Prevent the deformation of the film inside the fence. 4. Design the alignment accuracy marks on the four corners of the board.
29. Why should the board be pre-baked before registration exposure? And is it baked at low temperature (75 ± 5 ℃)?
Answer: Pre-baking of the board before the registration exposure: it is to fully evaporate the solvent in the ink, and also to prevent the board ink from sticking to the film without initial curing during the registration, which will cause the board surface to oil and film pollution. Off gear. The solder resist liquid photosensitive ink is easy to be cured when baked at a temperature of> 85 ° C. At the same time, because of its viscosity and fluidity during processing and printing, some pads and SMT are higher than the substrate and some ink in the component holes It will be relatively thin. If it is baked at a temperature higher than 80 ° C, it will be completely cured. In addition, it will be irradiated with high-energy ultraviolet light during registration exposure, which will cause the ink in this part to be completely crosslinked. It will not be easily dissolved by the sodium carbonate solution, resulting in unclean development.
30.How to improve the film negatives that appear on the board after the solder mask registration exposure?
Answer: 1. Prolong the board pre-baking time before alignment (can not increase the temperature). 2. Reduce the vacuum of the exposure machine (10-15%). 3. Reduce the exposure energy. 4. Control. Clean quality and service life of film surface. 5, control the frequency of scrubbing should not be too much.
31. What is the network distance?
Answer: From the principle of printing, when printing in a tight state, the screen is not stretched, and the dimensional accuracy of the printing can be obtained, but in fact, the oil film is prone to bleed and the printing cannot be performed cleanly, so The most basic requirement is that the screen must have a certain gap with the surface of the board to be printed. This is the so-called screen distance. (General mesh distance is controlled at 3-5mm).
32.What is a mesh?
Answer: The purpose is to show the value of the hole density of the wire mesh. It is expressed by the number of holes of 1 square inch. Now it is often expressed by the number of holes of 1c㎡. West European countries such as West Germany, Switzerland, and Italy are calculating the network. Meshes are measured in centimeters, while in Japan, inches are used. Usually “mesh” is also called “T”.
33. Why should the line be tested for indentation? And it must be ≥4mm?
Answer: The indentation of the circuit is to detect the parallel uniformity of the upper and lower pressing rollers when laminating the film by the laminator, and to adjust the pressing rollers according to the actual situation of the indentation test to obtain the best film pressing ability. The indentation test must be ≥4mm, because the normal area of the upper and lower indentations is that the area of the two indentations that are in contact with the board is exactly 4mm.
34.What is the maximum and minimum optical density of a film?
Answer: Regarding the optical density, the maximum optical density Dmin (minimum)> 4.0 and the minimum optical density Dmax <0.17 are required. The maximum optical density refers to the lower limit of the light barrier presented by the surface light blocking film in the ultraviolet light. When the bottom plate is opaque When the light blocking density Dmin 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 blocking effect of the transparent plate other than the light blocking film in the ultraviolet light. When the light blocking density Dmax = 0.17 in the opaque area of the basic plate, the light transmittance is about 70%, so Dmax (max. ) Is less than 0.17 to achieve good light transmission purposes.
35. What are the reasons for the circuit to cause infiltration?
Answer: 1. Poor performance of dry film, use beyond the expiration date; 2. The surface of the substrate is not clean or roughened, and the dry film is poorly adhered; 3. The film temperature is low, the transfer speed is fast, and the dry film is not adhered firmly; 4 1. Excessive exposure energy leads to brittle corrosion resistance; 5. Insufficient exposure energy causes slow development of the hair edges of the resist due to slow development speed; 6. The temperature of the chemical solution before plating is too high.
36. What is the principle of photoimaging of liquid photosensitive ink?
Answer: When the photosensitive ink is exposed to ultraviolet light, the photoinitiator decomposes into free radicals and attacks the resin to form free polymerization, which instantly increases the polymer molecules. At this time, the ink should be insoluble in 1% sodium carbonate, but it should be soluble in strong Alkali 5-10% sodium hydroxide, in order to achieve the purpose of development and timely rescue of the problem board, the printed board pad part is blocked by the film light blocking point unexposed ink development is removed, the exposed part is developed After reservation.
