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Tin Flakes/Splashes in SMT
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Authored By:Jose M. Servin, Miguel Dominguez, Tabata Nothel, Indira Vazquez, Pedro Juarez, Ivan Roman Continental Cuautla, Morelos, Mexico SummaryDuring the quality revisions, an increasing of electrical shorts reported in several testers was noticed. Initially, it was found than more of the 80% of the shorts corresponding to thin metal flakes/splashes. Due to the fact, the nonconformity increasing coincided to a new solder paste introduction, an analysis of the solder residues found in several processes was made, including printing, pick and place, reflow, and rework. Scanning Electron Microscopy with X-Ray Microanalysis (SEM/EDS) shows the flakes were made of pure tin. The residues were characterized and ruled out according to them. The other possible source of the metal flakes was component metallization. However, because the product has several components with tin metallization, an analysis of flake positions was done to delimit the possible component. In order to prove the D2PAK component was producing these flakes, several experiments were made to reproduce the issue in lab test and reflow with different conditions. It was found that the tin metallization was detached and flying out during the reflow melting time. This was the reason that this issue only happened in lead-free products and not leaded ones because only when metallization is melted, the flakes were expelled out. The root cause of this metallization detachment was the cleaning state of the base metal before tin deposition because tin metallization existed but it was not proper adhered. A possible oxide layer prevented tin from getting a good attachment. After removing the suspicious lots and improving component quality, this type of short disappeared from the quality charts. Additionally, sometime later, similar issue appeared in another product. Tin flakes causing shorts. Using the last experience, it was quickly analyzed and a SOIC component was the possible source of the issue. Similar actions were made to eliminate the issue. ConclusionsThe shorts that are well known in SMT process can have different causes from solder excess. In this article, a different source of shorts was found and shown. The Cause-Effect Tree tool was used and developed for solving the issue. A complete analysis of the process was carried out in order to determine the source of the metallic burrs. Other possible sources of burrs were found and actions were taken. The main cause of the shorts was the detachment of the tin metallization of a component. The activation process on the component manufacturing was the root cause of the issue and the action to improve it was done. Other actions to other possible contamination sources were done also. Initially Published in the SMTA Proceedings |
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