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The effects of temperature and ultrasonic-assisted time on the microstructures and mechanical properties of the electroless nickel-immersion gold (ENIG)/Sn-3.5Ag/ENIG sandwich structure in ultrasonic chip bonding were investigated. Ultrasonic-assisted solder bonding was performed for 0.5–4 s at 25, 150, and 250 °C. The bonding was successful under all conditions. At 25 °C, Au–Sn intermetallic compounds (IMCs) were formed at the upper interface, and AuSn4 was formed on Ni–Sn IMCs at the lower interface. As the bonding time increased, spalling of AuSn4 was increasingly observed at the upper interface. At 150 °C, a trend similar to that at 25 °C was observed, but as the time increased, the thickness of the joint decreased, Au–Sn IMCs at the upper interface disappeared, and thin Ni–Sn IMCs and Cu6Sn5 were formed. The results observed at 250 °C differed from those observed at 25 and 150 °C because ultrasonic-assisted solder bonding was performed with the solder in a molten state. Ni–Sn IMCs were formed at the upper and lower interfaces, residual Sn and Au–Sn IMCs were present inside, and the thickness of the joint was very small. At 250 °C, the shear strength was high regardless of the bonding time. Observations of the top views and cross-sections of the fracture surfaces showed that, at 25 and 150 °C, the fractures initially occurred at the lower interface but moved to the upper interface as the bonding time increased. At 250 °C, the fractures did not significantly change, regardless of the ultrasonic-assisted time.

The growth kinetics of intermetallic compound layers formed between Sn-3.5Ag solder and Cu substrate were investigated as a consequence of solid-state isothermal aging. Isothermal aging was carried out in a temperature range between 70°C and 200°C for 0 to 60 days. A quantitative analysis of the intermetallic compound layer thickness as a function of time and temperature was performed. The diffusion couples showed a composite intermetallic layer comprised of Cu6Sn5 and Cu3Sn. The growth of intermetallic compounds followed diffusion-controlled kinetics and the layer thickness reached only 9 μm after 60 day of aging at 150°C. The apparent activation energies were calculated for the growth of the total intermetallic compound (Cu6Sn5+Cu3Sn); Cu6Sn5 and Cu3Sn intermetallic are 65.4, 55.4 and 75.7 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]