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How does SMT solder paste printing reduce product defects caused by poor printing?

Time : 2020-04-22

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How does SMT solder paste printing reduce product defects caused by poor printing?

Everyone knows that more than 60% of SMT quality defects are caused by printing. In other words, printing is one of the main causes of poor quality. So, how to improve the printing quality and reduce the bad product caused by bad printing?

1. Matters needing attention in the storage and use of solder paste:
1. As we all know, solder paste storage needs to be carried out in the refrigerator, the temperature is preferably 3-8 ℃. The storage period shall be based on the production date of the solder paste, and the shelf life is 6 months. The temperature recovery time before solder paste is used by many people is 2 hours. In fact, the temperature recovery time of different brands of solder paste is also different. The technical information provided by the solder paste supplier should prevail. Although there are ice packs in the process of purchasing freight, the time is only a few hours. Especially in summer, the outdoor temperature is high, so you should put it in the refrigerator as soon as you receive the goods. The life of solder paste should usually be discarded after more than 8 hours of use. Put it on the screen paste printer for less than 4 hours, you can put it back in a closed container and store it at room temperature. It is recommended to use it within 24 hours. The recovered solder paste should be placed separately and cannot be mixed with new solder paste. In the production process, the amount of solder paste on the stencil is continuously decreasing with the amount of printing and the amount consumed on the PCB. New solder paste can be added in time, but the solder paste on the stencil must be removed and newly added The solder paste is mixed and stirred. For continuous production, it is recommended to add and stir for 2-3 hours. When adding, also pay attention to the amount of addition, generally 1 / 4-1 / 3 cans, do not add too much.

2. Print as much solder paste as possible, but also control a certain amount. First, it is easy to produce continuous welding, and second, it is possible to produce tin beads at the device welding (after reflow soldering). How much solder paste should be on the stencil? Everyone often adds a can. Actually, the size of the printed PCB should be considered. As long as the solder paste curtain on the squeegee is not hollow in the printing area, the solder paste curtain is uniform and without defects (The so-called solder paste curtain refers to the phenomenon that the solder paste naturally hangs when the scraper is lifted after printing). Once the solder paste curtain is defective or hollow, the solder paste should be added in time.

3. Setting of printing parameters. The first is the pressure of printing. The two classic parameters of printing are the doctor blade angle of 60 degrees and the printing pressure of 60N. In fact, it is not correct. The doctor blade angle is 60 degrees. This is given by the machine and cannot be easily adjusted. The pressure is adjustable and should be adjusted according to the printing effect. The judgment principle is as follows:
①Check the stencils you have used, and observe whether the printing surface of the stencils are scratched, to what extent, if so, and the severity is very strong, it means that the printing pressure you set is too large and should be reduced.

② Observe whether there is any bright gold in the solder paste on the screen during the printing process. If there is a description that the scraper has squeezed the solder ball in the solder paste into a tin sheet, there may be Defective solder paste, because the tin film will block the mesh, so the printing pressure should be reduced, and the solder paste must be replaced;

Automatic solder paste printer

3. Stencil (also called template)
The current SMT process requires the template to use laser-cut electropolished or electroformed templates. For the template of 0805 device, 0.18MM thick steel plate is recommended; for 0603 device template, 0.15MM thick steel plate is recommended; for 0402 device template, 0.12MM thick steel plate is recommended; for 0805 and 0603 device mixed For the template, it is recommended to use a 0.15MM thick steel plate; for sockets and shaped components, the pins require a sufficient amount of solder paste template. It is recommended to use a template with multiple thicknesses (multiple steps) Step-down or step-up.

1. Precautions for storage and use of templates:
①模板用后及时清洗,防止残留在模板上焊膏的干涸; ① Clean the template in time after use to prevent the drying of the solder paste remaining on the template;

②Be careful when cleaning the template, do not damage the template;

③ Put the washed template into the packaging box to avoid dust;

④ It should be placed in a special rack, not stacked;

⑤The product number is compiled on the template for easy search;

⑥ Before printing the template, you should first check whether the template corresponds to the PCB, and check whether the template is damaged;

⑦ It is recommended to attach the corresponding template to the empty pad without a patch. Its function can not only reduce the amount of solder paste, but also serve as a mark to distinguish it from other manufacturers;

⑧ In the printing process, pay attention to whether the stickers are dropped (especially if they are cleaned frequently or washed with water);

⑨The gap between the template and the PCB should not be set to a negative value; it is recommended to set ≥0, but it should not be too large.

Four, double-panel printing needs to pay attention to:
1. The printed surface of the gold-plated and bare copper plate is much flatter than that of the tin-plated plate, and the printed surface of the tin-plated plate is often not flat enough.

2. For double-sided boards with BGA, CSP, etc. or QFP pin pitch below 0.5MM, we suggest to do the other side (side without BGA, CSP, etc. or QFP) first. The second side, that is, there are BGA, CSP or QFP side to do;

3. The surface has been made, we think that the surface with BGA, CSP, etc. or QFP should be made in time to prevent the surface from oxidizing for a long time, and the environment will also deposit dust on the PCB surface, which will affect the quality of printing. ;

4. If the time does not allow, dustproof work should be done, and baking at 80 ℃ / 12 hours before patch printing, especially during the rainy season in coastal areas

5. When doing the printing on the second side, it is recommended to add an induction wind snake in front of the paste printer. When the PCB passes over the bridge and enters the paste printer, the damper is opened and the air is blown to blow off the PCB printing surface dust.
In addition to knowing these so-called printing secrets, the more important thing is to use them in actual work. Some secret executions require conditions, such as the different printing methods (manual printing, semi-automatic printing and fully automatic printing), the functions of the automatic paste pringter, the new and old degree (adjustable range, accuracy), etc. The accuracy will have a certain discount, or even not adjustable, and the so-called printing secret cannot be realized. Therefore, it is recommended that colleagues choose the automatic printing method when choosing the printing method.

③ Observe the thickness of the solder paste on the PCB after printing. If there is more virtual soldering after reflow, and the amount of tin on the device leads is too small, the height of the soldering tin cannot meet the requirements. I suggest to start by reducing the printing pressure. Pressure can increase the thickness of solder paste printing, reduce virtual soldering, extend the service life of the machine, save electricity, extend the use of stencils, and so on.
So, what is the best printing pressure? I suggest to start the experiment from 20N first, and use the above requirements as the principle to determine the printing pressure.

Second, the use of solder paste
1. In addition to certain requirements for storage, solder paste also has certain requirements for the use process;

2. Regarding the temperature recovery of the solder paste, as mentioned above, it must be noted that the bottle cap must not be opened when the solder paste is returned to the temperature to avoid the entry of air and water vapor

3. The stirring of solder paste is another important link after the solder paste has returned to temperature, and the role of stirring cannot be underestimated;

4. We recommend using a three-dimensional solder paste mixer;

5. Be sure to remove the inner cover of the solder paste tank before mixing, otherwise the solder paste in the tank will not have a certain space during mixing, and the mixing effect will not be achieved; at the same time, the outer cover must be tightened to prevent Escape during the mixing process and pollute the mixer;

6. The solder paste stirring time should be determined according to the characteristics of the mixer and the solder paste to be stirred; we recommend 3-4 minutes generally. The stirring time should not be too long. Too long a stirring time will destroy the characteristics of the solder paste, because the solder paste During the stirring process, high-speed friction occurs between the solder balls in the solder paste, which in turn generates a certain amount of heat.

7. If manual stirring is used, the metal stir knife can not be used. Stirring with the metal stir knife will squeeze the solder balls and may scrape the plastic inside the solder paste tank, which will affect the quality of the solder paste.