A high thermal conductivity base plate is provided for use in air cavity packages. The base plate is at least partially comprised of reinforced silver composite. The composite can include a matrix of pure silver or a silver alloy and reinforcement particles. The reinforcement particles can include high thermal conductivity, low CTE particles selected from the group consisting of diamond, cubic boron nitride (c-BN), silicon carbide (SiC), and any combinations thereof. In some embodiments, the base plate is entirely comprised of the composite. In other embodiments, the base plate has a core made of the composite. The core can include at least one outer layer on the core. The semiconductor package can include one or more dice or transistors on the base plate, an insulated frame on the base plate, and one or more leads on the insulated frame.