Transmission electron microscopy investigations on the growth of tin whiskers from CeSn3 substrate

Traditional transmission electron microscope (TEM) specimen of CeSn₃ was prepared and observed intermittently, and large amount of tin whiskers were found growing from the specimen due to the selective oxidation at room ambient. This method enables in-situ TEM study of tin whiskers without mechanical handling or focus ion beam (FIB) sampling. The growth rate of tin whisker could be more than 0.28nm/s, which was very fast than other reports. A reaction formula was proposed to summarize the oxidation of CeSn₃ at ambient condition. According to the formula, the reaction resulted in Ce(OH)₃ rather than reported CeO₂ at our room ambient condition (temperature: 18+2°C, relative humidity: 20+5%). The released tin atoms provide the source for tin whisker growth. Meanwhile, the oxidation induced 43% volume expansion, which introduced the compressive stress as the driven force for whisker growth. Tin atoms released from selective oxidation would diffuse or add to the whisker root under the compressive stress gradient. A growth model was proposed to illustrate the whisker growth process on Ce containing lead free solders, which would help to understand the mechanism of whisker phenomenon.