37.What are the reasons for the poor development of dry film and residual glue?
Answer: 1. Poor dry film quality, such as large or high molecular weight, accidental thermal polymerization during the use of dry 4 film, etc .; 2. Partial polymerization of the dry film exposed to white light; 3. Long exposure time or excessive energy. High; 4.The maximum light density of the light blocking point for film production is not enough, causing ultraviolet light to transmit and cause partial polymerization; 5.The temperature of the developing solution is too low or the concentration is too low, the developing speed is too fast, and the pressure is too low; 6. A large number of bubbles are generated in the medium to reduce the developing ability.
38. What factors will affect the PCB of the exposure machine?
Answer: Frame, light source, temperature control system, exposure control system, vacuum suction system
39 Why do ordinary pre-processors do abrasion test?
Answer: The wear marks of the pre-processor are used to check the balance and uniformity of the brush. At the same time, the wear mark test is performed to obtain the parameters of the grinding plate current for different plate thicknesses.
40. Why is the nozzle of the developing machine’s developing cylinder fan-shaped? Why is it not tapered?
Answer: Because there is a certain distance between the nozzle of the developing machine’s developing cylinder and the nozzle, the area that can be wetted by the sprayed medicine must be relatively wide and uniform. At the same time, it will also spray the holes in the hole. Uniform, if it is cone-shaped, the medicine sprayed by it is relatively inferior to the fan-shaped nozzle under the same process conditions.
41. Why does the developing machine need to add the developing cylinder automatically? Why is the platen machine not used?
Answer: First of all, the acid cleaning (sulfuric acid) only removes part of the oxide when grinding the plate. The most important thing is to add the effect of grinding and brushing. When the plate is sprayed by the acid tank, the plate is taken away from the acid tank The acid solution is only a small part, which does not affect its original concentration (3-5%). General oxides can be dissolved when they encounter 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 being developed, part of the sodium carbonate solution is taken out of the developing tank by the board during the process of dissolving the ink.
42. What is the difference between the development nozzle of the developing machine and the automatic swinging?
Answer: It will swing automatically, so its spray on the surface of the plate will be more uniform, especially the fine lines and swing can better solve the problem of unclean development and residual film, and it can strengthen the dissolution of the residual film in the hole. Improve development ability.
43. There are air bubbles on the edge of the circuit and the surface of the substrate after solder mask printing. Are there the same reasons? 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 printed board is quietly parked for a short time before pre-baking, ③ the ink viscosity is too high or the solvent in the ink is too much moisture, ④ The thickness of the ink printing layer is too thick, ⑤ the ink is not uniformly mixed, or the ink has a good static time.
The causes of air bubbles on the substrate surface are: ① uneven ink deployment or inadequate standing time of the prepared ink, ② moisture or dirt on the surface of the board, ③ ink viscosity is too high or the printing layer is too thick, ④ The oven temperature is uneven.
44.Why is the drying section of the plate mill set to be strong air drying, hot air drying, and cold air drying? Can their order be reversed?
Answer: It can’t be reversed, because in the reverse, cold wind drying does not have much effect in the first section. When strong wind drying in the first section, the water blown out from the hole is blown out by the strong wind, plus High temperature blowing and baking will easily volatilize, and there will be no water mark on the surface of the board. If the hot air blows dry in the last stage, the temperature of the surface of the board milled into the dark room will be relatively high (60-80 ° C). Generally, the temperature is between 18-24 ° C, so the board is prone to oxidation.
45.Why does the solder mask screen printer need to be shifted?
Answer: 1. It can prevent the ink on the printing plate (the same hole position) from being penetrated into the hole by repeated scraping by the scraper twice, and it can also increase the uniformity of ink coating.
46.Why do I need to add a defoamer to the developing tank’s chemical solution?
