Ni Barrier-Induced Cracks in Matte Sn Films

A nickel barrier layer between a matte tin film and a C194 substrate has been used to prevent whisker formation on the tin surface by blocking the formation of Cu₆Sn₅ intermetallics, which is one of the root causes of whisker growth. However, the introduction of the Ni barrier to the matte Sn/C194 system greatly changes the mechanical properties of the bended leads. In this paper, the effect of a Ni barrier on the mechanical stability of matte tin films deposited onto C194 leads of an SOIC-8 package is investigated. Backscattered electron imaging and focused ion beam imaging of the cross-section samples indicate that, during the forming process, the surface cracks in the matte tin films often initiate and propagate from the Ni barrier, which lacks cooperative deformation ability in comparison with the ductile copper alloy substrate and the matte tin film. The thicker Ni barrier could induce considerable mechanical damage to the matte Sn films of the integrated circuit package after reflow treatment due to the coefficient of thermal expansion mismatch. The thermal-humidity testing of the Sn/Ni/C194 samples reveals that the surface cracks formed may be attributed to the built-in tensile stress arising from the formation of Ni₃Sn₄ intermetallic compounds (IMCs). A thinner Ni barrier layer can withstand the forming stress, thermal mismatch stress, and IMC-induced stress without causing cracks in the matte Sn films and does not promote Sn whisker growth.