Formation Mechanism of Eutectic Cu6Sn5 and Ag3Sn after Growth of Primary β-Sn in Sn-Ag-Cu Alloy

In Sn-Ag-Cu solder balls, unusual microstructures consisting of both Sn-Ag3Sn and Sn-Cu6Sn5 binary eutectic stratures are sometimes observed. However, the formation mechanism of these unusual microstructures is still unclear. Therefore, in this study, the solidification process has been investigated to clarify the nucleation and growth of binary and ternary eutectic structures in Sn-1.0Ag-0.5Cu alloy by using thermal analysis and interruption tests. Cu-enriched zone was observed around beta-Sn in the liquid before the nucleation of Sn-Cu6Sn5 binary eutectic structure. Sn-Cu6Sn5 binary eutectic structure formed in these regions. Moreover, some Ag-enriched zones were observed around the Sn-Cu6SR5 binary eutectic structure in the liquid before the initiation of the ternary eutectic solidification. The unusual Sn-Ag3Sn binary eutectic structure formed in these regions just after the temperature reached the ternary eutectic point. The ternary eutectic structures were classified into three types depending upon the fluctuation of the remaining liquid composition at the ternary eutectic temperature. [doi:10.2320/matertrans.M2010325]