Interfacial Reactions and Joint Reliability in Ag-Cu-Ti Active Brazing of Al2O3-Metal Systems

Alumina is widely used in electronic packaging and high-temperature structural components because of its excellent heat resistance, electrical insulation, and wear resistance. However, reliable joining of dissimilar materials is difficult because of its poor wettability by metals and the difference in coefficients of thermal expansion. Ag-Cu-Ti active metal brazing enables joining without a separate metallization process by allowing Ti to react with the alumina surface and form Ti-O and Ti-Cu-O-based reaction layers. However, growth of the reaction layer does not always lead to improved strength. Excessive growth of brittle phases, metal-side intermetallic compounds, porosity, and residual stresses generated during cooling can become weaknesses in the joint. This review summarizes how processing conditions and filler design alter Ti migration, the composition, thickness, continuity, and substructure of reaction layers, and examines how these interfacial characteristics are connected to residual stress, fracture paths, joint strength, and thermal cycling stability. In addition, the effects of Ti content optimizing, heating-history control, multicomponent fillers, ductile Cu- or Ni-based interlayers, and W-, TiB-, and carbon-based reinforcements are compared. The literature analysis indicates that improving joint reliability requires the combined evaluation of the composition and continuity of Ti-O-rich and intermetallic compound-rich layers, the plastic deformation capacity of ductile interlayers, ceramic composition, and fracture location, rather than the total reaction-layer thickness alone. Future research should focus on standardized test methods, thermal cycling and long-term exposure evaluations, and quantitative optimization of process conditions based on interfacial substructures.