Effect of interfacial reaction on tin whisker formation of Sn/Ni films deposited on copper lead-frame

Ni film electroplated between matte tin film and Cu-based lead-frame was used as a diffusion barrier to prevent whisker formation on the tin surface through blocking the formation of Cu₆Sn₅ intermetallics. The interfacial microstructure of Sn/Ni and Sn/Cu which formed after reflow treatment and the following humidity/thermal testing were characterized by SEM and BSE. The testing results indicated that the reflow treatment helped to prevent tin whisker growth even after 10000 hours storage under 55°C/85% RH conditions. The intermetallic compounds (IMCs) formation due to the interface reaction between Ni barrier and the surface Sn film had significant influence on the mitigation of tin whiskers growth. The thickness evolution of IMCs at the interface indicated that Ni₃Sn₄ also served as a better diffusion barrier in Sn/Ni/Cu system than Cu₆Sn₅ did in Sn/Cu system and thus affected the internal stress state within the tin films. Accordingly, reflow treatment was recommended to suppress tin whisker growth to a great extent under thermal and humidity conditions.