Abstract
Ni-V alloys are frequently used as diffusion barrier materials in the electronic products. As a result, solder/Ni-V joints are often encountered in electronic products. Since Sn is the primary element of most of the electronic solders, Sn-Ni-V is an important ternary material system. However, its phase equilibria information is not available. Interfacial reactions are crucial for the reliabilities of the solder joints. There have been intensive investigations about solders/Ni interfacial reactions, but the interfacial reactions between solders and Ni-V substrates are seldom examined. This study investigates the phase equilibria of the Sn-Ni-V ternary system and the interfacial reactions between solders and the Ni-V substrates. The solders include pure Sn, eutectic Sn-Cu (Sn-0.7wt.%Cu) and eutectic Sn-Ag (Sn-3.5wt.%Ag) solders. The electromigration effect upon solders/Ni-V interfacial reactions is also examined in this study. Phase equilibria of the Sn-Ni-V system at 250oC were determined both by experimental determination and calculation with CALPHAD method. No ternary equilibrium phase is observed. At the Sn-rich corner, it is a L(Sn)+Ni3Sn4+VSn2(V2Sn3) three-phase region. The ternary elemental solubilities of Ni3Sn4 phase and VSn2(V2Sn3) phase are negligible, and those of both Ni and V in the liquid phase are also very low. The composition of VSn2(V2Sn3) phase is Sn-34at.%V, which is consistent to the structural prototype of CuMg2, and should be accordingly renamed as VSn2. The interfacial reactions in the solders/Ni-V couples are different from those of solders/Ni. When the V addition is higher than 5wt.%, a ternary meta-stable phase (T phase) is formed. It is an amorphous phase mixing with Ni3Sn4 fine grains. In the solid/solid reactions, unusual T/Ni3Sn4/T/Ni3Sn4 reaction phases were observed in the solders/Ni-V couples. It has two features: solid state amorphization and alternating reaction phases. In liquid/solid reactions, T phase formation makes the reaction phase detachment earlier in comparison with that of Ni substrate. Since Ni-V alloys are the commercial barrier layer materials practically used, and the solders/Ni-V interfacial reactions are different from those in the solders/Ni couples, the reliability assessments of flip chip products should be based on the interfacial reactions of solders/Ni-V. Electric current with 500A/cm2 density exhibits the same effect upon Sn/Ni-V interfacial reactions as upon those of Sn/Ni. The reaction phases are not altered, but the reaction rate is changed. At the interface where the electron flow and Sn atoms diffusion are co-current, the reaction rate is enhanced. In contrast, at the interface where the electron flow and Sn atoms diffusion are counter-current, the reaction rate is retarded. The result agrees with the momentum transfer theory.