Influence of phosphorus concentration in electroless plated Ni-P alloy film on interfacial structures and strength between Sn-Ag-(-Cu) solder and plated Ni-P alloy film

One of the critical issues which needs to be solved in the packaging technology of high speed and high density semiconductor devices is the enhancement of micro-solder joint reliability and strength. The reliability and strength of the solder joints depend on the interfacial structures between metallization and lead free solder. Both the interfacial structures and the strengths of the solder joints between plated Ni-P alloy films with various P concentrations and various solder materials have been investigated. The places where intermetallic compounds crystallized were found to vary according to the P concentration in plated Ni-P alloy films and the composition of the solder. Pyramidal intermetallic compounds that formed on plated Ni-P alloy films had the following compositions: Sn-3.5 mass%Ag/Ni-2 mass%P, Sn-3.5Ag-0.7 mass%Cu/Ni-P(2, 8 mass%) and Sn-50 mass%Pb/Ni-P(2, 8 mass%). Whereas intermetallic compounds were crystallized in the solder of the Sn-3.5 mass%Ag/Ni-8 mass%P sample. A P-enriched layer was formed between the plated Ni-P alloy films and the intermetallic compounds. The thickness of the P-enriched layers of each sample increased with the reaction time. In experiments using the same solder material, the P-enriched layer of the solder/Ni-8 mass%P sample was much thicker than that of the solder/Ni-2 mass%P sample. In experiments with plated Ni-8 mass%P alloy films, the P-enriched layers became thicker in this order: Sn-50 mass%Pb/Ni-8 mass%P; Sn-3.5Ag-0.7Cu/Ni-8 mass%P; Sn-3.5 mass%Ag/Ni-8 mass%P. The strengths of the solder joints decreased with the P concentration in plated Ni-P alloy films for all solder materials. However, it was found that the strength degradation ratio varied with the solder materials and they increased in the following order: Sn-50 mass%Pb; Sn-3.5Ag-0.7 mass%Cu; Sn-3.5 mass%Ag. Therefore, it was found that the solder joint strength is very sensitive to the thickness of the P-enriched layer at the solder joint and the solder joint strength decreased with the thickness of the P-enriched layer independent of the solder materials.