Electrophoretic Deposition of Ni/Au Coated Particles for Ultra-Fine Pitch Interconnection

Anisotropic Conductive Adhesives (ACAs), are a fine-pitch flip-chip packaging method that has been successfully utilised in flat screen assembly and smart cards for more than two decades, but it struggles to meet the challenge of ultra-fine pitch interconnection because of the increasing probability of short-circuits as the pitch reduces. Previous studies have shown that electrophoretic deposition can overcome this problem by selectively depositing the interconnection particles onto the target pads. However the proposed hydrochloric acid (HCl) based immersion charging method damages the particle, which is considered likely to significantly impact the conductivity and reliability of the resulting interconnections. This paper presents an alternative charging method that is harmless to the particles, and through use of specific test chips with different dimensions of pad array explores the pitch capability of electrophoretic deposition. The results proved that Ni/Au coated polymer core particles became positively charged when immersed into a NiCl2 solution, and the subsequent zeta potential tests illustrated that the conductivity and pH of the suspension strongly impact the zeta potential of the particles. Finally the charged particles were electrophoretically deposited onto chips with array of pads having diameters ranging from 10 to 90 µm, which demonstrates the feasibility of this ultra-fine pitch process.