Lead-free solders with high Sn content cause excessive interfacialreactions at the interface with under-bump metallization duringreflow. The interface formed after reflow affects thereliability of the solder joint. For this paper, we investigated the interfacialreactions of Sn0.7Cu and Sn36Pb2Ag solder on electrolytic Ni layer for differentreflow times. The traditionally used Sn36Pb2Ag solder was used as a reference. It was found that during as-reflowed, the formation of Cu-rich Sn–Cu–Ni ternary intermetallic compounds (TIMCs) at the interface of Sn0.7Cu solder with electrolytic Ni is much quicker,resulting in the entrapment of some Pb (which is present as impurity in the Sn–Cu solder) rich phase in the TIMCs. During extended time ofreflow, high (>30 at.%), medium (30-15 at.%) and low (<15 at.%) Cu TIMCs formed at the interface. The amount of Cu determined the growth rate of TIMCs. Cu-rich TIMCs had higher growth rate and consumed more Ni layer. By contrast, the growth rate of the Ni–Sn binary intermetallic compounds (BIMCs) in the Sn36Pb2Ag solder joint was slower, and the Ni–Sn BIMC was more stable and adherent. The dissolution rate of electrolytic Ni layer for Sn0.7Cu solder joint was higher than the Sn36Pb2Ag solder joints. Less than 3 μm of the electrolytic Ni layer was consumed during moltenreaction by the higher Sn contauning Sn0.7Cu solder in 180 min at 250 °C. The shear strength of Sn–Pb–Ag solder joints decreased within 30 min of reflow time from 1.938 to 1.579 kgf due to rapid formation of ternary Ni–Sn–Au compounds on the Ni–Sn BIMCs. The shear strength of Sn0.7Cu solder joint is relatively stable from 1.982 to 1.861 kgf during extended timereflow. Cu prevents theresettlement of Au at the interface. The shear strength does not depend on the thickness of intermetallic compounds (IMCs) andreflow time. Ni/Sn–Cu solder system has higher strength and can be used during prolongedreflow.
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Islam, M.N., Chan, Y.C. & Sharif, A. Interfacialreactions of Sn–Cu and Sn–Pb–Ag solder with Au/Ni during extended timereflow in ball grid array packages. Journal of Materials Research 19, 2897–2904 (2004). https://doi.org/10.1557/JMR.2004.0399