Selective formation of intermetallic compounds in Sn-20In-0.8Cu ball grid array solder joints with Au/Ni surface finishes

After Sn-20In-0.8Cu solder balls are reflowed on a ball grid array (BGA) substrate (substrate A) with an Au/Ni surface finish, scallop-shaped intermetallic compounds with a composition of 0.83[Cu6(Sn0.87In0.13)5] + 0.17[Ni3(Sn0.87In0.13)4] are formed at the solder/pad interface. The distribution of the intermetallics is not altered by gravity or by multiple reflows of the solder joints. As another substrate (substrate B) is further attached onto the primary reflowed BGA assembly to form a sandwich structure subjected to subsequent multiple reflows, the Cu6(Sn0.87In0.13)5 interfacial intermetallic scallops remain still on the side of substrate A while many Au(In0.91Sn0.09)2 intermetallics of cubic shape appear near the solder/Ni interface on the side of substrate B. When the Sn-20In-0.8Cu solder balls are assembled simultaneously in between two substrates (A and B), Au(In0.91Sn0.09)2 intermetallic cubes of equal proportion are observed to form on both sides of the assembly. In summarizing the results, it is proposed that the diffusion of Cu atoms in the Sn-20In-0.8Cu solder toward the Ni layers after Au thin-film dissolution on Au/Ni surface finishes led to the formation of Cu6(Sn0.87Zn0.17)5 intermetallic compounds, which prevailed over the gravitational effect so that no intermetallic sedimentation in the liquid solder would occur. The appearance of Au(In0.91Sn0.09)2 at the Ni/Sn-20In-0.8Cu interfaces was hindered by the preferential formation of Cu6(Sn0.87Zn0.17)5 until the Cu atoms in the Sn-20In-0.8Cu solder matrix were consumed to a lower content via the attachment of a second substrate to the assembly.