In order to standardize the solder paste printing process in SMT workshops and ensure the quality of solder paste printing, the following process guidelines have been formulated, which are applicable to solder paste printing in SMT workshops in Hong Kong. The engineering department is responsible for the formulation and modification of the guidelines; it is responsible for setting printing parameters and improving poor processes. The manufacturing department and the quality department implement the guidelines to ensure good printing quality.

 

1. Tools and accessories used in SMT solder paste printing process:

1. Printing machine

2. PCB board

3. Stencil

4. Solder paste

5. Solder paste stirring knife

 

2. SMT solder paste printing steps

1. Inspection before printing

1.1 Check the correctness of the PCB board to be printed;

1.2 Check whether the surface of the PCB board to be printed is complete, free of defects, and free of dirt;

1.3 Check whether the steel mesh is consistent with the PCB, and whether its tension meets the printing requirements;

1.4 Check whether the stencil is blocked. If there is any blockage, wipe the stencil with dust-free paper and alcohol, and dry it with an air gun. Use the air gun to keep a distance of 3-5 cm from the stencil;

1.5 Check whether the solder paste used is correct and whether it is used according to "Storage and Use of Solder Paste". Remarks: Pay attention to the temperature recovery time, stirring time, distinction between lead-free and lead-containing, etc.

 

2. SMT solder paste printing

2.1 Fix the correct stencil to the printing machine and debug OK;

2.2 Assemble a clean and good scraper to the printing machine;

2.3 Use a solder paste mixing knife to add solder paste to the stencil. The height of the first solder paste is about 1CM, the width is 1.5-2CM, and the length depends on the length of the PCB. The two sides should be about 3CM longer than the printing area, and should not be too long or too short ;In the future, add solder paste every two hours, and the amount of tin is about 100G;

2.4 Put in the PCB board for printing. The first 5 PCS boards to be printed require a full inspection. After the printing quality is OK, notify IPQC for the first inspection. After confirming that the printing quality is normal, notify the production line operator to start production;

2.5 During the normal printing process, the operator needs to check the printing effect every half an hour to check whether there are any bad phenomena such as less tin, tin connection, sharpening, shifting, and missing printing. SOP, row and socket" and other key inspections of printing effects;

2.6 Every time 5PCS is printed, the stencil needs to be cleaned once. If there are components with too dense pins "BGA, QFP, SOP, socket" on the PCB, the cleaning frequency should be increased and cleaned every 3PCS;

2.7 During the production process, if continuous 3PCS printing is found to be bad, the technician should be notified to debug; clean the badly printed PCB board. When cleaning poorly printed PCBs, do not directly scrape the surface of the PCB with hard objects to prevent scratches on the surface of the PCB. PCBs with gold fingers should avoid gold fingers. After repeated wiping with dust-free paper and a little alcohol, use an air gun Blow dry, check under a magnifying glass, no residual solder paste is OK;

2.8 During the normal printing process, it is necessary to regularly check whether the solder paste is overflowing, and gather the overflowing solder paste;

2.9 After the production is over, it is necessary to recycle auxiliary materials and tools such as solder paste, scraper, stencil, and clean the tooling and fixtures. Specifically, follow the "Storage and Use of Solder Paste" and "Guidelines for Cleaning Stencils";

 

3. Solder paste printing process requirements

3.1 The main defects of printing include: less tin, tin connection, sharpening, shifting, missing printing, excessive tin, collapse, dirty PCB board, etc.

3.2 The thickness of solder paste printing is stencil thickness -0.02mm～+0.04mm;

3.3 Ensure that there is no defect in the welding effect after the furnace;