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PCB board dry area problem solving chapter two

  • Author:o-leading.
  • Source:o-leading.com
  • Release on :2019-03-06


1. Why should the pre-baked board be pre-baked? And is it a low temperature (75 ± 5 ° C) baking?







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Answer: Pre-bake before the positional exposure: It is to volatilize the solvent in the ink more fully, and also to prevent the surface of the ink from sticking to the film when it is in the position, which causes the film to lose oil and the film pollution. Drop the point. The solder resist liquid photosensitive ink is easily cured by baking at a temperature of >85 ° C. At the same time, due to its viscosity fluidity during processing and printing, some of the pads and SMT are higher than the part of the substrate and some of the ink in the component holes. It will be thinner. If it is baked at a temperature higher than 80 °C, it will be completely cured. In addition, high-energy ultraviolet light will be irradiated during the alignment exposure, resulting in complete crosslinking of the ink in this part. It is not easily dissolved by the sodium carbonate solution, resulting in poor development.

2. How to improve the film of the film on the surface after the resistive welding?

Answer: 1. Appropriately increase the pre-bake time of the board before the alignment (can not increase the temperature), 2. Reduce the vacuum of the exposure machine (10-15%), 3. Reduce the exposure energy, 4. Control The quality and service life of the film surface. 5, the frequency of control friction should not be too much.

3. What is the network distance?

A: From the principle of printing, when printing in the close state, the screen will not stretch, and the dimensional accuracy of printing can be obtained. However, in fact, the oil film is prone to bleeding, and the printing cannot be carried out very cleanly. The most basic requirement is that the screen has a certain gap with the surface of the board to be printed. This is called the screen distance. (General grid distance is controlled at 3-5mm).

4. What is the net?

A: The purpose is to indicate the value of the density of the mesh, which is expressed by the number of meshes of 1 square inch. Now, the number of holes is 1cm2, and the Western European countries such as West Germany, Switzerland and Italy are in the calculation network. The number of frames is in centimeters, while in Japan it is based on inches. Usually the "mesh" is also called "T".

5. Why do lines have to be indented? And must be ≥ 4mm?

Answer: The indentation of the line is to check the parallel uniformity of the upper and lower pressures when the film is pressed, and adjust the pressure according to the actual conditions of the indentation test to obtain the best lamination ability. The indentation test must be ≥ 4mm, because the area of ​​contact between the two pressing squeezing and the pressing squeezing in the normal film is just 4mm, and if it is <4mm, the two squeezing and the slab The area of ​​the dry film contact is small, and the force applied to the dry film is reduced and ineffective, which may result in poor lamination, and the bonding force between the dry film and the plate surface is not good.










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6. What is the maximum and minimum optical density of the film?

Answer: Regarding the optical density, the maximum optical density Dmin (minimum) is required to be >4.0, and the minimum optical density is Dmax<0.17. The maximum optical density refers to the lower limit of the light barrier exhibited by the surface light-blocking film in the ultraviolet light, when the bottom plate is opaque. When the light-blocking density Dmin of the zone is equal to 4.0, the light transmittance is 0.03%, so Dmin exceeds 4.0 to achieve a good light-blocking purpose. The minimum optical density refers to the upper limit of the light barrier exhibited by the transparent sheet outside the light blocking film in the ultraviolet light. When the light blocking density Dmax=0.17 of the opaque area of ​​the bottom plate, the light transmittance is about 70%, so Dmax (maximum ) After less than 0.17, good light transmission can be achieved.

7. What causes the line to cause plating during plating?

Answer: 1. The dry film performance is poor, and it is used beyond the effective period; 2. The surface of the substrate is not clean or the rough surface is poor, and the dry film is poorly adhered; 3. The film temperature is low, the transfer speed is fast, and the dry film is not firmly attached; If the exposure energy is too high, the corrosion resistance is brittle; 5. The exposure energy is insufficient, the development speed is too slow, and the edge of the resist is raised; 6. The temperature of the treatment liquid before plating is too high.

