In order to standardize the solder paste printing process in the SMT workshop and ensure the quality of the solder paste printing, SMT has formulated the following process guidelines, which are suitable for the solder paste printing in the SMT workshop. Next, Sanjing introduces the printing steps of SMT solder paste.

1. Preparation and inspection before printing

Before printing, we need to prepare the tools and auxiliary materials used in the SMT solder paste printing process: printers, PCB boards, stencils, solder paste, solder paste mixing knives, etc. At the same time, check whether the surface of the PCB to be printed is intact and free of defects and dirt; check whether the stencil is consistent with the PCB and whether its tension meets the printing requirements; check whether the stencil is blocked or not. Wipe the stencil with dust paper dipped in alcohol, and dry it with an air gun. When using the air gun, keep a distance of 3-5cm from the stencil; check whether the solder paste used is correct, pay attention to the temperature recovery time, mixing time, and the distinction between lead and lead-free, etc. 

Second, add solder paste

Fix the stencil to the printing machine and debug it, install a clean and good squeegee on the printing machine, and then add the solder paste to the stencil with a solder paste stirring knife. Depending on the length of the PCB, both sides should not be too long or too short, generally 3cm longer than the printed area, and add about 100g of tin paste every two hours.

Three, PCB board printing

After the solder paste is added, it can be placed on the PCB for printing. The first 5PCS of the printing must be fully inspected. After the printing quality is passed, the IPQC first inspection will be carried out to confirm that the printing quality is normal, and then mass production will be carried out.

During the printing process, it is necessary to check the printing quality every half an hour to see if there are any undesirable phenomena such as tin loss, tin connection, sharpening, shifting, and missing printing. In particular, it is necessary to check the pin over-dense components, such as BGA, QFP, For SOP, power strips, etc., the stencil needs to be cleaned every 5PCS printed. If there are components with denser pins, the cleaning frequency should be increased. Here, by the way, the printing process requirements are introduced. The thickness of the printed solder paste is the thickness of the stencil -0.02mm~+0.04mm.

Fourth, clean poorly printed PCB boards

During the production process, if poor printing occurs several times in a row, it is necessary to re-adjust and clean the poorly printed PCB. It should be noted here that when cleaning poorly printed PCBs, do not directly scratch the layer with a hard object to prevent scratching the PCB surface circuit. For PCBs with gold fingers, avoid the gold fingers and use dust-free paper to add a little After repeated wiping with alcohol, dry it with an air gun, and check with a magnifying glass to ensure that no solder paste remains.

In addition, pay attention to whether there is solder paste overflow during the printing process. If so, it is necessary to gather the overflow solder paste.

After the production is over, the solder paste, scrapers, steel mesh and other auxiliary materials and tools should be recovered, and the tooling fixtures should be cleaned, which can be operated in accordance with the standard operating instructions.

The above is an introduction to the steps of SMT solder paste printing, I hope it can be helpful to everyone!