Microstructure, joint strength and failure mechanisms of SnPb and Pb-free solders in BGA packages

The microstructure, joint strength and failure mechanisms of SnPbAg, SnAg and SnAgCu solders on Cu/Ni/Au BGA pad metallization were investigated after multiple reflows or high temperature aging. In the SnPbAg system, a two-layer structure, i.e., Ni₃Sn₄ and (Au, Ni)Sn₄, was formed at the solder-substrate metallization interface after aging at 125, 150, and 175°C. However, such structure was not present in the two Pb-free solder systems. Only a layer of Ni₃Sn₄ intermetallic compound in the SnAg system and a layer of Cu-Sn-Ni-Au intermetallic compound in the SnAgCu system were found at respective interfaces, even after the two solder systems had been heat treated for 1000 h at the aforementioned temperatures. The formation of the (Au, Ni)Sn₄ ternary compound in the SnPbAg system was due to re-settlement of Au at the interface which led to brittle failure in this system during ball shear testing. In contrast, SnAg and SnAgCu systems failed exclusively inside the solder ball during shear testing after aging at 150°C for up to 1000 h. The two Pb-free solder systems showed good resistance to thermal aging with the solder ball shear strength being maintained at 1.60 to 1.70 kgf. The SnPbAg system degraded in mechanical performance with aging time and had strength as low as 1.20 kgf after aging at 150°C for 1000 h. The growth rates of intermetallic compound layers at 125, 150, and 175°C aging temperatures and the activation energy for the formation of different intermetallic compound layers were also determined in this investigation.