The whole process of PCB screen printing process, which includes: safe production, use equipment, required materials, process flow and control parameters, manufacturing process (principle, working conditions, screen preparation, screen production, ink mixing, scraper use , Screen printing positioning method, incoming board inspection, brushing, screen printing, pre-baking, exposure, development, curing), document and process review, inspection and testing projects. It can be said that the scope of PCB silk screen printing is very extensive.
It is no exaggeration to compare PCB silk screen printing to the coat of PCB board. As the saying goes, people rely on clothing, so having a good-looking appearance is the most intuitive feeling for users to look at printed boards. Users will comment on a board because of its appearance. In addition, the silk screen solder mask and the characters belong to the post-process in the lengthy manufacturing process of the printed board. The printed board that is about to be completed with a high value has errors in the post-process, and the loss is too great. It's not worth it. Besides, solder mask and characters are one of the most scrapped processes. Therefore, it is very important to stabilize the process of silk screen solder mask and characters, and to straighten out the management of this process, document control and equipment maintenance. .
After 1990, basically all PCB factories do not use traditional processes. There are reasons for this. The reasons include: the density of double-sided and printed boards is increasing, the high density of small holes and thin lines SMT is an irreversible trend in the development of printed boards, and the line width and spacing are 0.12~0.20mm narrow welding The ring is already in the majority, the silk screen thermosetting green oil is powerless, the appearance is not neat, the oil seeps on the pad, and the printing is skipped, which is greatly affected by the proficiency and emotion of the screen printing staff. All have eliminated thermosetting epoxy green oil and switched to liquid photosensitive solder resist ink process.
There are many specifications for PCB screen printing technology, which cannot be done casually. These specifications have promoted the development of PCB screen printing technology. Let's take a look at the specifications below:
All components, mounting holes, and positioning holes have corresponding silk screen labels. In order to facilitate the installation of the finished board, all components, mounting holes, and positioning holes have corresponding silk screen labels. The mounting holes on the PCB are screen printed with H1, H2… ...Hn is identified.
Screen printing characters follow the principle of left to right and bottom to top. Screen printing characters try to follow the principle of left to right and bottom to top. For devices with polarities such as electrolytic capacitors and diodes, try to keep the same direction in each functional unit. .
There is no silk screen on the device pad and the tin track that needs to be tinned, and the device number should not be blocked by the device after installation. (The density is higher, except for those that do not need silk screen printing on the PCB)
In order to ensure the soldering reliability of the device, it is required that there is no silk screen on the device pad; in order to ensure the continuity of the tin-lined tin track, it is required that there is no silk-screen on the tin-lined tin track; in order to facilitate device insertion and maintenance, the device tag number should not be After installation, the device is blocked; the silk screen cannot be pressed on the via holes and pads, so as to avoid the loss of part of the silk screen when the solder mask window is opened, which will affect the training. The screen spacing is greater than 5mil. The polarity of polarized components is clearly indicated on the silk screen, and the polarity direction mark is easy to identify.
The number of sheets of the PCB light drawing file is correct, each layer should have the correct output, and there should be a complete number of layers output.
The identifier of the device on the PCB must be the same as the identifier in the BOM list. The direction of the directional connector is clearly indicated on the silk screen.
There should be a barcode position mark on the PCB. If the space of the PCB board allows, there should be a 42*6 barcode screen printing frame on the PCB, and the position of the barcode should be considered for easy scanning.
The screen printing position of the board information such as the PCB name, date, version number, etc., should be clear. The PCB file should have the board name, date, version number and other information silk screen of the finished board, and the location is clear and eye-catching.
There must be complete relevant information and anti-static logo of the manufacturer on the PCB.
The production process of PCB screen printing stencil can be roughly divided into two aspects
1. Pull the net
Steps of drawing the net: cleaning the screen frame - level inspection - applying the primer - pulling the net - measuring the tension - applying the glue - lowering the net, sealing the edge - storage
Specific operation instructions:
Due to the repeated use of the screen frame, there are residual viscose, mesh and other debris around the screen frame, which must be removed to avoid affecting the adhesion between the mesh and the screen frame.
