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 approa ch to bumping directly on aluminum bondpads. An immersion tin layer fo r coating and soldering is plated on the copper. Due to the high alkal inity (pH > 12) of electroless Cu baths, a thick Ni layer of about 7 m u m is required on the aluminum for sealing. A shear strength of 180 c N and a contact resistance of less than 2 m Omega for the bumps were o btained. Because of the high hardness of nickel, conventional gang bon ding techniques are not applicable. The hardness of electroless copper is about 200 mHV(25), and after annealing in the range of 130-150 mHV (25). Thermocompression gang bonding of Au plated tape to the Ni/Cu/Sn metallization was carried out. The average pull strength was 50 cN. T he influences of the size and electroless solder plating of the deposi ted copper are also investigated.