Investigation of Electroplated Ni and Ni-Cu alloy UBM (Under Bump Metallurgy) with Lead-Free Solders for Flip Chip Packages

摘要

Lead-free solder bumping in the flip chip interconnections is one of the key issues for the high density and environmentally friendly IC chip packages. The design of UBM (under bump metallurgy) structure and materials is also important, when we take the interaction with solders and the reliability problems into account. In order to use tin-rich, lead-free solders, Ni is one of the most promising candidate materials to replace Cu in the UBM systems. However, the Ni metallizations often have high stresses which may cause the peeling off of the films or Si chip cratering. In the present paper, electroplated Ni or Ni-Cu alloy UBM/Sn-3.5Ag solder systems were investigated focusing on the stress of the UBM and interfacial reactions between the UBM and the solder. After the sputtering of the adhesion and the seed layers, Ni or Ni-Cu alloys were electroplated on Si wafers, and the electroplating conditions were varied to make deposits of various alloy compositions. Residual stresses of Ni UBMs were reduced with increasing the plating temperatures and decreasing the current density. Cu alloying in the Ni layers reduced the stress of the UBM but accelerated the interfacial reaction with lead-free solders. To reduce the stress of the UBM, Cu/Ni-Cu double layer UBM was introduced by the step electroplating method in a single bath, in which the Cu acted as a cushion layer.