The interfacial structure and strength of solder joints between Sn-9 mass%Zn solder and plated Au/Ni-P alloy film on a Cu substrate have been investigated. Three reaction layers with 0.2 to 0.5 µm thickness were formed along the interface between the plated Ni-P alloy films and Sn-9 mass%Zn solder. The outermost layer contains a Ni-Sn intermetallic compound. The middle layer contains approximately 40 mass%Au, 35 mass%Zn, 20 mass%Ni and 5 mass%Sn. The thickness of the Au layer is 0.1 µm, so the Au layer does not dissolve. The innermost layer contains about 63 mass%Zn, 25 mass%Ni, 10 mass%Au and 2 mass%Sn. The strength of the Sn-9 mass%Zn solder joints take almost the same values independent of P concentration. The strength of Sn-Zn solder joints with Sn-Zn/Ni-2 mass%P, Sn-Zn/Ni-4 mass%P and Sn-Zn/Ni-8 mass%P joints were found to be almost constantly independent of reflow cycles. Therefore, Sn-9 mass%Zn solder is considered to be an excellent solder material for plated Ni-P alloy films.

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. Therefore, research into the interfacial structures between electroless plated Ni-P alloy film and solder is very important. It has been reported that reliability degradation occurs at the interface between plated electroless Ni-P alloy film and solder, when a P-enriched layer is formed at the interface during the soldering process.8) However, no-one has yet confirmed the relationship between the interfacial structure, including intermetallic compound and the P-enriched layer, and the mechanical behavior of the solder joints.The first purpose of the present paper is to investigate the interfacial structures formed during the soldering of plated Ni-P alloy films having various P concentrations with solder materials. Next, solder joint strength was investigated as a function of the solder materials and P concentrations in plated Ni-P alloy films. Finally, the relationship between the interfacial structure and the solder joint strength was investigated.

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The interfacial energies between ␣ and ␥ iron were calculated for Kurdjumov-Sachs (K-S), Nishiyama-Wassermann (N-W), and one orientation relationship (OR), which may represent irrational OR, with varying interface orientation. The relaxation of atomic structure in the vicinity of the interface was performed by the Monte Carlo method using an embedded atom method (EAM) potential. Whereas the polar plot of calculated interfacial energies exhibited small and large cusps for the K-S and N-W ORs, an almost equiaxed energy surface with shallow cusps was obtained for the irrational ORs. The equilibrium shape of an ␣ particle, N-W oriented with the ␥ matrix, was a thick rectangular plate with broad facets containing monatomic ledges. In contrast, the equilibrium shape of a particle, K-S oriented with the ␥ matrix, was elongated nearly in the closed-packed directions with {112} ␥ type and another broad facet that achieves relatively good matching. The volume of these shapes in the Wulff space tends to be smaller for K-S than that of the N-W OR. The equilibrium shape of the grain boundary ␣ particle at the ␥ grain boundary was calculated under the assumption that the ␣ particle has K-S OR with one of the two ␥ grains using the modified Wulff construction proposed earlier. The variation of the shape of the ␣ particle with ␥ grain boundaries at an early stage of precipitation may primarily be ascribed to the change in the variant of the smallest nucleation activation energy and less prominently to the change in the ␥ grain boundary energy.