Study of Sn and SnAgCu Solders Wetting Reaction on Ni/Pd/Au Substrates

Wetting reactions of pure Sn and Sn-Ag-Cu solder balls on Au(100 A and 1000 A)/Pd(500 A)/Ni substrates were investigated. The (Au, Pd)Sn4 phase formed in the initial interfacial reaction between pure Sn and Au(100 A and 1000 A)/Pd(500 A)/Ni substrates. Then, the initially formed (Au, Pd)Sn4 compound layer either dissolved or spalled into the molten Sn solder with 3 s of reflowing. The exposed Ni under-layer reacted with Sn solder and formed an interfacial Ni3Sn4 compound. We did not observe spalling compound in the Sn-Ag-Cu case, either on the thin Au (100 A) or the thick Au (1000 A) substrates. This implies that the Cu content in the Sn-Ag-Cu solder can efficiently suppress the spalling effect and really stabilize the interfacial layer. Sn-Ag-Cu solder has a better wetting than that of the pure Sn solder, regardless of the Au thickness of the Au/Pd/Ni substrate. For both cases of pure Sn and Sn-Ag-Cu, the initial wetting (<3-s reflowing) on the thin Au (100 A) substrate is better than that of the thick Au (1000 A) substrate. Over 3-s reflowing, the wetting on the thicker Au layer (1000 A) substrate becomes better than the wetting on the thinner Au layer (100 A) substrate.