With the development of microelectronic packaging and increasingly specific service environment of solder joints, much stricter requirements have been placed on the properties of lead-free solders. On account of small size effect and high surface energy, nanoparticles have been widely used to improve the microstructure and properties of lead-free solders. Therefore, the composite solders bearing nanoparticles have recently attracted wide attention. This article reviewed the recent research on SnAgCu, SnBi, and SnZn composite solder alloys and introduced the effect of nanoparticles on their microstructure, mechanical properties, wettability, and reliability. The mechanism of nanoparticles strengthening was analyzed and summarized. In addition, the shortcomings and future development trends of nanoparticle-reinforced lead-free solders were discussed, which is expected to provide some theoretical reference for the application of these composite solder in 3D IC package.

Effect of laser power on formation mechanism of laser brazing diamond coating has been investigated for the first time and its interface microstructure was also investigated. The BNi-2 alloy solder was utilized as braze material to fabricate brazing diamond coating on 65Mn steel substrate by using fiber laser. The laser spot was scanned at a steady speed 3 mm/s. With increase in laser power, BNi-2 alloy solder layer gradually melts and spreads fully until a laser power of 1.1 kW was reached. A further increase in laser power results in intense graphitization of the surface of the diamond grits and thermal damage. The breakage of diamond is observed when laser power is increased to 1.3 kW. Four steps are observed in laser brazing process: melting, gathering, fusing and spreading. The microstructure investigation indicates that both Cr₃C₂ and B₄C carbides are formed at the interface between diamond and BNi-2 alloy matrix.

Medical titanium and alumina (Al₂O₃) bioceramic are widely utilized as biomaterials. A reliable brazed joint of titanium and alumina was successfully obtained using biocompatible Au foil for implantable devices in the present study. The interfacial microstructure and reaction products of titanium/Au/Al₂O₃ joints brazed under different conditions were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). In this study, the typical interfacial microstructure of the titanium/Au/Al₂O₃ joint was titanium/Ti₃Au layer/TiAu layer/TiAu₂ layer/TiAu₄ layer/Au + granular TiAu₄ layer/TiO_x phase/Al₂O₃ ceramic. With increasing brazing temperature or holding time, the thicknesses of Ti₃Au + TiAu + TiAu₂ layers adjacent to the titanium substrate increased gradually. Shear tests indicated that the joint brazed at 1115 °C for 3 min exhibited the highest shear strength of 39.2 MPa. Typical fracture analysis displayed that the crack started at the Al₂O₃ ceramic and propagated along the interface of TiAu₂ and TiAu₄ reaction layers.

Titanium and zirconia (ZrO₂) ceramics are widely used in biomedical fields. This study aims to achieve reliable brazed joints of titanium/ZrO₂ using biocompatible Au filler for implantable medical products. The effects of brazing temperature and holding time on the interfacial microstructures and mechanical properties of titanium/Au/ZrO₂ joints were fully investigated by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and X-ray diffraction (XRD). The results indicated that the typical interfacial microstructure of the titanium/Au/ZrO₂ joint was titanium/Ti₃Au layer/TiAu layer/TiAu₂ layer/TiAu₄ layer/TiO layer/ZrO₂ ceramic. With an increasing brazing temperature or holding time, the thickness of the Ti₃Au + TiAu + TiAu₂ layer increased gradually. The growth of the TiO layer was observed, which promoted metallurgical bonding between the filler metal and ZrO₂ ceramic. The optimal shear strength of ~35.0 MPa was obtained at 1150 °C for 10 min. SEM characterization revealed that cracks initiated and propagated along the interface of TiAu₂ and TiAu₄ reaction layers.