IN-PROCESS REDUCTION / MITIGATION OF TIN WHISKERS BY CONDENSING VAPOR REFLOW

摘要

Tin (Sn) whiskers are generally believed to be the result of natural relieving of high stress Sn coatings used in electronics packaging applications. When applied by plating methods to metallic surfaces, Sn often has regions of higher stress density. These regions can be due to the physical geometry of the components, such as bends or shoulders, or can be due to internal anomalies within plated layers. For example, parts with multiple plating steps, voids, pits, or irregularities can cause nucleation of crystal growth, which can be equated to snowflakes or ice formations nucleating at dust particles. While the physics of formation is an exciting area of research, this paper focuses on mitigating or reducing the formation of high stress Sn crystal growth. For years, it has been speculated and proposed that high temperature annealing can reduce Sn whisker growth. High temperature storage at or near the Sn melting point (232C) is believed to change the crystal orientation and reduce the stress on the Sn grains. This behavior is common place to change material properties in metals from solder to steel. One drawback to this approach, is that high temperature storage can reduce solderability of the Sn coating and can cause other temperature induced metallurgical changes such as intermetallic growth, consumption of base material layers by solid state diffusion reactions, and other issues. In addition, annealing is a time consuming process, carried out either preassembly on raw components, or post assembly after SMT processes. In previous years, many processes relied on the lead (Pb) content in Sn-Pb solders to suppress the plating stress and Sn whisker growth. Only a small amount of Pb (typically 0.5%) is required to show dramatic Sn whisker suppression, as Pb provides a built in slip plane within the solder. However, many people have made the transition to a lead free solder process, as driven by RoHS legislation in Europe, resulting in a rise in Sn whisker concerns. While there are many organizations that have worked on tests and methodologies to detect and measure Sn whisker growth, there does not seem to be industry agreement on practical methods for prevention. This paper will discuss the methodology and experimental results for an assembly level process that has shown to substantially reduce the occurrence and size of Sn whiskers, while still meeting a RoHs compliant process. Experimentation with low temperature RoHS reflow (230C) has prompted an investigation into the effectiveness of Vapor Phase reflow soldering on Sn whisker suppression. Evidence and test results will be presented along with theories on why vapor phase soldering has shown this behavior. The results of this study have shown orders of magnitude of improvement in Sn whisker formation for samples evaluated. Comparison with standard Sn-Pb assembly processes will be presented as a base line.