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TOF SIMS Analysis for SnxOy Determination on Lead-Free HASL PCBs
November 19, 2013 |Estimated reading time: 1 minute
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 colors. 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-of-flight 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 uncommon 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, not SnO2, 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 help us understand better the solderability topics in the electronics industry.
Scanning Electron Microscope and SEM/EDX for Analysis
SEM uses a focused beam of high-energy electrons to generate a variety of signals at the surface. When the primary electron beam interacts with the specimen, the electrons lose energy with a repetitive and absorption volume known as interaction volume. Its size will depend on the electron´s released energy, the atomic number, and the specimen density. The energy exchange between the primary beam and the sample electrons’ released energy results in the reflection of electrons with high energy and electromagnetic radiation that are detected by a specialized screen that collects the information. Read the full article here.Editor's Note: This article originally appeared in the October 2013 issue of SMT Magazine.