Microstructure, joint strength and failure mechanism of Sn-Ag, Sn-Ag-Cu versus Sn-Pb-Ag solders in BGA packages

The microstructure, joint strength and failure mechanisms of Sn-Ag and Sn-Ag-Cu versus Sn-Pb-Ag systems on Cu/Ni/Au BGA pad metallization have been investigated after multiple reflow and high temperature storage. Sn-Pb-Ag system gave rise to a two-layer structure, i.e. Ni₃Sn₄ and (Au,Ni)Sn₄, at the interface after aging at 150°C. However, such a structure was not detected in both lead-free systems. Only a layer of Ni₃Sn₄ phase in the Sn-Ag system and a layer of Cu-Sn-Ni-Au intermetallic compound in Sn-Ag-Cu system were found at the interfaces, even after 1000 hours at the afore-mentioned temperature. The formation of the (Au,Ni)Sn₄ ternary compound was due to re-settlement of Au at the interface which led to severe brittle failure in the Sn-Pb-Ag system. In contrast, Sn-Ag and Sn-Ag-Cu systems failed exclusively inside the solder after aging at 150°C up to 1000 hours. The solder ball joint strength of the three systems and failure modes were also evaluated. Both lead-free systems showed good resistance to thermal aging with a solder ball joint strength maintained at about 1.60 to 1.70 kgf. The Sn-Pb-Ag system, on the other hand, degraded in mechanical performance over aging time, reaching a strength as low as 1.20 kgf. The growth rates of intermetallic layers at 125, 150, and 175°C, and the activation energy were also determined in this study