Under bump metallurgy study for Pb-free bumping

The demand for Pb-free and high-density interconnection technology is rapidly growing. The electroplating-bumping method is a good approach to meet fine-pitch requirements, especially for high-volume production, because the volume change of patterned-solder bumps during reflow is not so large compared with the stencil-printing method. This paper proposes a Sn/3.5Ag Pb-free electroplating-bumping process for high-density Pb-free interconnects. It was found that a plated Sn/Ag bump becomes Sn/Ag/Cu by reflowing when Cu containing under bump metallurgy (UBM) is used. Another important issue for future flip-chip interconnects is to optimize the UBM system for high-density and Pb-free solder bumps. In this work, four UBM systems, sputtered TiW 0.2 mum/Cu 0.3 mum/electroplated Cu 5 mum, sputtered Cr 0.15 mum/Cr-Cu 0.3 mum/Cu 0.8 mum, sputtered NiV 0.2 mum/Cu 0.8 mum, and sputtered TiW 0.2 mum/NiV 0.8 mum, were investigated for interfacial reaction with electroplated Pb/63Sn and Sn/3.5Ag solder bumps. Both Cu-Sn and Ni-Sn intermetallic compound (IMC) growth were observed to spall-off from the UBM/solder interface when the solder-wettable layer is consumed during a liquid-state "reflow" process. This IMC-spalling mechanism differed depending on the barrier layer material.