Effect of Co-W and Co-Fe-W Diffusion Barriers on the Reliability of the Solder/Cu Interface during Reflow Conditions

Efficient diffusion barriers are necessary to prevent the formation of copper-tin intermetallic compounds (IMCs) in advanced packaging for Sn/Cu micro-bumps. This study investigated the interfacial properties of solder and Ni, Co-9W, Co-20W, Co-20Fe-10W, and Co-36Fe-17W barriers and determined the thickness of IMCs formed between Sn and these barriers after up to 15 reflows. Among the five barriers, Co-36Fe-17W proved to be the most effective in inhibiting the reaction of liquid Sn solder. At the Sn/Co-W interface, CoSn₃ IMC was formed, while at the Sn/Co-Fe-W interface, CoSn₃ IMC and FeSn₂ IMC were observed. The contact angles of these layers were measured and found to be 18°, 22°, 25°, 29°, and 27°, respectively. The results showed that an increase in W content in Co-W led to an increase in the contact angle, while the intrinsic wettability of Co-Fe-W decreased with an increase in Fe content. The shear strengths of the five joints were 27 MPa, 31 MPa, 25 MPa, 25 MPa, and 26 MPa, respectively, with different fracture modes observed. The Co-Fe-W-Sn layer was partially peeling from the diffusion barriers in SAC305/Co-20Fe-10W, and the fracture surfaces exhibited an irregular and rough state, which was attributed to the increasing Fe and W contents. These findings offer valuable insights for enhancing the reliability of electronic packages.

Graphical Abstract