TOF SIMS Analysis for SnO Determination

Jose Maria Servin Olivares, Cynthia Gomez Aceves
CONTINENTAL CUAUTLA
Av. Ignacio Allende Lote 20
Parque Industrial Cuautla
Cd. Ayala, Morelos 62749, Mexico

During the production of Lead-Free Hot Air Solder Leveling (LF HASL), non-wetting issues in several components were found including BGA pad. The common visual aspect of the suspicious pads was the typical yellowish and bluish color. However, during traditional Scanning Electron Microscopy/Electron Dispersive X-Ray Spectroscopy (SEM/EDX) analysis for wetting issues only a small increasing of copper was found but not related to the problem.

Because of that, Time-OfFlight Secondary Ion Mass Spectroscopy (TOF SIMS) analysis was proposed; using this technique, we could achieve better surface analysis in which we found the root cause on non-wetting just in nanometers of penetration.

With this non-common tool, the results showed something not detected before: the yellowish were zones with different thick oxide layers (about 250 nm) undetectable by SEM/EDX -four times higher than a normal oxide thickness. Consequently, solder paste flux was not able to clean that oxide thickness and joints were not formed properly. The oxide is expected to be Sn2O3 and not SnO, the most common tin oxide. In this part, we would also conclude the activation level of solder paste flux depends on the type of oxide.

With this information, an investigation was conducted to remove the oxide layer as much as possible, so a solder paste with a flux more suitable to eliminate it was implemented and a cleaning process was designed to reduce it. These actions decreased the defects. In conclusion, TOF-SIMS analysis is a tool to understand better the solderability topics in Electronics Industry.

Wetting issues are related to oxidation or oxide layers on the surface of metallizations. However, its detection and quantification are hard because oxide layer is too thin (less than 1 micron) in many cases. This type of thickness can be shadowed by the presence on the element in deeper layers when SEM/EDX is used to analyze. In our case, a LF HASL PCB presented several wetting issues. Several analyses were made but the one which gave important insights was TOF SIMS.

This tool can sputter and remove material layers measured in atomic thickness and determine the presence of inorganic and organic compound in orders of ppm or lower. The results showed the oxide layer was thicker in problematic areas compared to "good" ones. Cleaning actions were carried over to eliminate this oxide layer therefore. A good reduction of its presence and reduction of wetting issues were obtained.