Effect of transition metals on the interfacial reactions in electroless Ni(P)-solder interconnections

The effect of transition metal (TM) additive of Ni, Co or Zn on the interfacial reactions of the solder joints between Sn-Ag-Cu (SAC) solder and the Cu/Ni(P)/Au substrate was investigated, especially when subsequent to multi-reflowing. (Cu,Ni)₆Sn₅ formed at the interface of all the joints but the SAC-Ni of which the interfacial compound was (Ni,Cu)₆Sn₅. The interfacial compounds of the SAC-Co and SAC-Zn contained a small amount of alloying elements of less than 3at%. Two P-rich layers, Ni₃P and Ni-Sn-P were found have emerged at the interface of the SAC joints. Nanoindentation analysis indicates that the hardness and Young´s modulus of these two phases were slightly higher than Ni(P) substrate, which were in turn much greater than Cu-Ni-Sn compounds. Worthy of notice is that with TM additions the Ni-Sn-P phase between Ni₃P and interfacial compounds was absent even after 10-time reflows. For the SAC-Co joints, the growth of Ni-containing intermetallic compounds (IMCs) within the solder gave rise to the excess Ni dissolution which caused a discrete Ni₃P layer and over-consumed Ni(P) substrate underneath the grooves in-between (Cu, Ni)₆Sn₅ scallop grains at the interface.