Self-Healing of Kirkendall Voids and IMC Growth in the Interfacial Reaction of Novel Ni/Cu bi-layer Barrier and Solder

Single Ni layer is often inserted as diffusion barrier between Cu pillar and Sn-based solder to avoid excessive growth of brittle intermetallic compounds (IMCs) and consequent Kirkendall voids (KVs) in microbumps. However, with shrinking size of microbumps, Ni layer cannot maintain the inhibition performance as its thickness is reduced as well. In this work, Ni/Cu bi-layer barrier was employed at Cu-Sn interface, which was expected to reduce diffusion by rapidly generated Cu-Sn IMC retarding the diffusion of Ni. IMC growth behavior and interfacial reaction during isothermal aging were investigated. The self-healing phenomenon of KVs was detected during aging at 150 °C . It’s attributed to the transformation from Cu₃Sn to Cu₆Sn₅. The novel barrier exhibited excellent inhibition property compared with single Ni layer with slower IMC growth rate and less Cu substrate diffusion. Moreover, during 170 °C aging test, the Ni/Cu bi-layer barrier showed no sign of depletion until 600 h, while the single Ni barrier was completely depleted after 144 h. Such excellent inhibition property is beneficial to the future application of ultra-thin barrier layer in microbumps.