Interfacial reactions between Sn-Cu solders and Ni-Co alloys at 250 °C

Sn-xCu/Ni-yCo (x = 0.2–1.0 wt%, y = 10, 20, 40 at.%) interfacial reactions at 250 °C are examined in this study. Sn-Cu alloys are promising lead free solders, and Ni-Co alloys are the potential diffusion barrier layer materials in flip chip packaging. The Co and Cu effects on the Sn-Cu/Ni-Co interfacial reactions are examined. When the Co addition is 10 at.%, the reaction phases are the Ni₃Sn₄ and Cu₆Sn₅ phase, and Sn-Cu/Ni-10at.%Co interfacial reactions are similar to those of Sn-Cu/Ni. When the Co addition is 20 at.%, the CoSn₂ phase is formed, and the reaction path is Sn-Cu/Ni₃Sn₄/CoSn₂/Ni-20at.%Co. When the Co addition is 40 at.%, only Sn-Co binary phases are formed (CoSn₂ and CoSn₃), and Sn-Ni binary phases are not formed. The Cu₆Sn₅ phase is not formed until the Cu content is higher than 0.7 wt%. The Cu concentration effect is the main drawback of using Ni as the diffusion barrier layer material and the Sn-Cu solders. The Cu concentration effect of the Sn-Cu/Ni-Co interfacial reactions is not as pronounced as that of Sn-Cu/Ni.