The Observation of Intermetallic Compound Microstructure Under Sn Whisker in Lead-free Finish

Sn whiskers can grow from the pure Sn and high Sn-based finish and cause the electrical shorts and failures. Even with the wealth of information on whiskers, we have neither the clear understanding of whisker growth nor methods for its prevention. In this study, the whisker grain roots which connected with intermetallic layer were analyzed by high-resolution transmission electron microscopy (HR-TEM). In the Sn-Cu plated leadframe (LF) that was stored at ambient condition for 540 days, filament-shaped whiskers were grown on the Sn-plated surface and η'-Cu₆Sn₅ precipitates were widely distributed along the grain boundaries at the Sn matrix. The measured of the lattice fringes at the η'-Cu₆Sn₅ was 4.71Å at the coarse grain and 2.91Å at the fine grain. The Cu₃Sn which generates the tensile stresses was not observed. The formation of Cu₆Sn₅ precipitates and intermetallic layer were strongly related to whisker growth, but, the whisker growth tendency does not closely relate with the geometric morphology of irregularly grown intermetallic compound (lMC).