Tin whiskers are conductive, filamentary growths that can form on the surface of plated tin and tin solders. Tin whiskers can lead to electronic failures due to electrical shorting and plasma events, risks unacceptable for high reliability electronics. It has been known for decades that the addition of lead to tin plated films greatly reduces or eliminates whisker formation and growth, although the mechanism by which lead prevents whisker formation is not fully understood. Environmental legislation passed in Japan and Europe, including the Restriction of Hazardous Substances Directive, has driven the increased use of lead-free solders and plated films, particularly the use of pure tin. Even though such legislation has yet to be adopted in the United States, many companies are already switching to lead-free solders and plated films in order to remain competitive in the European and Asian markets. As more companies switch to lead-free products, it will become increasingly difficult for aerospace and military programs to procure electronics made with tin-lead surface finishes.
The objective of this project was to quantify the effectiveness of six alternative elements—germanium, nickel, cobalt, copper, antimony, and gold—that when added to tin in trace amounts were thought to greatly reduce or eliminate the formation of tin whiskers. Electrochemical methods for incorporating these candidate elements into the tin films were evaluated.
Samples were analyzed before and after aging in an environmental chamber to accelerate whisker growth. To determine the effectiveness of whisker suppression, three samples from each plated film group were inspected both with optical and scanning electron microscopy. The samples were inspected prior to aging, at 4,000 hours, and at 8,000 hours. Samples that had low whisker counts at 8,000 hours were returned to the environmental chamber for an additional 4,000 hours of aging. In addition, data was collected to determine if morphological and crystallographic changes of the films could be correlated with whisker growth or suppression. Electron backscatter diffraction analysis was performed by EBSD Analytical and focused ion beam analysis was performed by Evans Analytical Group.
The technical literature was searched to determine if methods for electroplating these doped-tin films already existed or if existing techniques could be modified. For films for which no method could be found, Boeing developed the needed techniques. Tin films doped with each of these elements to a weight percent of 1-3% were successfully plated. In addition, a thin film layer of gold was sputtered over the surface of pure plated tin and evaluated for whisker suppression. Pure tin samples and lead-doped tin samples were also plated and analyzed as a baseline.
The method used for plating the pure tin samples was a technique known to generate low whisker density. However, three of the elements investigated in this study—gold, germanium, and antimony—showed substantially lower whisker growth than the pure tin samples. Gold was effective both when added as a dopant to the electroplated film and when applied as a sputtered thin film layer. Not all of the dopants were successful in suppressing whisker growth; most notably the nickel- and cobalt-doped films had whisker densities substantially higher than the pure tin baseline. Interestingly, the lead-doped tin film had higher whisker density than the pure tin samples, although it should be noted that the weight percent of lead used in this project is substantially lower than most lead-tin solders or plated films.
Also, during the scanning electron microscope inspection, on the gold-doped and germanium-doped films, intermetallics or elemental gold and germanium precipitated out as fine particles leaving small nano-pores in the films. Although there were no obvious precipitates noted on the surface of the antimony-doped film, the film was very porous. Whether this change in porosity affected the whisker formation will require additional research.
The results from the morphological and crystallographic data were less clear. The electron backscatter diffraction analysis did show a general trend that indicated the lower the pole figure maximum intensity (the more random the texture), the more likely a film was to form whiskers.
Finding alternatives to pure tin for lead-free plating and solder is critical. Although strategies have been identified to reduce the chance of growing whiskers, none have been 100% effective. The use of conformal coatings, altering current density during plating, the use of barrier layers between the substrate and the plated film, and regulating the film thickness and texture have all had unsatisfactory results. On the other hand, there is an undeniable shift to lead-free electronics. The benefits of using tin-based solders—low melting point, high electrical conductivity, good corrosion resistance, and good solderability—make them attractive for use in many consumer electronics. Regardless of whether it is driven by environmental stewardship, a desire to be responsive to global regulations, or the need to remain competitive in a world market, the proliferation of lead-free tin electronics will continue.
Since it will become increasingly more difficult and expensive in the future to obtain tin-lead components for high-reliability systems such as military and aerospace platforms, it is important that effective and low cost strategies for controlling tin whisker risks be developed to ensure the reliability of military and aerospace hardware in the field.