PCB Tech - PCB silk screen production process specification requirements

The production process of PCB screen printing stencils can be roughly divided into two aspects: drawing and drying; PCB manufacturer provides PCB design, circuit board production and welding, SMT chip processing, PCBA processing, PCBA foundry materials, winding coils, etc. One-stop service.

There are many small operation methods in these two aspects, let's take a look together

1. Pull the net

Steps of pulling the screen: cleaning the screen frame-level checking-applying the bottom layer glue-pulling the net-measuring the tension-applying glue-lowering the net, sealing the edge-storing

Specific operation instructions:

Due to the repeated use of the screen frame, there are residual viscose, net yarn and other debris around the screen frame, which must be removed to avoid affecting the adhesion between the net yarn and the screen frame

Place the screen frame on the platform (need to be level) to check whether the screen frame is deformed, if it is deformed, it needs to be leveled

After cleaning, the undeformed screen frame and the net yarn will be spread and evenly coated with a layer of Nanbao right grease (without adding hardener) to enhance the adhesion between the net yarn and the net frame after drawing

After the first application of glue for about 10 minutes, place the screen frame on the drawing table, and adjust the relative position and height

Select the mesh, loosen the surrounding gripper, spread the mesh on the frame, and then evenly clamp the mesh into the gripper without wrinkles. Note that there should be looser mesh in the four corners, and the gripper must be locked. Tight, there can be no gap between the clamp and the clamp (for example, automatic lifting frame and manual netting)

Pulling the net: the first tension is 26, the tension is 24 when standing for 5 minutes; the second tension is 28, the tension is 26 when standing for 5 minutes; the third tension is 32, the tension is 30 when standing for 5 minutes; the fourth correction is 5 points Tension is 32, the gluing tension is 30 after standing for 20 minutes; the screen tension is 28 when the glue is cured in 15 minutes, and the screen can be made after 72 hours of standing (take the use of a one-meter x one-meter full-automatic production line as an example).

Use a small brush to evenly brush the glue that has been adjusted on the surface of the screen frame and the mesh. Do not drop the glue into the middle part of the screen. After the glue is dry for 8 minutes, you can use a spatula to paste the glue on the glued surface. After the glue is completely dried (open air blower should be used to strengthen the drying), then the net can be installed.

Remove the excess mesh around the screen, and mark the date, mesh and tension of the screen on the frame of the screen (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 it The waterproof tape is sealed above the mesh frame and the surface of the mesh, which also prevents the penetration of the medicine

2. Net Expo

Washing the net: Use grinder paste to remove grease (new net), ghost paste to remove graphics (old net), remove pulp powder to remove net pulp and blue oil, wash debris with anti-white water, rinse the net with detergent, and finally use high pressure Rinse with a water gun, and finally clean with pure water.

Grilling net-oven set temperature should be less than 48 degrees C.

Premium film: clean the net and clean it with pure water again. Increase the size of the project film by 20% and select the water film. Use a triangle ruler to press one end of the water film on the net, and then use the triangle ruler to slowly scrape it up, and then use a squeegee to gently press and wipe off the excess water. Portion drying

Use mesh pulp: dry the mesh plate and then sizing, use a scraping box, and scrape the box containing the mesh pulp onto the net, where the green oil is three times, (about once every 10 minutes or more) the base oil, the surface oil is second, and the blue gum first Tear off the film with 50K water film, and then apply 20 mesh pulp, 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, green oil, base oil, surface oil with 120T, 100T, 110T mesh, carbon oil 51T, blue glue 24T photosensitive line and thermosetting green oil 77T

18K water film is used for the film selection line (no mesh pulp is used because of the unevenness of the Internet due to the use of mesh pulp: dog teeth, fat oil, waves), green oil, base oil, and surface oil use mesh slurry, and carbon oil uses 50K water film

Paste the required engineering graphic film on the selected position of the screen, place it on the exploding machine for exploding, the time is selected (3000W spotlight), the line is generally 60-80 seconds, the green oil is 80-100 seconds, and the bottom character oil is 40- 60 seconds, carbon oil, blue gum 350-400 seconds

Rinse the net with pressurized water and dry it.

Seal the outer edge of the graphic film with blue oil (sealing paste) and dry it.

Check, repair the network, write down the completion date and each corresponding number, and record and save it.

Of course, this is just a general operation step. The ancients have said that it is better to have no books. If the actual operation strictly follows this method, it is extremely unwise. We should learn and apply, so that we can continue to improve and make better products. .

PCB silk screen specification requirements

PCB screen printing technology has many specifications, which can not be done casually. These specifications have promoted the development of PCB screen printing technology. Let's take a look at the specifications:

All components, mounting holes, and positioning holes have corresponding silk-screen marks. In order to facilitate the installation of the board, all components, mounting holes, and positioning holes have corresponding silk-screen marks. The mounting holes on the PCB use H1, H2... …Hn for identification.

Silk-screen characters follow the principle of left-to-right and bottom-up. Silk-screen characters should follow the principle of left-to-right and bottom-up as far as possible. 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. (Except for high-density PCBs that do not require silk screen printing)

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 enamel, it is required to have no silk print on the enamel tin; in order to facilitate the device insertion and maintenance, the device location number should not be The device is blocked after installation; the silk screen should not be pressed on the via hole and the pad, 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 silk 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 PCB light drawing files is correct, each layer should have the correct output, and there should be a complete output of the number of layers.

The identifier of the device on the PCB must be consistent with the identification symbol in the BOM list.

The direction of the directional connector is clearly indicated on the silk screen.

There should be a bar code position mark on the PCB. When the PCB board space permits, there should be a 42*6 bar code silk screen frame on the PCB. The position of the bar code should be considered to be easy to scan.

PCB board name, date, version number and other finished board information silkscreen position should be clear. The PCB file should be printed with the board name, date, version number and other information of the finished board, and the position is clear and eye-catching.

The PCB must have complete relevant information and anti-static signs of the PCB manufacturer.