In electronic packaging, flip chip technology with area array is the most effective method for high density interconnection. One of the important issues for the flip chip interconnection is development of lead-free solder bumping techniques which meet fine pitch and low cost and of under bump metallization(UBM) suitable for lead-free solder. In this paper Sn-3.5Ag lead-free solder using electroplating method for solder bumping and Ni-based UBM for lead-free solder were studied. Electroplating is one of the most popular and feasible methods for fine pitch flip chip solder bumping. Electroplate bumping process consisting of alloy plating of Sn-Ag solder bumps following electroplating of UBM layers was introduced. As well as pure Ni UBM, multi-layered Cu/Ni-Cu alloy/Cu UBM was suggested; Cu bottom layer acts as a cushion layer to reduce the stress of UBM, Ni-Cu alloy middle layer as a diffusion barrier, and Cu finish layer as a wetting layer. Solders and Ni, Cu/Ni-Cu alloy/Cu UBM successfully plated on the photoresist mold patterned by controlling the plating conditions. Under bump metallurgy played an important role as a plating base for the uniform bump height.
Solder bumps on Cu/Ni-Cu alloy/Cu UBM exhibited fairly good shear strengths even after 30sec reflow and no significant changes were observed as the reflow time. Whereas, the ball shear strength of Ni UBM samples were not high enough to be those of Cu/Ni-Cu alloy/Cu UBM samples in a short time reflow, but gradually improved with reflow time. After 300sec reflow, $Ni_3Sn_4$ IMC at Ni UBM/solder interface, simultaneously $(Ni,Cu)_3Sn_4$ IMC of columnar type and $(Cu,Ni)_6Sn_5$ IMC of diamond type at Cu/Ni-30Cu/Cu UBM/solder interface, $(Cu,Ni)_6Sn_5$ IMC of fine columnar type at Cu/Ni-60Cu/Cu UBM/solder interface, $(Cu)_6Sn_5$ IMC of large columnar type at Cu UBM/solder interface were formed.