Place the screen frame on the platform (it needs to be level) to check whether the screen frame is deformed. If there is any deformation, it needs to be leveled
After cleaning, apply a layer of Nanbaoquan grease evenly on the surface of the undeformed mesh frame and the mesh yarn (no need to add hardener) to enhance the adhesion between the mesh yarn and the mesh frame after drawing the mesh.
About 10 minutes after the first gluing, place the screen frame on the drawing table, and adjust the relative position and height.
Select the mesh, loosen the clamping nozzles around, spread the mesh on the frame, and then evenly clamp the mesh into the clamping nozzle without wrinkling, pay attention to the loose mesh at the four corners, and the clamping nozzle must be locked. Tight, there can be no gap between the clip and the clip (automatic lifting frame, manual pulling net as an example)
Netting: the first tension is 26, and the tension is 24 for 5 minutes; the second tension is 28, and the tension is 26 for 5 minutes; the third tension is 32, and the tension is 30 for 5 minutes; the fourth correction is 5 points Tension 32, after standing for 20 minutes, the gluing tension is 30; after 15 minutes of glue curing, the lower mesh tension is 28, and the screen can be made after standing for 72 hours (take the screen plate used in the one-meter & TImes; one-meter automatic production line as an example).
Use a small brush to brush the adjusted glue evenly on the surface of the screen frame and the mesh. Do not drop the glue into the middle of the screen. After the glue is dry for 8 minutes, you can use scraper glue to apply it on the glued surface. After the glue is completely dry for about 10 minutes (the open air blower should be used to strengthen drying), the net can be removed.
Remove the excess mesh around the screen, and mark on the screen frame, date, mesh and tension when unloading (to observe the tension change) In order to prevent the infiltration of white water, seal the inner corner of the screen frame with red glue, and then use The waterproof tape is sealed above the interface between the mesh frame and the mesh, which also prevents the penetration of the potion.
2. Net drying
Washing the net: use grinding paste to remove grease (new mesh), ghost paste to remove graphics (old mesh), de-sizing powder to remove mesh pulp and blue oil, wash debris with anti-white water, rinse the mesh with detergent, and finally use high pressure Rinse with water gun and finally rinse with pure water.
Baking wire - oven set temperature should be less than 48 degrees C.
Affordable film: Wash the net again with pure water. Select the water film according to the size of the engineering film patch by about 20%, press one end of the water film with a triangle ruler on the net, then slowly scrape it up with a triangle ruler, and then gently press it with a rubber scraper, and wipe the excess water with a towel. portion drying
Use mesh pulp: dry the mesh board and then sizing, use a scraper box, and scrape the mesh pulp box onto the mesh, in which the green oil is three times, (about once every 10 minutes or more) the base oil, the face oil is twice, and the blue glue first Apply 50K water film to tear off the film, and then apply the mesh sizing for 20 times, scraping three times each time (we have used an automatic sizing machine), and dry after the sizing is completed.
The selection of mesh yarn is general line, 120T, 100T, 110T mesh yarn for green oil, base oil and top oil, 51T for carbon oil, 24T for blue plastic photosensitive line and 77T for heat-cured green oil
18K water film is used for the selection line of the film (no mesh pulp is used due to uneven Internet access: dog teeth, fat oil, waves), mesh pulp is used for green oil, base oil, and face oil, and 50K water film is used for carbon oil.
Paste the required engineering graphic film on the selected position of the stencil, and place it on the exposure machine for exposure. The time is selected (3000W spotlight), the line is generally 60-80 seconds, the green oil is 80-100 seconds, and the bottom surface character oil is 40- 60 seconds, carbon oil, blue glue 350-400 seconds
Rinse the net with pressurized water and dry. Seal the outer screen edge of the graphic film with blue oil (sealing paste) and dry. Check, repair the net, write the completion date and the corresponding number and record, archive and store.
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