Growth and strength of the solid solution phase (Ag) with indium

Over the past several years, we have developed several fluxless bonding processes using the silver-indium (Ag-In) system. With innovative design, the joint achieves high melting temperature, higher than 600°C, even when the bonding is performed at low temperature, 180°C. The joint has the structure of Ag/(Ag)/Ag₂In/(Ag)/Ag where (Ag) is the solid solution of Ag with In. Shear test results show that Ag₂In is the weak region in the structure. To increase the joint strength, an idea is to convert Ag₂In compound into (Ag) by continuous reaction with the adjacent (Ag) regions. When this happens, the melting temperature of the joint will increase to 790°C. In this research, we studied this converting process with systematic experiments. The joint is made between two copper (Cu) substrates to ensure that the samples break in the joint rather than in the substrate during shear test. The upper Cu was electroplated with 15μm thick Ag layer. The lower Cu was electroplated with 15μm thick Ag layer, followed by 8μm indium, which was the capped by 0.1μm Ag layer to avoid In oxidation. The samples were bonded at 180°C in 100 millitorr for 5 minutes. SEM & EDX analysis show that the joint has structure of Ag/(Ag)/Ag₂In/(Ag)/Ag, where the Ag₂In region is 15μm. Samples are well bonded between Cu substrates without crack or voids. To convert Ag₂In region into (Ag), samples were annealed at 200°C for 200, 300, 500 hours, respectively. 200°C was chosen because it is more compatible with the surviving temperature of most devices. The samples were cut in cross section, polished, and examined using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) to study its microstructure and composition. At 200 hours, the sample is still well bonded. The joint turned into Ag/(Ag)/alloy/(Ag)/Ag, where alloy is a m- xture of Ag₂In and Ag₃In compounds. At 300 hours, the joint structure has not changed but the alloy region becomes thinner. At 500 hours, the alloy region is almost gone. The annealing process continues. After the alloy region turns into (Ag), six samples will go through shear test. Shear strength will be measured and fracture mode identified. We expect the (Ag) phase to be significantly stronger than the Ag₂In compound. This bonding and annealing processes thus provide a new way to produce strong and high-temperature joints at low temperature, 200°C.