Latest advancement of fully additive process for 8 µm ultra-fine pitch chip-on-film (COF) by nano-size Ni–P metallization

This paper presents the first demonstration of a precision formation of conductor patterns as fine as 8 µm pitch for flex-based electronics assemblies. We have developed a novel fully additive process (FAP) by combining an ultra-thin chemical surface modification on polyimide (PI) to achieve a uniform Ni–P metallization and subsequent high-speed electrolytic pattern copper plating. On top of the modified interface, an ultra-thin anchoring layer (< 5 nm) constituting amide and carboxyl bonding is able to compensate the hydrophilic nature of the modification layer to obtain reliable adhesion even after being subjected to 300 °C heat treatment, as verified by XPS and TEM analysis at 1000kX. This exceptional property eliminates the trade-off concern by achieving both high-speed signal transmission and enhanced interfacial bond strength on a smooth surface (Ra < 60 nm). Experimental results revealed that the bending endurance and trace deformation characteristic of the fine pitch circuit by this FAP is comparable to conventional COF substrates and is also able to produce high-quality diffusion bonds with the formation of nano-twin structure on the Au–Au bonding interface. Insulation resistance measurements showed no metal residues and no current leakage between 16, 20, and 25 µm pitch COF after temperature humidity bias. On the basis of these results, FAP by direct Ni–P metallization proposed in this work provides unique opportunities for ultra-fine pitch and high electrical performance interconnects.