Investigation of Sn/Cu/Ni Ternary Alloying in Lead Free Solder Bump Applications

Solder bump is widely used in advanced packaging. Solder joint reliability and solder reaction has been studied extensively. In particularly, the increasing usage of lead free solder bump has generated intensive investigation of optimizing under bump metallization (UBM) structures. In this paper, the effects and experimental results of IMC (intermetallic compound) in solder bump design will be presented. Besides the sputtered thin film barrier and seed metal, emphasis is focused on the impacts of material and thickness of plated UBM. The IMC formation between plated UBM and plated solder will affect solder bump´s microstructures and long-term reliability. The experiment results compare Ni (nickel) diffusion rate in solder with and without the presence of Cu (copper). It was found that a thin layer of Ni is effective in controlling the diffusion rate of Cu. The data supports the idea of usage of thin Ni underneath a thin layer of Cu for lead free solder bump structure. The thin plated copper on thin Ni is good enough for reducing diffusion rate into Sn rich solder due to the ternary IMC formation