Effect of Co addition into Ni film on shear strength of solder/Ni/Cu system: Experimental and theoretical investigations

Depositing Ni-based films on Cu pads has been accepted as an effective strategy to address the reliability challenges due to the development of plastic ball grid array (PBGA) in electronic packaging technology. Modification mechanisms of Co on Ni film and Sn-37 wt%Pb/Ni/Cu solder joints were researched from experimental and theoretical methods in this work. Experimental results suggested the additional minor Co element (12.46 at. %) led to the formation of Ni-Co solid solution. After reflowing, Ni3Sn4 intermetallic compound (IMC) was formed within Sn-37Pb/Ni/Cu system while (Ni,Co)(3)Sn-4 IMC was produced at solder/Ni-Co interface. N-anoindentation tests indicated that the doping of Co enhanced the mechanical properties of Ni film and Ni3Sn4 IMC. Our single-lap shear tests demonstrated that the shear strength of Sn-37Pb/Ni/Cu reflowing joints increased from 24.68 to 29.72 MPa with the addition of Co. Moreover, theoretical investigations based on first-principles calculations were carried out to uncover the Co strengthening mechanism. The calculated results noted that the improved stability of coating/IMC interface and mechanical properties of coating and IMC phases were the main reasons for the enhanced shear strength of Sn-37Pb/Ni-Co/Cu solder joints.