The Strength of High-Temperature Ag–In Joints Produced Between Copper by Fluxless Low-Temperature Processes

Two copper (Cu) substrates were bonded using silver (Ag) and indium (In) and annealed at 200–250 °C to convert the joints into the solid solution (Ag) for enhanced strength and ductility. Cu–Cu pair was chosen so that the samples break in the joint during shear test. The upper Cu was electroplated with 15 μm Ag. The lower Cu was plated with 15 μm Ag, followed by In and 0.1 μm Ag to inhibit indium oxidation. Two designs were implemented, using 8 μm and 5 μm In, respectively. The Cu substrates were bonded at 180 °C in 100 mTorr vacuum without flux. Afterwards, samples were annealed at 200 °C for 1000 h (first design) and at 250 °C for 350 h (second design), respectively. Scanning electron microscope with energy dispersive X-ray analysis (SEM and EDX) results indicate that the joint of the first design is an alloy of mostly (Ag) with micron-size Ag₂In and (ζ) regions, and that of second design has converted to a single (Ag) phase. Shear test results show that the samples are very strong. The breaking forces far exceed requirements in MIL-STD-883 H standards. Fracture incurs inside the joint and is a mix of brittle and ductile modes or only ductile mode. The joint solidus temperatures are 600 °C and 900 °C for the first and second designs, respectively.