Optimization of Hybrid Silica Composition in Polyimide Composite Films for Balanced Processability and High‐Frequency Dielectric Performance

Abstract

Hybrid silica‐filled polyimide (PI) composites were developed for low‐dielectric copper‐clad laminate (CCL) applications by systematically varying the hollow‐silica‐to‐nano‐silica ratio while maintaining a constant total silica content. The influence of hybrid filler composition on bulk rheological behavior, film‐forming characteristics, mechanical properties, thermal dimensional stability, environmental dielectric reliability, and high‐frequency transmission performance was systematically investigated to establish the processing–structure–property relationships of the composites. Increasing the nano‐silica fraction progressively enhanced filler–filler interactions and network development, resulting in increased viscosity and thixotropic behavior. At an intermediate hollow‐silica‐to‐nano‐silica ratio of 75:25 (Sample #2), a moderate filler‐network response was achieved while sufficient flow and leveling were retained, resulting in surface quality comparable to that of the hollow‐silica‐only formulation. Sample #2 exhibited the highest Martens hardness and B‐stage tensile strength and elongation, together with a 20.1% reduction in the coefficient of thermal expansion from 202.4 to 161.7 ppm/°C. After environmental reliability testing at 85°C/85% RH for 96 h, Sample #2 exhibited only minor changes in dielectric constant and dielectric loss and maintained stable high‐frequency transmission characteristics in the fabricated CCL. In contrast, excessive nano‐silica loading produced a strongly interconnected filler network, which restricted flow and surface leveling and was accompanied by increased surface roughness, inferior mechanical and interfacial properties, and greater dielectric degradation after environmental exposure. These results demonstrate that optimizing the hollow‐silica‐to‐nano‐silica ratio, rather than simply increasing the nano‐silica fraction, provides an effective strategy for balancing processability, mechanical reliability, thermal dimensional stability, environmental dielectric stability, and high‐frequency electrical performance in low‐dielectric PI composite CCLs.