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Formation and Growth of Intermetallic Compounds at the Interface between Pb-Free Solder and Cu-Zn Alloy UBM
Keywords: Cu-Zn alloy, UBM, IMCs
SAC solder containing small amount of Zn has been revealed to be improved impact reliability and depressed of IMCs growth in solder joints of electronic packaging.In this study, Instead of adding Zn into the solder, we used Cu-Zn alloy UBM for Pb-free solder to improve the difficult control quaternary alloy system and the easy oxidation of solder alloy system including Zn. Comparing with Cu UBM, the morphology and growth of interfacial IMCs between Sn-Ag-Cu (SAC) lead-free solder and Cu-Zn alloy UBM electrodeposited were investigated during soldering at 260 for 20s and isothermal aging at 150 up to 1000h and investigated the mechanical strength of the solder joints using high speed ball shear test and microhardening test for mechanical property. Cu-20wt% Zn alloy was deposited on the Cu pads using electroplating, and the thickness of Cu-Zn alloy layer electrodeposited was various from 2 to 6. The morphology at the interface between solder joints before and after aging was analyzed using SEM, EDS, FIB and EPMA. It was found that the growth of Cu-Sn IMC layer at the solder joints was remarkably depressed regardless of Cu-Zn layer thickness due to the effect of Zn element in Cu-Zn alloy electrodeposited. While the various microstructure was found in the solder interface with Cu-Zn layer thickness. When Cu-Zn layers thickness was thin, a typical bilayer of Cu6Sn5 and Cu3Sn was observed at the interface between solder joints during isothermal aging. But with the Cu-Zn alloy layer increasing, the formation of Cu3Sn layer was strongly suppressed during isothermal aging. Only Cu6Sn5 phase was formed on the Cu-Zn alloy layer with approximately 6. Kirkendall voids did not form at the solder/UBM interface regardless of the thickness of Cu-Zn layer. Mechanical strength of the solder joints in the high speed ball shear test and fracture modes will be discussed.
Chang-Yul Oh, Student
Hanyang University
Seoul 133-791,

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