Interfacial Reactions of Sn-Ag-Cu Solders Modified by Minor Zn Alloying Addition

The near-ternary-eutectic Sn-Ag-Cu alloys have been identified as a leading Pb-free solder candidate to replace Pb-bearing solders for microelectronic applications. However, recent investigations on the processing behavior and solder joint reliability assessment have revealed several potential reliability risk factors associated with the alloy system. The formation of large Ag3Sn plates in Sn-Ag-Cu joints, especially solidified in a relatively slow cooling rate, is one issue of concern. In the previous studies, the implications of large Ag3Sn plates on solder joint performance and several methods to control them were discussed. The addition of minor amounts of Zn was found to be effective in reducing the amount of undercooling required for tin solidification and thereby to suppress the formation of large Ag3Sn plates. The Zn addition also caused the changes in the bulk microstructure as well as the interfacial reaction. In this study, an in-depth characterization of the interfacial reactions of Zn-modified Sn-Ag-Cu solders on Cu and Au/Ni(P) surface finishes is reported. The effects of Zn addition on modification of the interfacial IMCs and their growth kinetics are also discussed.