Maskless bumping by electroless plating for high pin count, thin, and low cost microcircuits

Without using a photolithographic process, a maskless bumping method has been achieved by palladium activation, nickel electroless plating, and ultrasonic soldering. A palladium chloride solution was developed for selective deposition. The average bump shear strength was 26.8 g/bump; this was maintained after 1000 hours at 150°C and after 300 thermal cycles. Nickel diffusion into the aluminum pads by 150-450°C annealing resulted in high adhesion strength and low electrical contact resistance. The bump structures were analyzed by AES (auger electron spectrometer), SIMS (secondary ion mass spectrometer), XPS (X ray photo spectrometer), and XRD (X ray diffractometer). The bumps were applied to high power transistors, LCD driver LSIs, and ASICs. Additionally, 376 pin count, 50 micrometer pitch bumps were successfully fabricated.