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In flip chip technology, Al/Ni(V)/Cu under-bump metallization (UBM) is currently applicable for Pb-free solder, and Sn-Ag-Cu solder is a promising candidate to replace the conventional Sn-Pb solder. In this study, Sn-3.0Ag-(0.5 or 1.5)Cu solder bumps with Al/Ni(V)/Cu UBM after assembly and aging at 150°C were employed to investigate the elemental redistribution and reaction mechanism between solders and UBMs. During assembly, the Cu layer in the Sn-3.0Ag-0.5Cu joint was completely dissolved into solders, while Ni(V) layer was dissolved and reacted with solders to form (Cu 1Ày ,Ni y ) 6 Sn 5 intermetallic compound (IMC). The (Cu 1Ày ,Ni y ) 6 Sn 5 IMC gradually grew with the rate constant of 4.63 3 10 À8 cm/sec 0.5 before 500 h aging had passed. After 500 h aging, the (Cu 1Ày ,Ni y ) 6 Sn 5 IMC dissolved with aging time. In contrast, for the Sn-3.0Ag-1.5Cu joint, only fractions of Cu layer were dissolved during assembly, and the remaining Cu layer reacted with solders to form Cu 6 Sn 5 IMC. It was revealed that Ni in the Ni(V) layer was incorporated into the Cu 6 Sn 5 IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. During the period of 2,000 h aging, the growth rate constant of (Cu 1Ày ,Ni y ) 6 Sn 5 IMC was down to 1.74 3 10 À8 cm/sec 0.5 in the Sn-3.0Ag-1.5Cu joints. On the basis of metallurgical interaction, IMC morphology evolution, growth behavior of IMC, and Sn-Ag-Cu ternary isotherm, the interfacial reaction mechanism between Sn-3.0Ag-(0.5 or 1.5)Cu solder bump and Al/Ni(V)/Cu UBM was discussed and proposed.

In general, formation and growth of intermetallic compounds (IMCs) play a major role in the reliability of the solder joint in electronics packaging and assembly. The formation of Cu-Sn or Ni-Sn IMCs have been observed at the interface of Sn-rich solders reacted with Cu or Ni substrates. In this study, a nanoindentation technique was employed to investigate nanohardness and reduced elastic moduli of Cu 6 Sn 5 , Cu 3 Sn, and Ni 3 Sn 4 IMCs in the solder joints. The Sn-3.5Ag and Sn-37Pb solder pastes were placed on a Cu/Ti/Si substrate and Ni foil then annealed at 240°C to fabricate solder joints. In Sn-3.5Ag joints, the magnitude of the hardness of the IMCs was in the order Ni 3 Sn 4 Ͼ Cu 6 Sn 5 Ͼ Cu 3 Sn, and the elastic moduli of Cu 6 Sn 5 , Cu 3 Sn, and Ni 3 Sn 4 were 125 GPa, 136 GPa, and 142 GPa, respectively. In addition, the elastic modulus of the Cu 6 Sn 5 IMC in the Sn-37Pb joint was similar to that for the bulk Cu 6 Sn 5 specimen but less than that in the Sn-3.5Ag joint. This might be attributed to the strengthening effect of the dissolved Ag atoms in the Cu 6 Sn 5 IMC to enhance the elastic modulus in the Sn-3.5Ag/Cu joint.

The Ni-based under-bump metallurgies (UBMs) are of interest because they have a slower reaction rate with Sn-rich solders compared to Cu-based UBMs. In this study, several UBM schemes using Ni as the diffusion barrier are investigated. Joints of Sn-58Bi/Au/electroless nickel (EN)/Cu/Al 2 O 3 and Sn-58Bi/Au/electroplated nickel/Cu/Al 2 O 3 were aged at 110°C and 130°C for 1-25 days to study the interfacial reaction and microstructural evolution. The Sn-Bi solder reacts with the Ni-based multimetallization and forms the ternary Sn-Ni-Bi intermetallic compound (IMC) during aging at 110°C. Compositions of ternary IMC were (78-80)at.%Sn-(12-16)at.%Ni-(5-8)at.%Bi in joints of Sn-58Bi/Au/Ni-5.5wt.%P/Cu, Sn-58Bi/Au/Ni-12wt.%P/Cu, and Sn-58Bi/Au/Ni/Cu. Elevated aging at 130°C accelerates the IMC growth rate and results in the formation of (Ni,Cu) 3 Sn 4 and (Cu,Ni) 6 Sn 5 adjacent to the ternary Sn-Ni-Bi IMC for the Sn-58Bi/Au/Ni-12wt.%P/Cu and Sn-58Bi/Au/Ni/Cu joints, respectively. The Cu content in the (Cu,Ni) 6 Sn 5 IMC is six times that in (Ni,Cu) 3 Sn 4 . Electroplated Ni fails to prevent Cu diffusion toward the Ni/solder interface as compared to EN-based joints. Cracks are observed in the Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al 2 O 3 joint aged at 130°C for 25 days. It is more favorable to employ Ni-12wt.%P for the Sn-58Bi/Au/EN/Cu joint. Electroless nickel, with the higher P content of 12 wt.%, is a more effective diffusion barrier during aging. In addition, P enrichment occurs near the interface of the EN/solder, and the degree of P enrichment is enhanced with aging time. The Au(Sn,Bi) 4 , with pyramidal and cubic shape, is observed in the Sn-58Bi/

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