8. What is the principle of liquid photosensitive ink photodevelopment imaging?

Answer: When the photosensitive ink is exposed to ultraviolet light, the photoinitiator decomposes into a free radical to attack the resin to form a free polymerization, which instantly increases the polymer molecule. At this time, the ink should be insoluble in 1% sodium carbonate, but soluble in strong Alkali 5-10% sodium hydroxide, so as to achieve the purpose of developing and timely saving the problematic board, the printed board pad portion is removed by the film stop point to block the unexposed ink during development, and the exposed portion is developed. Reserved after.

9. What are the causes of dry film development and excess glue?

Answer: 1. The quality of dry film is poor, such as large or high molecular weight, accidental thermal polymerization during the use of dry film; 2, dry film is exposed to white light to cause partial polymerization; 3. exposure time is too long or energy 4; The maximum optical density of the film blocking point is insufficient, causing partial polymerization due to ultraviolet light transmission; 5. The developer temperature is too low or the concentration is too low, the development speed is too fast, and the pressure is too small; A large number of bubbles are generated to reduce the developing ability.







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10. What factors affect the PCB?

A: frame, light source, temperature control system, exposure control system, vacuum system

11. Why do general pretreatment machines have to do the wear scar test?

A: The wear marks of the pretreatment machine are used to detect the balance and uniformity of the brush, and also the wear scar test to obtain the plate current parameters required for different plate thicknesses.

12. Why are the nozzles of the developing unit developing cylinders fanned? Why not a cone?

Answer: Because there is a certain distance between the nozzle of the developing cylinder of the developing machine and the nozzle, it is a fan-shaped area. The area where the sprayed syrup can wet is definitely wider and uniform, and it also compares the spray in the hole. Uniform, if it is tapered, the syrup sprayed under the same process conditions, its uniformity is relatively poor compared to the fan-shaped nozzle.

13. Why should the developing machine add an automatic developing cylinder? Why not use the grinding machine?

Answer: Firstly, pickling (sulphuric acid) in the grinding machine only removes part of the oxide. The most important thing is to add the effect of the brush. When the board is sprayed through the acid tank, the board is taken away from the acid cylinder. The acid solution is only a small part, which does not affect its original concentration (3-5%). The general oxide can be dissolved when it encounters 3-5% acid. The developing solution of the developing machine is (sodium carbonate). When it is dissolved in water, it is not a 100% completely aqueous solution. It still forms some tiny particles. When it is developing, part of the sodium carbonate solution is carried out of the developing cylinder by the board during the process of dissolving the ink.

14. What is the difference between the developing nozzle of the developing machine and the automatic swinging?

A: It will automatically swing, then it will be more uniform on the surface of the board, especially the fine lines, with the swing can better solve the problem of poor development and residual film, and at the same time strengthen the dissolution of the remaining film in the hole, Improve the developing ability.

15. After the solder resist printing, there are bubbles at the edge of the line and bubbles on the substrate surface. Are the reasons for the same? why?

Answer: The causes of air bubbles at the edge of the line are: 1 the line conductor is too high or the side erosion is relatively large, 2 the board is too short before the pre-baking, the ink is too high, or the ink in the ink is too much. 4 The ink printing layer is too thick, 5 inks are unevenly distributed or the prepared ink is not static enough.









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The reasons for the bubble on the surface of the substrate are as follows: 1 the ink is unevenly distributed or the prepared ink is not allowed to stand for a long time, 2 the surface of the board has moisture or dirt, 3 the viscosity of the ink is too high or the printed layer is too thick. 4 oven oven temperature is not uniform.

16. Why is the drying section of the grinding machine set to be blown dry, hot air blown, and cold air blown, can their order be reversed?

A: It can't be reversed. Because in the opposite direction, the cold wind blows dry in the first stage. It doesn't have much effect. When the strong wind blows in the first stage, the water blown out from the hole is blown out by the strong wind, plus High temperature blowing and baking are easy to volatilize, and there will be no water marks on the surface of the board. If the hot air is blown in the last stage, the surface temperature of the board ground into the darkroom is relatively high (60-80 ° C), while the darkroom The temperature is generally between 18-24 ° C, then the board is prone to oxidation.

17. Why should the resistance screen printing machine have a shift setting?

Answer: 1. It is possible to prevent the ink on the printing plate (the same hole position) from being repeatedly punched by the scraper twice into the hole, and also increase the uniformity of ink coating.

