Study On The Interfacial Morphology And Mechanical Properties Of Narrow Gap Cu/Sn-0.7Cu-xNi/Cu Solder Joints

Sn-0.7Cu lead-free solder is widely applied in microelectronic packaging spheres due to its relatively rich raw material, low cost, and impurity sensitivity. However, compared with the traditional lead-contained solder, there are some disadvantages existed in Sn-0.7Cu solder for its elevated melting temperature, poor fluidity, and thermal aging stability. To further optimize the microstructure and enhance the service reliability, the following two measures has been proposed to modify the properties of Sn-0.7Cu solder: the first one is alloying by doping trace elements, such as Ni, Ge, Nd and Zn; the second one is to fabricate particle enhanced composite solder by adding ceramic particles, oxide particles, nanotubes, et al.Sn-0.7Cu-xNi(x=0.025, 0.05, 0.1, 0.2, 0.4 wt. %) composite solders are fabricated via mechanical mixing of different weight percentage of Ni particle with the Sn-0.7Cu solder paste. Effect of Ni concentration on the phase precipitation behaviors, microstructure, interfacial IMC layer, processing property and mechanical properties of the Cu/Sn-0.7Cu-xNi/Cu solder joints is investigated, and the appropriate addition of the Ni content is 0.05 wt. %. Further, Cu/Sn-0.7Cu/Cu and Cu/Sn-0.7Cu-0.05Ni/Cu joints are adopted for aging treatment to study the interfacial morphology and mechanical properties, and the dynamic equation is established for H-shaped IMC and IMC layer growth behavior.The matrix microstructure, interfacial IMC layer, and mechanical properties of Cu/Sn-0.7Cu-(0-0.4)Ni joints were investigated. Phase precipitation behavior is similar to that of the Sn-0.7Cu solder for the Sn-0.7Cu-(0.025-0.2)Ni, while the process is changed and new phase Ni3Sn4 is emerged with further adding 0.4 wt. % Ni particle. Refined dot-shaped IMC particles are uniformly dispersed among the primary ?-Sn matrix. The number of dot-shaped IMC is decreased, while coarse corncob-shaped IMC is precipitated with adding Ni content from 0.1 wt. % to 0.4 wt. %. Densified thin scalloped-like IMC layer is obtained for Cu/Sn-0.7Cu-(0-0.05)Ni/Cu solder joint. The better wettability is obtained with metallic luster bright surface after adding 0.05 wt. % Ni particle. The highest microhardness value(15.32 Hv) is obtained due to the combined effects of dispersion strengthening and grain refinement strengthening with 0.05 wt. % Ni content. Tensile properties are improved after adding Ni content to 0.05 wt. % because of the existence of the refined dot-like IMC particles, acting as the second phase particles to obstacle dislocation movement. Large amount of dimples is existed on the fracture surface, the better tensile heavier is obtained for Cu/Sn-0.7Cu-0.05Ni/Cu joint. The appropriate addition of Ni concentration is 0.05 wt. %.The influence of aging temperature on microstructure and mechanical properties of Cu/Sn-0.7Cu/Cu and Cu/Sn-0.7Cu-0.05Ni/Cu joints were studied. Multiple morphologies of IMC particles are observed on solder matrix after aging treatment, including H-shaped, E-shaped, U-shaped, corncob-shaped, dot-shaped, pole-shaped, and block-shaped. The size of H-shaped(Cu, Ni)6Sn5 IMC is increased with increasing aging temperature, while the morphology is scarcely changed. Compared with Cu/Sn-0.7Cu/Cu joint, the layer morphology of Cu/Sn-0.7Cu-0.05Ni/Cu is transformed from scallop-like as soldered to layer-like without serrated-like during aging treatment, and the number and size of voids and cracks is decreased. Mechanical properties, including hardness, tensile strength, and elongation, is deteriorated with increasing aging temperature. Fracture mode for both joints is changed from ductile to brittle behavior, simultaneously, the fracture position is also transferred from solder matrix to solder/IMC layer interface, and the better plastic property is obtained at the same aging treatment for Cu/Sn-0.7Cu-0.05Ni/Cu joint. Both of tensile strength and plastic property is enhanced with addition of Ni particle during the aging treatment.The microstructure and mechanical properties were studied for both of Cu/Sn-0.7Cu/Cu and Cu/Sn-0.7Cu-0.05Ni/Cu joints during isothermal aging treatment. H-shaped IMC is increased slowly and then quickly with prolonging aging time. Though general trend of IMC growth for Cu/Sn-0.7Cu-0.05Ni/Cu joint is similar to that of Cu/Sn-0.7Cu/Cu joint during aging treatment, growth rate is generally slower, whice is suppressed by the reduced diffusion rate of Cu and Ni due to the addition of Ni particle, and the initiation and propagation of voids and cracks is also suppressed after adding Ni. During aging treatment, Interfacial morphology for Cu/Sn-0.7Cu/Cu joint is changed from thin scallop-like as soldered to needle-like extending into solder matrix, and finally to smooth layer-like, while morphology of Cu/Sn-0.7Cu-0.05Ni/Cu is directly transformed from scallop-like to smooth layer-like with the lowest surface energy. The mechanical properties are deteriorated with increasing aging time. In the same aging conditions, the better tensile property is obtained, and the decreased elongation is also suppressed for Cu/Sn-0.7Cu-0.05Ni/Cu solder joint. Compared with Cu/Sn-0.7Cu-0.05Ni/Cu joint, both of tensile strength and ductile property are enhanced during aging treatment, indicating that the addition of 0.05 wt. % Ni can enhance the joints reliability through improving the interfacial IMC layer by inhibiting IMC layer overgrowth and optimizing interfacial morphology. Growth activation energy of interfacial IMC layer for Cu/Sn-0.7Cu/Cu and Cu/Sn-0.7Cu-0.05Ni/Cu is 23.42kJ/mol and 69.56kJ/mol, respectively.