3.2 Process control when using solder paste
(1) Strictly use the solder paste within the validity period, store the solder paste in the refrigerator on weekdays, and keep it at room temperature for more than 6 hours before use. Afterwards, open the lid and use it. The used solder paste is stored separately. qualified.
(2) Before production, the operator uses a special stainless steel rod to stir the solder paste to make it uniform, and periodically measure the viscosity of the solder paste with a viscosity tester.
(3) After the printing of the first printed board or equipment on the shift is adjusted on the day, the printed thickness of the solder paste shall be measured using the solder paste thickness tester. The test points shall be selected at the top, bottom, left, right, and middle of the test surface of the printed board. Record the value and require the paste thickness to be within -10% of the template thickness - +15% of the template thickness.
(4) In the production process, 100% inspection of the solder paste printing quality is performed. The main content is whether the solder paste pattern is complete, whether the thickness is uniform, and whether the solder paste is tipped.
(5) Clean the template according to the process requirements after the shift is completed.
(6) After the printing experiment or printing failure, the solder paste on the printed board is required to be thoroughly cleaned and dried with an ultrasonic cleaning device to prevent the occurrence of solder balls after reflow due to residual solder paste on the board when it is used again.
3.3 Common Printing Defects and Solutions
Solder paste printing is a very complicated process. It is influenced by materials and has a direct relationship with the equipment and parameters. Through the control of various fine links in the printing process, defects that often occur in printing can be prevented. The following brief Introduce some of the most common defects and their corresponding prevention or solutions when printing solder paste.
3.3.1 Incomplete Printing
Incomplete printing means that solder paste is not printed on some places on the pad. The reason may be:
(1) The opening is blocked or part of the solder paste sticks to the bottom of the template.
(2) The viscosity of the solder paste is too small.
(3) There are larger size metal powder particles in the solder paste.
(4) Scraper wear.
Prevent solutions:
Clean the openings and the bottom of the stencil, select the solder paste with suitable viscosity, and allow the solder paste to print effectively covering the entire printed area; select the solder paste that corresponds to the particle size of the metal powder and the opening size; check the replacement squeegee.
3.3.2 Pull Tip
The tip is printed on the pad after the solder paste is a small mountain-like peak, may be due to the blade gap or the viscosity of the solder paste is too large. Prevention or solution:
Reduce the gap between the squeegees or select the appropriate viscosity for the solder paste.
3.3.3 Collapse
After printing, the solder paste collapsed on both sides of the pad. The reason may be:
(1) Squeegee pressure is too high.
(2) The printed board is not positioned well.
(3) The solder paste viscosity or metal content is too low.
Prevention or solution:
Adjust the pressure; re-fix the printed board; select the appropriate viscosity of the solder paste.
3.3.4 solder paste is too thin
The reason may be:
(1) The template is too thin.
(2) Squeegee pressure is too high.
(3) The solder paste has poor fluidity.
Prevention or solution:
Select the appropriate thickness of the template; select the appropriate paste size and viscosity; reduce the pressure of the blade.
3.3.5 Inconsistent thickness
After printing, the thickness of solder paste on the pad is inconsistent. The reason may be:
(1) The template and the printed board are not parallel.
(2) Inconsistent agitation of the solder paste makes the particle size inconsistent.
Prevention or solution:
Adjust the relative position of the template and the printed board; fully stir the solder paste before printing.
3.3.6 Glitches on the edges and surfaces
The reason may be that the viscosity of the solder paste is low, and the wall of the template opening hole is rough.
Prevention or solution:
Choose a solder paste with a slightly higher viscosity; check the etched quality of the template opening before printing.
4 Conclusion
In order to ensure the quality of surface mount products, we must analyze and study the key factors that affect all aspects of production and formulate effective control methods. Solder paste printing as the key process is of utmost importance. Only by formulating suitable parameters and mastering the laws between them can we obtain high quality solder paste printing quality.
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