Under-tin and under-printing in the SMT process

The phenomena of less tin and missed prints in the SMT (Surface Mount Technology) process are common quality problems, and they have an important impact on the quality and reliability of products. The following is a detailed analysis of these two phenomena:

 tin deficiency

There are many reasons for less tin, such as the thickness of the template, the shape of the hole and the sidewall roughness, the size of the tin powder particles, scraper pressure, solder paste rheology, and so on. The performance of tin deficiency is mainly a graphic defects, as shown in Figure1-1

Figure 1-1 tin deficiency phenomenon

Main reason:

1. Template issues:

The thickness of the stencil is one of the main reasons for the lack of tin. More accurately, it is the stencil opening area ratio is small, the process generally requires an area ratio of more than 0.66. Here we need to pay attention to two points, first, this data itself is a 70% transfer rate data; second, with the introduction of the micro-pad, sometimes it is difficult to achieve the level of 0.66, therefore, the less tin has become a common problem of poor printing. The sidewall of the stencil is not smooth is also easy to cause a major cause of less tin. The rougher the sidewall, the more difficult the release of solder paste, the more likely to adhere to the solder paste, which is one of the reasons for the use of FG stencils (the surface of the nanoprocessing after the formation of a special layer of functional protective film, with the effect of repulsion, fine crystal stencils), electroforming stencils. Stencil opening cross-section shape also has an impact on the less tin, the use of inverted trapezoidal cross-section shape is conducive to the release of solder paste. 2. Solder paste problems: stencil openings blocked by the common cause is the printing process using the lack of rheology of the solder paste. This can be caused by a variety of reasons, such as the use of too high or too low viscosity solder paste, or the use of expired solder paste, or the use of solder paste is not properly defrosted, or solder paste in the solvent evaporation is too much, as well as by the flux and the reaction of the solder powder caused by the formation of crust problems. The latter two cases may be the result of leaving solder paste on the stencil for too long, or using solder paste that has been printed on, or printed under heightened conditions. Leakage is an extreme case of underprinted solder, as shown in Figure 1-2, and is usually caused by a large portion of the opening being clogged or by a foreign object on the pad.

Figure 1-2 Print leakage phenomenon

The phenomenon of tin deficiency and missed prints in the SMT process is the result of a combination of factors. To effectively solve these problems, we need to start from the stencil, PCB board, solder paste, printing machine, placement pressure and reflow soldering and other aspects of the comprehensive measures to improve and optimise.