SMT processing solder paste application fault elimination countermeasures
Solder paste printing is a relatively complex process technology, which is affected by both materials and printing equipment and process parameters. The soldering quality defects of most SMT products are directly or spaced from the solder paste printing process. Although the quality of solder paste printing can be improved by strengthening design and process control, defects and failures are still inevitable. For this reason, it is also an important task in the quality control of solder paste coating to analyze the causes and take countermeasures to solve the problems of solder paste coating.
SMT processing
Table 5.7 Causes of Poor Printability of Solder Paste
The cause of the problem
Position deviation The position accuracy of the device itself is not good
Poor uniformity of solder paste entering the opening of the screen during printing
A bad pulling force formed by the scraper and its friction factors on the mesh plate
Missing printing The opening of the stencil is blocked or part of the solder paste sticks to the bottom of the stencil. The viscosity of the solder paste is too small
The fineness of solder paste metal powder particles is too coarse
Worn scraper, uneven knife edge
The quality of the solder paste is not good and the storage period has expired
Solder paste sharpening, printing and scraping gap is too large, solder paste viscosity is too large
Insufficient amount of solder paste. Excessive printing pressure generates a digging force on the opening of the stencil. Excessive printing pressure causes the flux to overflow, resulting in deterioration of release due to insufficient printing pressure, and the solder paste stays on the surface of the stencil. Deterioration of release
Excessive amount of solder paste. Insufficient printing pressure and residual solder paste on the surface of the stencil will increase the printing volume. Solder paste will seep to the reverse side of the stencil, which will affect the adhesion of the stencil and increase the printing volume.
Poor printing shape (rounded corners, sags) The 71 value of the solder paste is too large (the thixotropic coefficient is too small)
Solder paste penetration Poor adhesion between the substrate and the stencil. Excessive pressure during printing will cause solder paste to overflow
1 It is caused by a kind of pulling force formed by the scraper and its friction factor on the mesh plate
