Highly Thermally Conductive Epoxy Composite With Small‐Sized Boron Nitride/Alumina Hybrid Fillers for Fine‐Pitch Underfill Application

Advanced electronic packaging requires underfill materials with both high thermal conductivity and excellent processability, a demand that is further intensified by rising power densities and finer pitches. In this study, epoxy (EP) composites incorporating a hybrid filler system consisting of 1 μm hexagonal boron nitride (BN) platelets and 300 nm spherical alumina (Al 2 O 3 ) are systematically investigated to address this challenge. The incorporation of spherical Al 2 O 3 enables higher total filler loadings while maintaining processable viscosity. The optimized 65BN‐80Al 2 O 3 /EP composite achieves a thermal conductivity of 2.28 W m −1 K −1 at a total filler loading of 37.6 vol%, which is approximately 9 times that of neat epoxy and 30.3% higher than that of the 65BN/EP composite. The measured electrical and thermomechanical properties further support the potential of these composites as underfill candidates. Overall, this hybrid filler strategy based on small‐sized, multi‐morphology ceramic fillers offers a promising approach to addressing thermal management challenges in fine‐pitch electronic packaging.