use fresh, stirred ink on one side and old ink that has been sitting for a long time on the other. , the solution is to try to avoid the above two situations.
8. Cracking. Due to insufficient exposure during the exposure process, small cracks appear on the board. The solution is to measure the exposure, so that the comprehensive value of parameters such as exposure lamp energy and exposure time reaches the level of 9-11 exposure, and there will be no cracks within this range.
9. Bubbles. After developing, air bubbles will be generated between the lines of the printed board or on the side of a single line. Main reasons: Bubbles between two or more lines are mainly due to narrow line spacing and line height. During the screen printing process, the solder resist cannot be printed on the substrate, resulting in the presence of air or moisture between the solder resist and the substrate. During curing and exposure, the gas expands thermally and causes a single wire to cause the wire to be too high. When the squeegee is in contact with the line, the line is too high, and the angle between the squeegee and the line increases, so that the solder resist cannot be printed to the root of the line. There is gas between the side of the wire root and the solder mask, and bubbles will be generated after heating. The solution is as follows: when screen printing, it should be visually checked whether the screen printing material is completely printed on the substrate and the sidewall of the circuit, and the current during electroplating should be strictly controlled.
10. Ghosting: There are regular ink dots next to the pads on the entire printed circuit board. The reason is that the positioning of the printed board is not firm during the screen printing, and the residual ink on the screen is not removed in time and accumulated on the printed board. The solution is to fix the paper firmly with the positioning pins, and remove the residual ink on the screen in time.
During the modification process, due to the serious defects of some printed boards that could not be repaired, the original solder resist was dissolved with sodium hydroxide aqueous solution, and then reworked after screen printing and exposure. If the defects of the printed board are small, such as a small copper dew point, it can be carefully repaired by moistening with a fine brush and adjusting the solder mask.
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