In the electronics field, the demand for polymer-based interface materials with high thermal conductivity is increasing. In this study, a three-dimensional thermally conductive framework was fabricated using a bidirectional freezing technique, incorporating hexagonal boron nitride (h-BN) as the primary filler and polyethylenimine (PEI) as the binder. Moreover, a phosphate ester hyperbranched flame retardant (DTFR) was synthesized. It was miscible with the epoxy (EP) prepolymer, allowing the flame retardant to be immersed into the three-dimensional thermal conductivity skeleton along with the EP prepolymer. The thermal conductivity of EP composites was increased to 1.35 W m