Effect of substrate metallization on mechanical properties of Sn-3.5Ag BGA solder joints with multiple reflows

The microstructural evolutions and shear properties of the Sn-3.5Ag (in wt%) BGA solder joints were investigated in terms of the effects of various substrate metallization methods with multiple reflows. A continuous scallop-shaped Cu6Sn5 intermetallic compound (IMC) layer was formed at the solder/Cu interface, whereas a reaction layer between the solder and electrolytic Ni/Au substrate was analyzed to be a continuous Ni3Sn4 IMC layer. The thickness of the IMC layers increased with the number of reflows. The reaction products between the solder and electroless Ni/immersion Au (ENIG) substrate were three distinctive layers consisting of Ni3Sn4, Ni-Sn-P and Ni3P. The spalling of the Ni3Sn4 IMC and the growth of Ni-Sn-P and Ni3P layers were observed with the number of reflows. The shear properties of the Sn-3.5Ag/Cu solder joints were nearly consistent with the multiple reflows. The electrolytic Ni/Au substrate provided the excellent mechanical properties of the Sn-3.5Ag BGA solder joints. The shear properties of the Sn-3.5Ag/ENIG solder joints significantly decreased with the multiple reflows.