Comparison of interfacial reactions and reliabilifies of Sn3.5Ag, Sn4.0Ag0.5Cu, and Sn0.7Cu solder bumps on electroless Ni-P UBMs

Abstract

Screen-printing of solder bumps combined with electroless Ni-P plating of UBMs is one of the most cost effective flip chip bumping technologies. As many Pb-free solder pastes such as ternary and quaternary alloys have been applied, demands for understanding of interaction between electroless Ni-P UBM and various Pb-free solders also are increasing. Because of higher process temperature and higher Sn content than Pb63Sn solder, faster interfacial reaction and faster consumption of the UBM occur resulting in reliability problems of solder joint. In addition, it is expected that small addition of Cu in solder would greatly affects the formation and growth of IMCs, and consumption of the electroless Ni-P UBM. In this paper, interfacial reactions and reliability of 3 widely used Pb-free solder bumps (Sn3.5Ag, Sn4.0Ag0.5Cu, and Sn0.7Cu) on electroless Ni-P UBMs were investigated. UBM consumptions, ball shear strengths, and fracture modes were compared with each solder alloy. Finally, the best compatible Pb-free solder alloy with electroless Ni-P UBM will be suggested. It was observed that consumption of Ni-P UBM in Sn3.5Ag solder bump was much faster than those in Cu-contained solder bumps (SnAgCu and SnCu). Due to the small addition of Cu in Cu-contained solders, firstly forming IMC at the interface was not Ni3Sn4 but Cu6Sn5. Therefore, formation of Cu6Sn5 IMC in SnAgCu and SnCu solders greatly reduced consumption of Ni-P UBM. Compared with Sn4.0Ag0.5Cu and Sn0.7Cu, higher Cu content was more effective to reduce consumption of the UBM. In order to estimate reliability of each solder bump, ball shear strengths and shear modes were measured and compared, respectively. Ball shear strength seemed to be very dependent to mechanical properties of solder bumps.