Pb-free bumping technology and UBM (under bump metallurgy)

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. This paper suggests a Sn/3.5Ag Pb-free electroplated bumping process for high density Pb-free interconnection. The Sn/3.5Ag alloy electroplating process was successfully developed in cooperation between KAIST and Fraunhofer IZM. Another important issue for the future flip chip interconnection is to optimize UBM (under bump metallurgy) systems for high-density and Pb-free solder bumps. In this work, four different types of UBM systems (sputtered TiW 0.2 μm/Cu 0.3 μm/electroplated Cu 5 μm, sputtered Cr 0.15 μm/Cr-Cu 0.3 μm/Cu 0.8 μm, sputtered NiV 0.2 μm/ Cu 0.8 μm, and sputtered TiW 0.2 μm/ NiV 0.8 μm) were selected, processed, and compared in terms of the interfacial reaction of electroplated Pb/63Sn and Sn/3.5Ag solder bumps. Both Cu-Sn or Ni-Sn IMC (intermetallic compound) growth showed a tendency to spall-off from the UBM/solder interface when the solder wettable layer was completely consumed during the liquid state ´reflow´ process. This IMC spalling mechanism appeared different depending on the barrier layer material