Solid state interfacial reactions in SAC305/Ni-Co couples

Because of the relatively low reactivity with solders, Ni-based materials are the most commonly used diffusion barrier layer materials in flip chip packaging technology for Al metallization integrated circuits. However, due to the high diffusivity of Cu, the diffusion barrier materials need to be re-evaluated for Cu/low k integrated circuits. Some literatures had indicated that Co is with superior barrier capability against Cu diffusion. Thus, Ni-Co alloys are the potential diffusion barrier layer materials of UBM for flip chip packaging. This work examines the interfacial reactions between the commercial Sn-3.0wt%Ag-0.5wt%Cu solder and Ni-Co alloys for the evaluation of Ni-Co alloys as the new diffusion barrier layer materials. The Co contents examined are 10, 20, and 40at%, and the reaction temperatures are 150 and 200°C. At 150 and 200°C, the SAC305/Ni-Co interfacial reactions are in solid/solid state, which is crucial to the reliability of the soldering joints. The SAC305/Ni-Co interfacial reactions are different from those of SAC/Ni. It depends on the Co additions. When the Co additions are 10 and 20at%, the reaction path is SAC305/CoSn₂/Ni₃Sn₄/Ni-Co. When the Co addition is 40at%, the reaction path is SAC305/Cu₆Sn₅/α-CoSn₃/CoSn₂/Ni-Co. Unusually, the reaction phases in SAC305/Ni-20at%Co couples are alternating reaction phases, CoSn₂/Ni₃Sn₄/CoSn₂/Ni₃Sn₄/CoSn₂/Ni₃Sn₄.... The SAC305/Ni-Co solid state interfacial reactions are also different from those of liquid/solid. The reaction paths of the SAC305/Ni-Co couples are illustrated using the Sn-Ni-Co and Sn-Cu-Co ternary phase diagrams. The growth rate of the reaction phases is SAC305/Ni-10at%Co>;SAC305/Ni-20at%Co>;SAC305/Ni-40at%Co.