Investigating the synergistic effects of hybrid and bimodal filler networks on the out-of-plane thermal conductivity of polymer composites

As 3D-integrated architectures such as high-bandwidth memory (HBM) increase in power density, managing vertical heat dissipation during burn-in testing (when temperatures can reach 280 °C) is critical. Traditional prepregs frequently lack sufficient thermal conductivity, causing thermal warpage or false device failures. In this study, we investigated monofiller, hybrid filler, and bimodal-morphology filler systems to increase the out-of-plane thermal conductivity (K_⊥) of styrene–ethylene–butylene–styrene (SEBS)-based polymer composites and prepregs at low filler loadings. While hybrid systems (h-BN/Al₂O₃) provide isotropic thermal bridges, they are hindered by high interfacial thermal resistance (R_int) at heterogeneous contacts. To address this issue, we developed a bimodal system using chemically identical h-BN fillers with different morphologies. This configuration facilitates an isotropic conduction network while reducing R_int through homogeneous filler-to-filler interfaces. At a total loading of 25 wt%, the aggregated bimodal prepreg achieved a K_⊥ of 0.56 W/m·K, which is an increase of 31% over the baselines of the monofiller. These results indicate that bimodal h-BN architectures comprise an effective method for developing high-performance, reliable substrates for next-generation semiconductor testing.