Deformation characteristics of Sn-3Ag-0.5Cu/Cu/Ni-xCu/Ti joints after mechanical test

In BGA packages and flip-chip packages, a Ni layer is often used in the under bump metallurgy (UBM) to serve as a diffusion barrier. However, unstable phase of Ni₃Sn₄ compound formed between Ni UBM and Sn-rich solder, leading to a critical spalling issue. To avoid the formation of a (Ni, Cu)₃Sn₄ interlayer between Ni versus the solder, it is known that to increase Cu concentration in the solder was an effective way for the reactive interface [1-8]. In this study, a systematic design of UBM with Cu/Ni-xCu/Ti (x = 0-20 wt.%) were adopted to react with Sn-3Ag-0.5Cu ball (phi = 300mum). Multi-reflow altered the morphology of the IMCs formed at the interface. FE-EPMA was used to quantitatively analyze intermetallic compounds and to observe fracture surface. Furthermore, the pull and shear tests were employed to measure the mechanical properties of Sn-3Ag-0.5Cu/Cu/Ni-xCu/Ti joints. It was revealed that the shear strength increased with Cu contents in Ni layer and reflow times. The thickness of (Cu,Ni)₆Sn₅ IMC affected the shear strength. The shear mode changed from ductile (failure inside the bulk solder) to partially brittle (failure at the solder/IMC or IMC/UBM interfaces) in the Sn-3Ag-0.5Cu/Cu/Ni-xCu/Ti (x = 0-20 wt.%) joints with the reflow times up to 5. The result of pull test was different from that of shear test, and force direction of tool was critical. The pull strength increased with the number of reflow times. Although the thickness of (Cu, Ni)₆Sn₅ increased with reflow times, it was too thin to affect pull strength. The Cu concentration of solder did play a critical role in pull strength. It was argued that the Cu concentration of solder could decrease Sn grain size, leading to the higher pull strength. The similar tread of shear and pull results was also found in Chen´s [9] and Lehma´s [10] studies. The results of this study suggest that Cu-rich Ni-xCu UBM can be used - to suppress interfacial spalling and to improve both shear and pull strength in the solder joint.