Flip Chip Metallurgies for Lead-Free Solders

Abstract

The most commonly used lead-free solders contain large amounts of tin, which makes them incompatible with the conventional Cu-based underbump metallization (UBM) schemes. The tin in the solder reacts with the copper layer of the UBM, depleting the UBM of copper and causing loss of adhesion and a weak interface. Use of new under bump metallization schemes with Ni or CuNi alloys as the solderable layer were investigated in this study. Instead of Cr, a Tibased adhesion layer was used to decrease the amount of stress in the CuNi layer. Flip chip solder joints were made in which three Sn-Bi-Ag based lead-free solders were reflowed to several UBM pads of different compositions. The resulting interfacial microstructures were examined by SEM/EDX analysis of cross-sectioned samples. The joints were also mechanically tested in fatigue and shear to assess the quality and reliability of the interface.

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Korhonen, T.M., Hong, S.J., Korhonen, M.A. et al. Flip Chip Metallurgies for Lead-Free Solders. MRS Online Proceedings Library 515, 79–83 (1998). https://doi.org/10.1557/PROC-515-79

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