Herein, a facile method of fabrication of high-density-polyethylene (HDPE)- hexagonal Boron Nitride (hBN) composite foams with high thermal conductivity, low dielectric constant, and low dielectric loss is presented. We have demonstrated that the generation of a microcellular structure can enhance the total thermal conductivity and decrease the real permittivity and dielectric loss of the hBN-based polymer composites. The microcellular HDPE-hBN composite foams were manufactured though melt mixing and SCF-treatment followed by physical foaming in an injection molding process. The introduction of SCF-treatment and foaming further exfoliated and dispersed hBNs and changed their orientation and arrangement within the polymer matrix. The HDPE-hBN composite foams exhibited more isotropic and higher total thermal conductivity and lower dielectric constant as compared to their solid counterparts. On the other hand, the solid injection molded HDPE-hBN composites exhibited a great anisotropies thermal conductivity due to the flow-induced orientation of hBN platelets. The solid injection molded HDPE-23.3 vol.% hBN composite exhibited an in-plane thermal conductivity of 10.11 W/(m.K), while the through-plane thermal conductivity was as low as 0.28 W/(m.K). This study presented a promising and industrially viable technique to develop low-k materials (dielectric constant less than 3) with tailorable thermal conductivity in-plane and through-plane.