Solder bumper formation using electroless plating and ultrasonic soldering

Aluminium electrodes on a Si wafer were surface-treated in a sequence involving palladium activation, Ni-P electroless plating, ultrasonic soldering with Pb-1Sn solder, and dipping in Sn-37Pb to form second-stage solder bumps. The average bump height and the shear force were 20.3 μm and 26.8 g/pad, respectively. The shear force did not decrease after a heating test (150°C fore 1000 h) or thermal cycle test (-65°C&rlarr2;room temperature&rlarr2;150°C, 300 cycles). Aluminum diffused to amorphous Ni-P through the Pd layer. Ni-Sn intermetallic compounds were formed at the Pb-1Sn/Ni-P interface. Low-cost polyester tab-automated bonding was utilized to melt low-temperature second solder on inner lead bonding. No bridge occurred when this work process was applied to an LSI with 376 electrodes, 100-μm wide with 105-μm pitch