Answer: Because the developing solution in the developing tank increases with the use time and the amount of plate passing, the solution will gradually decrease, and the remaining ink in the developing tank will accumulate more. With the repeated spraying of the solution, it will be used. There will be impurities and foam. If too much foam remains on the surface of the board, it will be difficult to wash it off when passing through the washing section of the developing machine, which will cause defects in the appearance of the board. Adding a defoamer will dissolve the foam in the developing tank. .
47.How does the viscosity of carbon oil affect the quality of PCB?
Answer: The viscosity of carbon oil directly affects the resistance value of carbon oil
48.What are the factors that affect the quality of screen printing?
Answer: 1.Ink properties: viscosity, fineness and fluidity of the ink; 2.Screen status: selection and use of the mesh, tension and coating of the photosensitive adhesive; 3.Screen printing conditions: printing pressure, blade hardness, Angle and speed of printing; 4, human factors: the operator’s technical skills and quality awareness concepts; 5, environmental factors: indoor temperature, temperature, purification degree and so on.
49.What is the dry film speed?
Answer: The dry film photosensitive speed refers to the amount of light energy caused by the photoresist under the irradiation of ultraviolet light, the polymerization reaction of the photopolymerization monomer to form a polymer with a certain resistance. When the light source intensity and light distance are fixed, the speed of light exposure is represented by the length of the exposure time. A short exposure time is fast.
50.What is the tolerance of dry film exposure time?
Answer: After the dry film is exposed for a period of time, after development, the photoresist layer has been fully or mostly polymerized. Generally speaking, the formed pattern can be used. This time is called the minimum exposure time. Continue to lengthen the exposure time to make the photoresist polymerize thoroughly, and the size of the pattern obtained after development still matches the size of the film. This time is called the maximum exposure time. Generally, the optimal exposure time for dry film is selected between the minimum minimum exposure time and the maximum exposure time. The ratio of the maximum exposure time to the minimum exposure time is called exposure time latitude.
51.What is the effect of too thick solder mask or excessive thickness?
Answer: When the solder resist ink is too thick (higher than the chip or IC level), post-processing will cause the solder paste to be defective due to poor brushing or soldering, and will cause component mounting failure or even failure to mount. Ink Passing the book will cause the insulation of the circuit not to be thick enough, and it will cause leakage phenomenon, which will affect the product performance of the later process.
52.What is the effect of conventional solder mask inks that are too thick or too thick?
Answer: Because solder resist ink is not only a protective layer, but also an insulating layer. If it is too thick, the circuit is prone to leakage due to excessive voltage or certain environmental influences. When the distance between the two lines is small To a certain degree, an arc will be generated between the two lines. If the insulation layer of the line passes through the book, it will be easily broken and a short circuit will occur. In solder mask printing, the corners of the circuit are difficult to print with oil. If the ink at the corners is thick enough, the thickness of the ink in other places such as the large copper surface is even thicker.
53.Why does the solder plug hole ink use a source liquid instead of adding oil and water?
Answer: Because the source liquid plug hole contains relatively less water, the high viscosity plug hole will not cause the plug hole to be full because of the flow, and it will volatize with the liquid moisture during post-curing. The degree of shrinkage is relatively small. At the same time, it will not cause the body’s moisture to volatilize outwards and form protrusions or cracks due to the high temperature during post-curing. If oil and water are added, its viscosity will be lower. The fluidity is relatively strong, and the easy flow when plugging will cause the plug to be full and the solder resist oil on the board to be poor, and the moisture in the ink liquid will rapidly volatilize during post-curing, which will cause plug oil or plug defects.
54.What is dry film developability and development resistance?
Answer: Dry film developability refers to the quality of the dry film after the film is exposed to the best working condition, the quality of the image obtained after exposure and development, that is, the circuit image is clear, the unexposed part should be removed without residue, and left after exposure The resist (dry film) on the board should be smooth and firm without edges. Dry film development resistance refers to the degree to which the exposed dry film is resistant to over-development. The development resistance reflects the latitude of the development process.