Tailoring and Assessing Copper Nanoparticle Seeded Electroless Copper Plating on Polymer Microspheres

Electroless copper plating is a vital technique for depositing uniform and conductive copper layers, particularly for fabricating copper-coated polymer microspheres used in anisotropic conductive adhesives. However, achieving high-quality coatings on polymer microspheres remains challenging due to their inherent catalytic inertness and the lack of reliable evaluation metrics. Herein, copper nanoparticles are utilized as catalytic seeds for electroless copper plating on styrene-divinylbenzene (PSDVB) microspheres via a prereduction step using ammonia borane. After tailoring the bath composition and processing conditions, the optimized PSDVB@Cu microspheres with a dense 0.047 μm thick copper layer, 50.60 wt % copper content, and a low resistivity of 6.04 × 10^-6 Ω·m can be obtained. Crucially, two novel quantitative indicators, named oxidation degree and densification degree, are introduced to comprehensively assess coating quality. A clear correlation is established among the densification degree, oxidation degree, and electrical performance. Specifically, the microspheres become conductive only when the densification degree exceeds 0.2 and the oxidation degree falls below 3.0. With a densification degree above 0.4 and an oxidation degree below 1.4, the resistivity stabilizes near 10^-6 Ω·m. Beyond elucidating a tunable electroless copper plating process, this work provides a quantitative framework for coating assessment, paving the way for the standardized fabrication of high-performance copper-coated microspheres in advanced electronics.