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Sn-Ag-Cu composite solder has been prepared by adding Ni nanoparticles. Interfacial reactions, the morphology of the intermetallic compounds (IMC) that were formed, the hardness between the solder joints and the plain Cu/immersion Ag-plated Cu pads depending on the number of the reflow cycles and the aging time have all been investigated. A scallop-shaped CuSn IMC layer that adhered to the substrate surface was formed at the interfaces of the plain Sn-Ag-Cu solder joints during the early reflow cycles. A very thin CuSn IMC layer was found between the CuSn IMC layer and the substrates after a lengthy reflow cycle and solid-state aging process. However, after adding Ni nanoparticles, a scallop-shaped (Cu, Ni)-Sn IMC layer was clearly observed at both of the substrate surfaces, without any CuSn IMC layer formation. Needle-shaped AgSn and sphere-shaped CuSn IMC particles were clearly observed in the β-Sn matrix in the solder-ball region of the plain Sn-Ag-Cu solder joints. Additional fine (Cu, Ni)-Sn IMC particles were found to be homogeneously distributed in the β-Sn matrix of the solder joints containing the Ni nanoparticles. The Sn-Ag-Cu-0.5Ni composite solder joints consistently displayed higher hardness values than the plain Sn-Ag-Cu solder joints for any specific number of reflow cycles-on both substrates-due to their well-controlled, fine network-type microstructures and the homogeneous distribution of fine (Cu, Ni)-Sn IMC particles, which acted as second-phase strengthening mechanisms. The hardness values of Sn-Ag-Cu and Sn-Ag-Cu-0.5Ni on the Cu substrates after one reflow cycle were about 15.1 and 16.6 Hv, respectively-and about 12.2 and 14.4 Hv after sixteen reflow cycles, respectively. However, the hardness values of the plain Sn-Ag-Cu solder joint and solder joint containing 0.5 wt% Ni nanoparticles after one reflow cycle on the immersion Ag plated Cu substrates were about 17.7 and 18.7 Hv, respectively, and about 13.2 and 15.3 Hv after sixteen reflow cycles, respectively. [ABSTRACT FROM AUTHOR]

Highlights: [•] Soldering of sub-micron scale solder thickness Cu/Sn–Zn/Cu and Sn–Zn/Cu were studied. [•] The “Cu6Sn5 concumption based” Cu3Sn growth and Zn diffusion were discussed. [•] Fast formation of Kirkendall voids in these structures were observed and studied. [ABSTRACT FROM AUTHOR]

In order to identify the effect on the properties and behavior of tin-zinc-bismuth (Sn-8 wt% Zn-3 wt% Bi or Sn-13.6 at.% Zn-1.6 at.% Bi) based solders produced by adding nickel (Ni) nano-particles, the interfacial microstructure between plain and composite solders with newly developed immersion silver (Ag) plated copper (Cu) substrates has been investigated as a function of reaction time, at various temperatures. For plain Sn-8Zn-3Bi solder joints, a scallop-shaped Cu-Zn-Ag intermetallic compound layer was found to adhere to the surface of the immersion Ag-plated Cu substrate. However, after addition of Ni nano-particles into the Sn-8Zn-3Bi solder, Cu-Zn-Ag (at the bottom) and (Cu, Ni)-Zn (at the top) intermetallic compound layers were observed at the interfaces. In addition, these intermetallic compound layer thicknesses increased substantially with increases in the temperature and reaction time. In the solder ball region, needle-shaped α-Zn rich phase and spherically-shaped Bi-particles appeared to be homogeneously distributed throughout a beta-tin (β-Sn) matrix. However, after the addition of Ni nano-particles, needle-shaped α-Zn rich phase appeared that exhibited a fine microstructure, due to the heterogeneous nucleation of the Ni nano-particles. The calculated activation energy for the Cu-Zn-Ag intermetallic compound layer for the plain Sn-8Zn-3Bi solder/immersion Ag-plated Cu system was 29.95 kJ/mol-while the activation energy for the total [Cu-Zn-Ag + (Cu, Ni)-Zn] intermetallic compound layers formed in the Sn-8Zn-3Bi-0.5Ni (Sn-13.6 at.% Zn-1.6 at.% Bi ~1 at.% Ni) composite solder/immersion Ag-plated Cu system was 27.95 kJ/mol. Addition of Ni nano-particles reduces the activation energy which enhanced the reaction rate as we know that lower the activation energy indicates faster the reaction rate. [ABSTRACT FROM AUTHOR]

Highlights: [•] Sub-micron solder thickness Cu/Sn/Cu sandwich structure and Sn/Cu binary samples were studied by reflow and aging, to explore the unique “solder-depletion” phenomenon. [•] The “Cu6Sn5 consumption based” Cu3Sn growth was also studied, and it showed very different growth rate and characteristics. [•] Optimized thickness for ultra-thin solder layer interconnection were studied by experiments. Formation of micro-voids was observed and discussed. [•] Effects of acid cleaning (before electroplating) on the microstructures was also discussed. [Copyright &y& Elsevier]