Kinetics of interfacial reaction between Sn-3.0Ag-0.5Cu solder and Co-4.0P or Co-8.0P metallization

Co-4.0 at.% P and Co-8.0 at.% P metallization layers with a thickness of 3.5 μm were electroplated on printed circuit boards as surface finishes. Ball grid array Sn-3.0Ag-0.5Cu (SAC305) solder balls were connected with Co-P metallization by reflow, and solid-state interfacial reaction in the solder joints aged at 110 and 150°C up to 1000 h was systematically investigated. CoSn₃ and (Cu,Co)_xSn_y interfacial intermetallic compounds (IMCs) were observed at the interface of SAC/Co-4.0P joints, while Co-Sn-P IMC was found at the interface of SAC/Co-8.0P joints. The growth of three IMCs was all governed by diffusion-controlled mechanism according to the analysis on kinetics. The growth coefficients K of the combination of CoSn₃ and (Cu,Co)_xSn_y were 1.23×10^-19 and 24.03×10^-19 m²/s at 110 and 150°C, respectively, while the K values of Co-Sn-P IMC were 0.75×10^-19 and 2.42×10^-19 m²/s at 110 and 150°C, respectively. For comparison, SAC/Cu joints were prepared and analyzed, and the K values of the combination of Cu₆Sn₅ and Cu₃Sn interfacial IMCs were 19.88×10^-19 and 76.18×10^-19 m²/s at 110 and 150°C, respectively. Therefore, the Co-Sn-P IMC in SAC/Co-8.0P joints had the smallest growth rate. The activation energy E_a for CoSn₃ plus (Cu,Co)_xSn_y and Co-Sn-P was calculated to be 100.2 and 39.5 kJ/mol, respectively. In addition, the shear strength of SAC/Co-P and SAC/Cu joints was measured. SAC/Co-8.0P joints had the largest shear strength after being annealed at 150°C, which was about 20% larger than that of SAC/Cu joints. In summary, electroplated Co-P film with an optimal composition is a promising candidate for metallization used in lead-free soldering.