Electroless Nickel/Copper Plating as a New Bump Metallization

An electroless bumping method was developed both for Rip-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 P/spl mu/m is required on the aluminum for sealing. A shear strength of 180 cN and a contact resistance of less than 2 m/spl omega/ 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 carded out. The average pull strength was 50 cN. Further investigations of the deposited copper are the influence of the size and electroless solder plating.