Electroless nickel/copper plating as a new bump metallization

An electroless bumping method was developed both for flip chip and TAB applications. Electroless Ni/Cu plating is a maskless low-cost approach to bumping directly on aluminum bondpads. An immersion tin layer for coating and soldering is plated on the copper. Due to the high alkalinity (pH>12) of electroless Cu baths, a thick Ni layer of about 7 μm is required on the aluminum for seating. A shear strength of 180 cN and a contact resistance of less than 2 mΩ for the bumps were obtained. Because of the high hardness of nickel, conventional gang bonding techniques are not applicable. The hardness of electroless copper is about 200 mHV₂₅, and after annealing in the range of 130-150 mHV₂₅ thermocompression gang bonding of Au plated tape to the Ni/Cu/Sn metallization was carried out. The average pull strength was 50 cN. The influences of the size and electroless solder plating of the deposited copper are also investigated