Cavity packaging structure and forming method thereof

The invention discloses a cavity packaging structure and a forming method thereof, the cavity packaging structure comprises a metal substrate, the metal substrate comprises a middle area and an edge area surrounding the middle area, the middle area comprises a mounting area, a causeway structure is formed on the edge area of the upper surface of the metal substrate, the causeway structure surrounds the middle area, and the surface of the surface of the metal substrate is provided with a cavity; pins extending to two sides are arranged in the causeway structure; the annular groove is positioned in the metal substrate in the middle area and surrounds the mounting area; the first buffer layer is located in the annular groove, and the thermal expansion coefficient of the first buffer layer is smaller than that of the metal substrate; the semiconductor chip is mounted on the upper surface of the mounting area and is electrically connected with the pins through metal leads; the upper cover is attached to the causew