Analysis of the factors affecting the quality of t

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Analysis of factors affecting the quality of solder paste printing (IV)

3.2 process control during the use of solder paste

(1) the solder paste shall be used strictly within the validity period. The solder paste shall be kept in the refrigerator at room temperature for more than 6 hours before use, and then it can be opened for use. The used solder paste shall be stored separately, and the quality shall be determined during reuse

(2) before production, the operator uses a special stainless steel rod to stir the solder paste to make it uniform, and regularly uses a viscosity tester to spot measure the viscosity of the solder paste

(3) after the first printed board or equipment is printed on duty that day is adjusted, the solder paste thickness tester shall be used to measure the solder paste printing thickness, and the test points shall be selected at the top, bottom, left, right and middle of the test surface of the printed board, and the values shall be recorded, The thickness range of solder paste is required to be between -10% and +15% of the template thickness

(4) during the production process, 100% inspection shall be conducted on the printing quality of solder paste, mainly including whether the solder paste pattern is complete, whether the thickness is uniform, and whether there is solder paste sharpening

(5) clean the formwork according to the process requirements after the completion of the work on duty

(6) after the printing experiment or printing failure, the solder paste on the printed board shall be thoroughly cleaned with ultrasonic cleaning equipment and dried to prevent the following operating defects on the board during reuse: solder balls after reflow soldering caused by residual solder paste

3.3 common printing defects and solutions

solder paste printing is a very complex process, which is not only affected by materials, but also directly related to equipment and parameters. Through the control of each small link in the printing process, the common defects in printing can be prevented. The following briefly introduces several common defects in solder paste printing and the corresponding prevention or solutions

3.3.1 incomplete printing

incomplete printing means that some parts of the pad are not printed with solder paste. The possible causes are:

(1) the opening is blocked or part of the solder paste is stuck at the bottom of the template

(2) the viscosity of solder paste is too small

(3) there are large metal powder particles in the solder paste

(4) the scraper is worn

prevention solution:

clean the opening and the bottom of the template, select the solder paste with appropriate viscosity, and make the solder paste printing effectively cover the whole printing area; Select the solder paste with the particle size of metal powder corresponding to the opening size; Check and replace the scraper

3.3.2 sharpening

sharpening is that the solder paste on the pad after printing is in the shape of a hill peak, which may be caused by too large scraper clearance or solder paste viscosity. Prevention or solution:

properly reduce the scraper clearance or select the solder paste with the proper viscosity

3.3.3 collapse

after printing, the solder paste collapses to both sides of the pad. The possible causes are:

(1) the scraper pressure is too high

(2) the printed board is not firmly positioned

(3) solder paste viscosity or metal content is too low

prevention or solution:

adjust the pressure; Fix the printed board again; Select the solder paste with proper viscosity and check the ambient temperature (10 (3) 5 ℃) and humidity (relative humidity) every day as long as the measuring circuit has no defect and the degree of defect is not greater than 80%)

3.3.4 the solder paste is too thin

the possible causes are:

(1) the template is too thin

(2) the scraper pressure is too high

(3) poor fluidity of solder paste

prevention or solution:

select the template with appropriate thickness; Select solder paste with appropriate particle size and viscosity; Reduce the scraper pressure

3.3.5 inconsistent thickness

after printing, the thickness of solder paste on the pad is inconsistent. The possible causes are:

(1) the template is not parallel to the printed board

(2) the solder paste is not stirred evenly, resulting in inconsistent particle size

prevention or solution:

adjust the relative position between the template and the printed board; Fully stir the solder paste before printing

3.3.6 burrs on edges and surfaces

may be caused by low viscosity of solder paste and rough hole wall of template opening

prevention or solution:

select solder paste with slightly higher viscosity; Check the etching quality of the template opening before printing

4 conclusion

in order to ensure the quality of surface mount products, it is necessary to analyze and study the key factors affecting each link of production and formulate effective control methods. As a key process, solder paste printing is the most important. Only by formulating appropriate parameters and mastering the laws between them, can we get high-quality solder paste printing quality

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