The undercooling behavior of pure Sn, Sn–0.7Cu, Sn–3.5Ag, and Sn–3.8Ag–0.7Cu solder alloys was observed in terms of various under bump metallurgies (UBMs). Four different UBMs (electroplated Cu, electroplated Ni, electroless Ni(P), and electroless Ni(P)/immersion Au) were used. The amount of the undercooling of Pb-free solder alloys was reduced when reacted with electroplated Cu UBM and Ni-based UBMs. The Ni-based UBMs were more effective than Cu UBM in reducing the undercooling of Pb-free solders. When Ni3Sn4 was formed during the interfacial reactions with Ni-based UBMs, the reduction of undercooling was significant, especially for pure Sn and Sn–3.5Ag. The effects of UBMs on the undercooing of Pb-free solder alloys are discussed by comparing intermetallic compounds formed during interfacial reactions with UBMs. In addition, the microstructural changes as well as the microhardness of four solders with or without UBMs are discussed, which could be related to their undercooling behaviors.
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We gratefully acknowledge Dr. Hae Jin Kim and Dr. Jin Bae Lee at the Korea Basic Science Institute (KBSI) for their work on DSC measurements as well as Han-Na Park at KBSI for the SEM observation and EDS analysis.
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Cho, M.G., Kang, S.K. & Lee, H.M. Undercooling and microhardness of Pb-free solders on various under bump metallurgies. Journal of Materials Research 23, 1147–1154 (2008). https://doi.org/10.1557/jmr.2008.0133