Influence of reducibility of reductant on nickel nanoparticles formation on polyimide

Currently, consumer electronics, flexible and portable electronic products are the most popular because of further miniaturization accompanying more functionality per device and low costs. To produce high volume, high-density interconnect substrates, flexible printed circuits (FPCs) will become more and more important. For the fabrication of FPCs, polyimides (Pis) possess many outstanding properties, such as high thermal stability, good mechanical strength, low dielectric constants, and superior chemical resistance in acidic environments.^[1] To metalize PI surfaces, there are many processes such as physical vapor deposition, chemical vapor deposition, plasma modification, laser modification have been proposed². In consideration of the costs, a wet process seems to be a good approach because of lower expense and higher deposition rate. In this study, copper metallization on polyimide films was carried out via a wet chemical process. This process included the chemical reaction of KOH with PI, ion exchange of doped K⁺ with Ni²⁺, doped Ni²⁺ reduction by aqueous dimethylamine borane (DMAB) to form nickel nanoparticles (NNPs), and electroless copper (ELC) deposition catalyzed by NNPs film. Here, nickel is chosen to be the catalysts for triggering off electroless copper plating deposition instead of commonly-used expensive palladium catalyst. Furthermore, nickel also serves as barrier layer to avoid copper diffusion into polymer, causing destruction of adhesion. Some additives are added into the DMAB solution for facilitating the oxidation of DMAB and then releasing electrons to nickel ions doped in PI, accelerating charge transfer to doped ions to form continuous NNPs film which blocks the diffusion of reductant into PI. Owing to the additive´s assistance, faster charge transfer causes faster diffusion of nickel ions to the PI surface and less nickel ions remaining in PAA. Besides, continuous and dense NNPs film is f- - ormed without heat treatment due to the addition of the additive in the DMAB solution. The modified NNP has better catalytic activity for electroless Cu plating. As to the relationship among additive, reductant and formation of NNPs, X-ray photoelectron spectrometry (XPS), field emission scanning electron microscopy (FESEM), cross-sectional transmission electron microscopy (TEM), etc are employed for characterization, respectively.