As the electronics industry prepares to switch to lead-free manufacturing, there are countless lectures and seminars on this topic. Surprisingly, most of these so-called "training courses" did not discuss in detail the impact of the lead-free process on the screen printing process and believed that the original process could continue to be used. But the fact is not the case! And the result of my research is also the same.
The printing process can be simplified into two parts: the filling of the mesh with the solder paste and the release of the solder paste. For the lead-containing materials used today, there are already well-documented documentation. We understand how various materials interact with different types of stencils or methods for various types of bulk extrusion applications.
However, since lead-free solder alloys significantly change solder paste characteristics, and these lead-free materials are not as mature as tin-lead solder pastes, this makes lead-free printing and tin-lead printing processes significantly different. Lead-free solder pastes are generally more viscous and adhere more easily to the hole walls. Therefore, for this new material, the mold release properties are quite different from the previously specified tin-lead solder paste!
To learn more about the batch extrusion printing process of lead-free solder pastes, DEK designed two different research projects (still more projects are underway) to evaluate the traditional squeegee and closed print heads in lead-free batch extrusion. The effect of press printing, and compare the impact of screen materials and manufacturing methods on lead-free printing.
The first study investigated the bulk extrusion printing process of lead-free solder pastes using conventional scrapers and closed print heads, and compared the parameters such as printing speed, solder paste pressure, aperture size, and demolding speed.
Our study concludes that the closed print head technology is compatible with all lead-free materials used in the study. In fact, the results show that, contrary to standard squeegee printing, closed-print head technology can enhance the ability of lead-free printing processes. This is because the downward pressurization of the closed print head can increase the hole filling ability, ensure that the mesh fills and the material completely adheres to the circuit board pad. Therefore, in the demoulding process, the adhesion of the lead-free material to the pad is greater than the adhesion to the hole wall, thereby greatly enhancing the release performance of the solder paste.
The use of closed printhead technology can also bring other advantages, at least to reduce solder paste waste. Since lead-free solder pastes are more expensive than lead-containing materials, closed-end printheads can thus save costs.
Second, we began to study whether the stencil materials and manufacturing methods have an impact on lead-free printing. In the experiment, we evaluated 7 different screens: 3 standard laser-cut stainless steel screens (in which different laser types and local production methods were used) and 1 sheet made of laser-cut electroforming nickel sheets. Hybrid screens, a standard electroformed screen, a laser-cut KaptonÂ® polyamide screen, and a laser-cut stainless steel screen with high nickel content.
The results of the study were very interesting: three laser-cut stainless steel stencils produced different results, which proves that the stencil manufacturing process and production method have a great influence on the printing process characteristics. The final study showed that electroforming nickel-based stencil-printed lead-free solder pastes performed best, and laser-cut nickel stencils ranked second. This shows that the use of nickel as a base metal seems to allow a more robust and powerful printing process. The study also shows that screen technology, screen materials and manufacturing methods have a great influence on the printing of lead-free materials. Therefore, customers must pay special attention to the selection of network board suppliers.
The last point to explain is the accuracy of the press. It is well known that tin-lead solder paste can self-align during reflow and correct spot misalignment of solder paste. In contrast, lead-free solder pastes tend to exhibit lower wetting forces during reflow and are less likely to self-align. Therefore, the wet process accuracy (relative to alignment accuracy) and repeatability of the printing system are important factors for successful lead-free printing. In the case of lead-free materials, defects may occur if the solder paste is misaligned during the printing process.
Well, you have gone through all the courses and learned how to optimize your reflow process, replace your wave soldering furnace and reprocess lead-free solder joints, and then return to the beginning of the process to study the effect of lead-free processes on screen printing. . After all, 60% of defects can be traced back to errors in the printing process - if you continue to believe that the original process will not be affected by the lead-free process, the defect rate may be higher.
Source; by DEK Global Application Engineering Technology Manager
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