Intermetallic reactions between lead-free SnAgCu solder and Ni(P)/Au surface finish on PWBs

Due to its toxicity, Pb is likely to be eliminated eventually from electronic products and, therefore, it is important to understand and control the compatibility of the Sn-Ag-Cu solder alloys with Ni(P)/Au metallizations. Transmission electron microscopy and scanning electron microscopy were employed to analyze the interfacial microstructure. The intermetallic compound Cu₆Sn₅, containing a small amount of dissolved Ni, was found to form preferentially on the Ni coating. This compound layer served as a barrier for the reaction of Sn with the Ni coating. On the Ni(P) side, a nickel phosphide was identified. Thermodynamic evaluation of the Cu-Ni-Sn system was carried out to rationalize the enrichment of Cu at the solder/finish interface. Effects of the interfacial reactions on joint reliability are discussed