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Detailed explanation of solar resistance welding process in PCB circuit board production process
The solar resistance soldering process in printed board is a printed board with solder mask after screen printing. The pad on the printed board is covered with the photographic plate so that it is not exposed to ultraviolet rays during exposure, and the solder resist is more strongly adhered to the surface of the printed board after ultraviolet irradiation, and the pad is not exposed to ultraviolet rays. The light can expose the copper pad so that the lead and tin can be applied during the leveling of the hot air.

Solar resistance welding processes can be roughly divided into three operating procedures:

The first procedure is exposure. First of all, before starting exposure, check whether the polyester film and the glass frame of the exposure frame are clean. If it is not clean, wipe it clean with an antistatic cloth immediately. Then, turn on the power switch of the exposure machine, and then open the vacuum button to select the exposure program and shake it. Exposure shutter, before the official exposure, should first let the exposure machine "air exposure" five times, "empty exposure" role is to make the machine can enter the working state of saturation, the most important is to make the UV exposure lamp energy into the normal range . If you do not "exposure" the exposure lamp's energy may not enter the optimal working condition. Exposing the board will cause problems. After five times of “air exposure”, the exposure machine has entered the best working condition. Before using the photo-bottom plate for alignment, it is necessary to check whether the quality of the bottom plate is qualified. Check if there is any pinhole and exposed part of the film on the bottom plate, and it is consistent with the pattern of the printed board, because this will check the photo plate to avoid the PCB rework or scrap for some unnecessary reasons.

The solar resistance soldering is usually visually positioned. Using a silver salt master plate, the pad of the master plate is aligned with the pad hole of the printed board, and the exposure can be performed by fixing with a tape. The solar resistance welding that will be encountered in the alignment is usually visually positioned. Using a silver salt master, the pad of the master is aligned with the pad hole of the printed board and can be exposed by tape. There are many problems encountered in the alignment, for example, because the bottom plate is related to factors such as temperature, humidity, etc. If the temperature and humidity are not controlled properly, the photographic plate may shrink or magnify the deformation so that when the solar radiation is soldered, the photographic film The bottom plate does not exactly match the PCB pad. When the base is shrinking, see how much the difference between the bottom pad and the PCB pad is. If the difference is small, lead and tin can be applied during hot air leveling. Then, there is no big problem for selenium soldering. If there is a large difference, only the reprint will be repeated and try to make the pads overlap. Before the alignment, it should also pay attention to whether the bottom of the film surface is reversed, should ensure that the film surface in the alignment when the downward, if facing up, so that the film surface was scratched, resulting in the bottom exposed, so that drying out Printed boards do not need to have solder resist at the exposure, which will cause the printed boards to be scrapped. In addition, it is also important to note that sometimes the imposition of the bottom plate will not coincide with the printed plate pattern, usually the imposition plate is cut along the edge of the plate, and then single-slot alignment, the entire printed board after exposure. The above problems are problems that should be taken care of before the official exposure of selenium solder mask.

Then, perform a solar resistance soldering and check that the printed board is sucked by the vacuum box before exposure. The vacuum suction pressure should be sufficient without any exposed gas. If the dew will make the UV light shine into the pattern along the side of the board, resulting in exposure of the shade, the development can not afford, and sometimes encountered in the case of single-sided exposure, in this case, the side with no graphics on a single side with a black cloth and The UV light emitted by the exposure lamp is separated. If no black cloth is used, the UV light is transmitted through the no-pattern side to the pad so that the solder resist in the hole of the pad is exposed and cannot be developed. When exposing printed boards with inconsistent patterns on both sides, first screen-printing a solder mask, and then performing single-sided exposure, after the development, the other side of the screen printing solder mask, because if both sides screen printing exposure, there is a complex pattern of pad There are many parts that need to be shaded, and there are few parts that need to be shaded on the other side, so that the ultraviolet light passes through one side and shines on the other side. The side with more shading is irradiated with ultraviolet light, and it does not appear to be distorted during development, which may cause rework or scrapped. In the process of exposure, there is also a case where the printed board after the screen printing is not dried during curing. In this case, the solder resist is stuck on the photofinished board when the positioning is performed, and the printed board is also To rework, therefore, found that quit, especially if most of the printed boards are not dried, they must be placed in an oven and dried again. These situations are problems that are easily exposed during the process of exposure. Therefore, they must be carefully checked, discovered in time, and resolved in a timely manner.

The second step is development. The developing operation is generally performed 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, and a better developing effect can be obtained. Development is the use of a developing solution to remove the solder resist on the pad. The developing solution is one-hundredth of anhydrous sodium carbonate, and the liquid temperature is usually between thirty and thirty-five degrees Celsius. Before the official development, the temperature of the developing machine is increased so that the solution reaches a predetermined temperature so as to achieve the best development effect. The development machine is divided into three parts: the first section is the spray section, which is mainly used to spray anhydrous sodium carbonate with high pressure to dissolve the un-exposed solder resist;

The second section is the water washing section. Firstly, the high pressure pump is used to wash the water. The remaining solution is washed first, then into the circulating water and thoroughly washed; the third section is the blow-drying section, and there is one air knife before and after the drying section. The hot air blows the board dry, and the temperature of the blow-drying section can also dry the board. The correct development time is determined by the development point. The development point must be kept at a constant percentage of the total length of the development section. If the development point is too close to the exit of the development section, the unexposed solder mask layer cannot be sufficiently developed. The residue of the unexposed solder mask layer may remain on the surface of the plate. If the spot is too close to the entrance of the developing section, the exposed solder resist layer may be etched due to the contact with the developing solution when it is too long. Become hairy and lose luster. Usually shows that the point of control in the total length of the development section within 40% -60%, in addition, pay attention to the development of the board is very easy to scratch, the usual solution is to develop the board, put the board operators to wear Gloves, on the board to be light and put, there are different sizes of the printed board, so try to put together as much as possible size, when placing the board, the board and the board to maintain a certain distance between to prevent transmission At that time, the board was crowded, causing "card board" and other phenomena. After finishing the photo, place the printed board on a wooden tray.