18. Why should the anti-foaming agent be added to the developing tank of the developing machine?

Answer: Because the developing solution in the developing cylinder increases with the use time and the amount of the plate increases, the solution will gradually decrease, and the ink remaining in the in-cylinder development will accumulate more, with repeated spraying of the long-term solution. There will be impurities and foam. If the foam remains on the surface of the board too much, it will be difficult to clean it in the washing stage of the developing machine, which will cause defects in the appearance of the board. Adding the defoaming agent dissolves the foam in the developing cylinder. .

19. What is the effect of the viscosity of carbon oil on the quality of the PCB?

A: The viscosity of carbon oil directly affects the resistance of carbon oil.

20. What are the factors that affect the quality of screen printing?

Answer: 1. Ink properties: viscosity, fineness and fluidity of the ink; 2. Screen state: selection of mesh, use of tension and coating of photoresist; 3. Screen printing conditions: printing pressure, hardness of the blade, Angle and printing speed; 4, human factors: operator's operational skills and quality awareness; 5, environmental factors: indoor temperature, temperature, purification and so on.







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21. What is the dry film speed?

Answer: The dry film speed is the amount of light energy of a polymer that has a certain resisting ability when a photopolymerizable monomer is polymerized under ultraviolet light. In the case where the intensity of the light source and the light distance are fixed, the speed of the light is expressed as the length of the exposure time, and the short exposure time is the speed of the light.

22. What is the dry film exposure time latitude?

Answer: After a period of exposure to the dry film, the developed or photoresist layer has been polymerized in whole or in part. Generally, the formed pattern can be used. This time is called the minimum exposure time. The exposure time is continued to lengthen, the photoresist is polymerized thoroughly, and the size of the pattern obtained after development is still consistent with the size of the film, which is called the maximum exposure time. Usually the optimum exposure time for the dry film is chosen between the minimum and minimum exposure times and the maximum exposure time. The ratio of the maximum exposure time to the minimum exposure time is called the exposure time latitude.

23. What is the effect of the solder mask ink being too thick or the book?

Answer: When the solder resist ink is too thick (higher than the chip or IC position), the post-process processing will cause the solder paste to be poorly soldered or poorly soldered, which will result in poor component mounting or even insufficiency. If the book is opened, the insulation of the line will not be thick enough, and leakage will occur, which will affect the performance of the post-process.

24. What effect does the conventional solder mask ink have on the thick or the book?

Answer: Because the solder resist ink is not only a protective layer, but also an insulating layer. If the voltage is too large or a certain environmental influence, the circuit is prone to leakage, when the distance between the two lines is small. When it reaches a certain level, an arc will be generated between the two lines. If the insulation layer of the line is broken, it will be easily broken down and a short circuit will occur. In the solder resist printing, the corner portion of the line is more difficult to print oil. If the ink in the corner portion is thick enough, the thickness of the ink on other large areas such as the copper surface is thicker.

25. Why should the solder paste plug ink be used instead of adding oil?

Answer: Because the source liquid plug hole has relatively less water, the high viscosity will not cause the plug hole to be full due to the flow, and the liquid will volatilize during the post-cure. The degree of shrinkage is relatively small, and at the same time it does not form a protrusion or a cracking phenomenon due to the rapid evaporation of water in the body due to the high temperature during post-cure. If the oil is added, its viscosity is relatively low, and the ink is low. The fluidity is relatively strong, and it is easy to flow when the hole is plugged, which may cause the plug hole to be insufficient and the plate surface soldering oil to be poor. When the post-cure occurs, the water in the ink liquid will rapidly evaporate, causing the plug hole to be oily or poorly plugged.

26. What is dry film developability and developability?

Answer: Dry film developability refers to the quality of the image obtained after filming, exposure and development of the dry film in the best working condition, that is, the circuit image is clear and the unexposed part should be removed without residue, and left after exposure. The resist layer (dry film) on the board surface should be smooth and solid without edges. Dry film development resistance refers to the degree of overexposure of the exposed dry film, and the development resistance reflects the latitude of the development process.