SEVERAL STUDIES OF NaCl PRESENCE ON TIN METALLIZATION

摘要

In several rejects with non-wetting issues, foreign elements have been found. These elements are mainly Cl, Na, S, Si, Fe, and Cr. However, the connection between these elements and the failures is not clear. In some cases, these rejects are massive but in most cases, these are sporadic. Nevertheless, sporadic cases would be more significant because they can escape easily to the final customer. Specifically, in this article, a study about the NaCl presence on Sn metallization is made. The selected component is a SOD123 with leads made of Alloy 42 (FeNi) with Sn metallization. The first step of the study is the application of an electrochemical stripping procedure to reduce the metallization thickness to specific ones. Several groups with different thicknesses were made, therefore. With these groups, different DOE's were made testing in combination with some contamination types -including NaCl- and position on PCB. The most significant factors were NaCl presence at low thicknesses -even in specification limits-. In other set of experiments, the components were oxidized according to J-STD 002's steaming procedure until 16 hrs. However, no issues were found. In another test, it was used tap water to steam the components. The results show several non-wetting issues steaming even with only 2 hours, and the components had the presence of several foreign elements including Na and Cl. Besides that, wetting balance and wetting tests were made with Cu and Sn slides. Cu slides were set in boiling water and salt for several minutes. It was found the wetting diminished along with boiling time. The conclusions are the presence of NaCl can provoke wetting issues on component metallization. However, the only salt presence is not enough. It is necessary other factors, such as low thickness or time, humid conditions, and energy in such combination to create issues.