a nickel / palladium / gold metallization stack is formed upon connection pads of integrated circuits at the wafer level through an electroless plating method . the metallization stack can be formed over copper or aluminum interconnect pads ; the lower nickel layer bonds securely to the copper or aluminum interconnect pads , while the intermediate palladium layer serves as a diffusion barrier for preventing the nickel from out - diffusing during subsequent thermal cycles . the upper gold layer adheres to the palladium and readily receives a variety of interconnect elements , including gold bumps , gold wire bonds , solder bumps , and nickel bumps . the electroless plating process permits connection pads to be formed using fine geometries , and allows adjacent connection pads to be formed within 5 micrometers of each other .