Fabrication of Nickel Microbump on Aluminium Substrate Using Electroless Nickel Plating

Abstract

Tape automated bonding (TAB) and flip chip bonding technologies have been progressing with the advancement of surface mounting technologies. The formation of metallic microbump is required to connect outer lead with each pad electrically for these mounting technologies. In general, aluminium has been used as a circuit or electrode material for large scale integrated circuits (LSI) fabrication. Recently, bump formation or via-hole filling by electroless nickel deposition was reported. A zincate process is generally used prior to electroless nickel plating on aluminium substrate. However, an organic resist cannot be applied because the zincate bath is strongly alkaline. Accordingly, we investigated electroless nickel plating on aluminium substrates using a nickel displacement process and fabrication of nickel bumps on aluminium electrode by electroless nickel plating. Furthermore, direct electroless nickel plating without nickel displacement on aluminium and nickel bump formation were studied. As major results, catalytic nickel site with a good adhesion on bulk aluminium substrate can be formed by the process of nickel displacement plating. A straight-walled nickel bump about 20 μm in width and 15 μm in height may be formed by optimization of nickel displacement plating bath composition and operating conditions. In addition, more uniform nickel bumps can be fabricated on an aluminium substrate by activation with dimethyl amine borane (DMAB) instead of nickel displacement plating.