Epoxy Composites With Improved Thermal Conductivity and Low Viscosity for Electronic Packaging via Nanodiamond‐Decorated Boron Nitride Nanosheets

Electronic packaging underfill materials require high thermal conductivity (TC) and low viscosity, but combining these in epoxy (EP) remains a critical bottleneck. Prefabricated skeleton networks or high filler content enhance EP's TC yet severely compromise viscosity. Therefore, improving TC at a relatively low filler content without significantly sacrificing viscosity is highly desirable. Here, a dense, uniform layer of nanodiamonds (ND) is decorated on the boron nitride nanosheets (BNNS) surface to fabricate BNNS@ND, which is then incorporated into EP to develop a BNNS@ND/EP composite. The anchored ND is proposed to suppress BNNS stacking, thereby favoring uniform dispersion of BNNS@ND and the formation of continuous heat‐conducting pathways at relatively low filler content in EP. The ND coating is also considered to improve interfacial compatibility between BNNS and EP, as well as to alleviate the viscosity increase of EP composite associated with BNNS stacking. Consequently, compared to pure EP, incorporating merely 30 phr BNNS@ND increases the TC by 522.2% to 1.12 W/(m·K) and raises the glass transition temperature ( T g ) by 25.1% to 133.8°C of BNNS@ND/EP, while the composite retains a low viscosity of 16.7 Pa·s. This work offers a feasible approach for electronic‐packaging polymer composites with exceptional TC, T g , and viscosity.