Characterization of Intermetallic Compound Formation on In/Bi/Sn Solder Bumps Used in Pb-Alloy Josephson Chip Packaging

An AuIn2intermetallic compound was discovered and its effect on bonding in Pb-alloy Josephson chip packaging was investigated. This was achieved by applying an In/Bi/Sn ternary alloy solder, which was characterized by X-ray and thermal analysis. Both bulk and vacuumdeposited In/Bi/Sn films were used, and in both cases the dominant compound was BiIn2. Indium, -SnIn, and -InSn were also detected. Bi3In5appeared as the In content decreased. The sequentially deposited thick solder film had a simple structure, but the structure at the interface between solder and Pd/Au was very complicated. The dominant compounds at this interface were AuIn2and Bi3In5. Joining strength using In/Bi/Sn solder depended on the solder-bump fabrication process. The fracture interface was analyzed using electron probe microanalysis (EPMA) and Auger electron spectroscopy (AES). Fracture occurred at the interface between Pd and Au. It was found that AuIn2intermetallic compound formation reduced chip bonding strength. Low-temperature annealing of Pd/Au thin film before solder evaporation reduced AuIn2intermetallic compound formation.