Behavior of tin whisker formation and growth on lead-free solder finish

The evaluation results of whiskers on three kinds of lead-free finish materials under the reliability test. The whisker growth in the Sn–Bi finish was shorter than that in the pure Sn and Sn–Cu finish. After the temperature cycling, the grown whisker shape was bent-type in the pure Sn and Sn–Cu finish and hillock-type in the Sn–Bi finish. The different composition of solution causes whisker to grow in different shape. In FeNi42 LF, the 7.0∼10.0 μm diameter and the 20.0∼35.0 μm long whisker was grown under TC 600 cycles. In the case of Cu LF, only the formation of nuclei grew on the surface. After TC 600 cycles, the amount of 0.76∼1.14 μm thick Cu6Sn5 and ∼0.27 μm thin Cu3Sn were formed between the Sn and Cu interfaces. However, the ∼0.25 μm thin Ni3Sn4 formed on the FeNi42 LF. The main growth factor of a whisker is irregular growth of the IMC in the Cu LF, and the thermal expansion coefficient mismatch in FeNi42 LF.