The following is the third operating procedure for solar resistance soldering:

Repairing the board includes two aspects: one is to repair the defect of the image, and the other is to remove the defects that are not related to the requested image. During the board repair process, the glove should be worn to prevent the hand sweat from contaminating the board surface. Common board defects include:
1. Skip marks are also called flying white. Mainly due to the plating current is too large, thick plating, resulting in high graphics lines, when the screen printing plate, due to the scraper knife and the screen printing frame at a certain angle screen printing, so both sides of the line due to the line is too high, Do not ink: cause skipping, another reason is that the scraper knife has a gap, no ink at the gap, resulting in skipping, the main solution is to control the plating current and check whether the scraper knife has a gap.

 2. Oxidation. There are signs of blackening on the copper foil lines under the solder mask of the printed board. The causes are: the water is not dried after the board is wiped, and the surface of the printed board before the printed solder mask is splashed or molded by hand. The method is to check whether the copper foil on both sides of the printed board is oxidized during screen printing.

3. Surface uneven, did not pay attention to timely printing paper screen, remove the residual ink of the screen, resulting in uneven surface, the solution is to promptly print the paper to remove the residual ink screen.
4. Hole solder resist. The reason is caused by the lack of timely printing of paper during screen printing, resulting in excessive residual ink on the screen, and printing of residual ink into the hole under the pressure of the doctor blade. The solution is to print the paper in time, and the mesh number is too low. , Can also cause holes in the solder mask, to use high mesh mesh screen printing, printing material viscosity is too low, for the use of high viscosity printing material, scraper screen printing angle size, appropriate adjustment of the scraper screen printing angle, scraper The knife edge is rounded and the sharpening blade is sharpened.

5. There are pinholes in the graphics. The reason is that there are dirt on the photographic plate, so that the part of the printed board that should be exposed to light during the exposure process is not exposed to light, resulting in pinholes in the pattern. The solution is to check the cleanliness of the photographic plate during the exposure process.
6. Dirt on the surface. Because the printing plate screen printing room belongs to the clean room, therefore, there should be an electrostatic wire at the screen printing outlet to play the role of adsorption of flying hair and other debris in the air. Therefore, in order to reduce the surface dirt, it is necessary to Fully ensure the cleanliness of the cleanroom and properly implement some specific measures: If you enter the clean room, you must fully ensure the cleanliness of the operator, avoid unrelated personnel from walking through the clean room, and regularly clean the clean room.
7. The two colors are inconsistent. The reasons for this may be the large difference in the number of knives on the two sides of the screen, and the mixed use of new ink and old ink. One side may use new ink that is stirred, and the other side uses old ink that has been stored for a long time. The solution is to avoid the appearance of the above two situations.

8. Cracked. Due to the lack of exposure during the exposure process, resulting in small cracks on the board surface, the solution is to measure the amount of exposure so that the integrated value of the exposure lamp energy, exposure time and other parameters reach the 9-11 level between the exposure series, in this range There will be no cracks.
9. bubble. Between the lines of the printed board or the sides of a single line, air bubbles are generated after development. Main reason: Bubbles between two or more lines are mainly due to narrow line spacing and high lines. When screen printing, solder resist can not be printed on the substrate, resulting in the presence of air or moisture between the solder resist and the substrate. During the curing and exposure, the gas expands due to heat, causing the single line to be mainly caused by high lines. 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 on the line root. There is a gas between the side of the root of the line and the solder mask, and bubbles are generated after the heat is applied. The solution is: when the screen is printed, check whether the screen material is completely printed on the substrate and the sidewalls of the line, and strictly control the current during plating.

10. Ghosting: There are regular ink dots along the entire printed circuit board next to the pad. The reason is that the positioning of the printed board is not secure during screen printing and the residual ink on the screen is not removed in time to the printed board. The method is to use a positioning pin fixed and timely printing paper to remove the residual ink on the screen.

In the process of revision, because some of the defects of the printed board are very serious and cannot be repaired, the original solder resist is dissolved by heating with an aqueous solution of sodium hydroxide, and then re-screened and then exposed for rework, if the printed board The defects are small, such as a small copper dew point, which can be carefully repaired with a fine brush pen with good solder resist.

The above is the entire process of the printed circuit board c. Although this process is a relatively simple procedure in each process of the printed circuit board, it also plays an important role. The solar resistance soldering process controls the printed circuit board. In the appearance and holes, efforts are made to “beautiful” for the “pretty coat” of the printed circuit board, so that the printed board looks more comfortable and plays a protective role, controlling the quality of the holes and making the holes in the printed board. No solder resist will appear, and the quality of the printed board will be guaranteed. Therefore, it is said that the solar resistance soldering process of the printed board is a very important process.